Significant Figures: Rules, Definition, Calculator, Questions, Examples

Q: What's the difference between rounding and truncating when dealing with significant figures?

Rounding involves considering the next digit and adjusting the last significant figure up or down accordingly, while truncating simply cuts off extra digits without considering them. Rounding is generally preferred as it's more accurate. For example, rounding 3.146 to 2 sig figs gives 3.1, while truncating gives 3.1 as well, but rounding 3.156 gives 3.2, while truncating would give 3.1.

Q: How do you express numbers with many zeros in scientific notation to clarify significant figures?

Scientific notation is useful for clarifying significant figures in numbers with many zeros. For example, 100,000 could be written as 1.00 × 10^5 (3 sig figs) or 1.0 × 10^5 (2 sig figs), depending on the precision of the measurement.

Q: Can a number have zero significant figures?

No, a number cannot have zero significant figures. Even measurements that are very close to zero must have at least one significant figure. For instance, 0.00001 has one significant figure, not zero.

Q: Why is it important to keep track of significant figures throughout a multi-step calculation?

Keeping track of significant figures throughout a calculation ensures that the final result accurately reflects the precision of the original measurements. If sig figs are only considered at the end, intermediate rounding errors can accumulate, potentially leading to an inaccurate final result.

Q: How do significant figures apply to logarithms and exponential functions?

For logarithms, the number of significant figures in the result equals the number of significant figures in the argument. For exponential functions, the result should have the same number of significant figures as the exponent. For example, log(1000) = 3.000 (4 sig figs if 1000 has 4 sig figs), and e^2.34 should have 3 significant figures.

Q: How do you handle significant figures in addition and subtraction?

For addition and subtraction, the result should have the same number of decimal places as the least precise measurement (the one with the fewest decimal places). For example, 12.52 + 1.7 = 14.2, not 14.22, because 1.7 limits the precision to one decimal place.

Q: What's the rule for significant figures in multiplication and division?

In multiplication and division, the result should have the same number of significant figures as the measurement with the fewest sig figs. For instance, 2.34 × 5.6 = 13 (two sig figs), not 13.104, because 5.6 has only two significant figures.

Q: Why do we round the final answer in calculations involving significant figures?

We round the final answer to maintain the appropriate level of precision indicated by the original measurements. Keeping extra digits would imply a higher level of precision than actually exists in the data, potentially leading to misinterpretation of results.

Q: How do exact numbers affect significant figure calculations?

Exact numbers, such as counting numbers or defined constants, have infinite significant figures and don't limit the precision of a calculation. For example, in 2π × 3.14 cm, π is considered exact, so the result's significant figures are determined solely by 3.14 (three sig figs).

Q: Why doesn't the number of decimal places always equal the number of significant figures?

The number of decimal places and significant figures are different concepts. Decimal places count digits after the decimal point, while significant figures include all meaningful digits, regardless of the decimal point position. For instance, 0.00100 has 3 significant figures but 5 decimal places, while 1000.0 has 5 significant figures but only 1 decimal place.

Significant Figures

Edited By Vishal kumar | Updated on Jul 02, 2025 05:33 PM IST

Imagine you're baking a cake, and the recipe says you need 1.5 cups of sugar. You have only a measuring cup marked in whole numbers. It is important to be accurate to get the cake right. Because significant figures are a measure of precision, they determine which digits in a measured or calculated value are reliable, thus building confidence in the quality of the result. Actually, in this article, we are going to see what are significant figures, and how they provide accuracy in measured values.

This Story also Contains

What is Significant Figure?
Rounding off
Significant Figures in Calculation
Solved Example Besed On Significant Figures

Significant Figures

The concept of significant figures comes under the chapter Physics and Measurement which is a crucial chapter in Class 11 physics. It is not only important for board exams but also for competitive exams like the Joint Entrance Examination (JEE Main), National Eligibility Entrance Test (NEET), and other entrance exams such as SRMJEE, BITSAT, WBJEE, VITEEE and more. Over the last ten years of the JEE Main exam (from 2013 to 2023), a total of two questions have been asked on this concept. And for NEET three questions were asked from this concept.

What is Significant Figure?

The figures of a number that expresses a magnitude to a specified degree of accuracy. All non-zero digits are significant

For Example-

42.3 -Three significant figures

238.4 -four significant figures

33.123 -five significant figures

Zero becomes a significant figure if it exists between two non-zero digits

For example-

2.09 - Three significant figures

8.206 -four significant figures

6.002 -four significant figures

For leading zero(s), the zero(s) to the left of the first non-zero digits are not significant.

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For example-

0.543 - three significant figures

0.069 - two significant figures

0.002 -one significant figure

The trailing zero(s) in a number without a decimal point are not significant. But if the decimal point is there then they will be counted in significant figures.

For example-

4.330- four significant figures

433.00- five significant figures

343.000- six significant figures

Exponential digits in scientific notation are not significant.

For example- 1.32 X 10^-2- three significant figures

Rounding off

While rounding off measurements, we use the following rules by convention:

Rounding off of figures during calculation helps to make the calculation of big digits easier.

(1) If the digit to be dropped is less than 5, then the preceding digit is left unchanged.

Example: x=7.82 is rounded off to 7.8, again x=3.94 is rounded off to 3.9.

(2) If the digit to be dropped is more than 5, then the preceding digit is raised by one.

Example: x = 6.87 is rounded off to 6.9, again x = 12.78 is rounded off to 12.8.

(3) If the digit to be dropped is 5 followed by digits other than zero, then the preceding digit is raised by one.

Example: x = 16.351 is rounded off to 16.4, again x = 6.758 is rounded off to 6.8.

(4) If the digit to be dropped is 5 or 5 followed by zeros, then the preceding digit is left unchanged if it is even.

Example: x = 3.250 becomes 3.2 on rounding off, again x = 12.650 becomes 12.6 on rounding off.

(5) If the digit to be dropped is 5 or 5 followed by zeros, then the preceding digit is raised by one if it is odd.

Example: x = 3.750 is rounded off to 3.8, again x = 16.150 is rounded off to 16.2.

Significant Figures in Calculation

1. Rules for addition and subtraction

The result of an addition or subtraction in the number having different precisions should be reported to the same number of decimal places as are present in the number having the least number of decimal places.

For example:-

1) 33.3+3.11+0.313=36.723 but here the answer should be reported to one decimal place as the 33.3 has the least number of the decimal place(i.e only one decimal place), therefore the final answer = 36.7

2) 3.1421+0.241+0.09=3.4731 but here the answer should be reported to two decimal places as the 0.09 has the least number of decimal place(i.e two decimal places), therefore the final answer=3.47

2. Rules for multiplication and division

The answer to a multiplication or division is rounded off to the same number of significant figures as is possessed by the least precise term used in the calculation:-

For example:-

1) 142.06 x 0.23=32.6738 but here the least precise term is 0.23 which has only two significant figures, so the answer will be 33.

Solved Example Besed On Significant Figures

Example 1: What is true for significant figure

1) The higher no. of significant figures, the higher the accuracy

2) All non-zero digits are significant

3) Both A and B

4) only B

Solution:

Significant figures -

The figures of a number that express a magnitude to a specified degree of accuracy

Higher accuracy means there are higher no of significant figures.

Hence, the answer is the option is (3).

Example 2: Find the true match -

Measurement	No. of significant figures
1) 2165.4	P) 3
2) 238.4	Q) 5
3) 2.05	R) 4

1)1 -Q, 2 - R, 3- P

2)1 - R, 2 -P, 3 - Q

3)1 -P, 2 - R, 3 - Q

4)1 - P, 2 - Q, 3 - R

Solution:

As we have studied all non-zero digits are significant and a zero becomes a significant figure if it exists between two non-zero digits

42.3 -Three significant figure

238.4 -four significant figure

2165.4 -five significant figures

Hence, the correct option is (1).

Example 3: The diameter and height of a cylinder are measured by a meter scale to be 12.6±0.1 cm and 34.2±0.1 cm, respectively. What will be the value of its volume in the appropriate significant figure?

1) 4300±80 cm3
2) 4264.4±81.0 cm3
3) 4264±81 cm3
4) 4260±80 cm3

Solution:

v=πd24 h=4260 cm3Δvv=2Δdd+ΔhhΔv=2×0.1v12.6+0.1v34.2=0.212.6×4260+0.1×426034.2=80∴ Volume =4260±80 cm3

Hence, the answer is the option (4).

Example 4: Which of the following has the maximum no. of significant figures?

1) 234.000
2) 0.000303
3) 234×105
4) 12×10−5

Solution:

Leading Zeros-

0.000303 has 3 significant figures
Exponential digits in scientific notation are not significant.

234×105 has 3 significant figures
12×10−5 has 2 significant figures

Trailing Zeros -

234.000 has 6 significant figures

All zeros to the right of a decimal point are significant

So 234.000 has the maximum number of significant figures.

Hence, the correct option is 1.

Example 5: For the four sets of three measured physical quantities as given below. Which of the following options is correct?

(i) A1=24.36,B1=0.0724,C1=256.2
(ii) A2=24.44,B2=16.082,C2=240.2
(iii) A3=25.2,B3=19.2812,C3=236.183
(iv) A4=25,B4=236.191,C4=19.5

1) A4+B4+C4<A1+B1+C1=A2+B2+C2=A3+B3+C3
2) A1+B1+C1=A2+B2+C2=A3+B3+C3=A4+B4+C4
3) A1+B1+C1<A3+B3+C3<A2+B2+C2<A4+B4+C4

4) None of these

Solution:

A1+B1+C1=24.36+0.0724+256.2=280.6324=280.6A2+B2+C2=24.44+16.082+240.2=280.722=280.7A3+B3+C3=25.2+19.2812+236.183=280.6642=280.7A4+B4+C4=25+236.191+19.5=280.691=281

Answer should be A_1+B_1+C_1<A_2+B_2+C_2=A_3+B_3+C_3<A_4+B_4+C_4

Hence, the answer is option (3).

Summary
Significant figures improve precision and accuracy in measurements and calculations, which is essential for scientific experiments and competitive exams. The greater the number of significant figures, the more precise the measurement.

Frequently Asked Questions (FAQs)

1. What's the difference between rounding and truncating when dealing with significant figures?

Rounding involves considering the next digit and adjusting the last significant figure up or down accordingly, while truncating simply cuts off extra digits without considering them. Rounding is generally preferred as it's more accurate. For example, rounding 3.146 to 2 sig figs gives 3.1, while truncating gives 3.1 as well, but rounding 3.156 gives 3.2, while truncating would give 3.1.

2. How do you express numbers with many zeros in scientific notation to clarify significant figures?

Scientific notation is useful for clarifying significant figures in numbers with many zeros. For example, 100,000 could be written as 1.00 × 10^5 (3 sig figs) or 1.0 × 10^5 (2 sig figs), depending on the precision of the measurement.

3. Can a number have zero significant figures?

No, a number cannot have zero significant figures. Even measurements that are very close to zero must have at least one significant figure. For instance, 0.00001 has one significant figure, not zero.

4. Why is it important to keep track of significant figures throughout a multi-step calculation?

Keeping track of significant figures throughout a calculation ensures that the final result accurately reflects the precision of the original measurements. If sig figs are only considered at the end, intermediate rounding errors can accumulate, potentially leading to an inaccurate final result.

5. How do significant figures apply to logarithms and exponential functions?

For logarithms, the number of significant figures in the result equals the number of significant figures in the argument. For exponential functions, the result should have the same number of significant figures as the exponent. For example, log(1000) = 3.000 (4 sig figs if 1000 has 4 sig figs), and e^2.34 should have 3 significant figures.

6. How do you handle significant figures in addition and subtraction?

For addition and subtraction, the result should have the same number of decimal places as the least precise measurement (the one with the fewest decimal places). For example, 12.52 + 1.7 = 14.2, not 14.22, because 1.7 limits the precision to one decimal place.

7. What's the rule for significant figures in multiplication and division?

In multiplication and division, the result should have the same number of significant figures as the measurement with the fewest sig figs. For instance, 2.34 × 5.6 = 13 (two sig figs), not 13.104, because 5.6 has only two significant figures.

8. Why do we round the final answer in calculations involving significant figures?

We round the final answer to maintain the appropriate level of precision indicated by the original measurements. Keeping extra digits would imply a higher level of precision than actually exists in the data, potentially leading to misinterpretation of results.

9. How do exact numbers affect significant figure calculations?

Exact numbers, such as counting numbers or defined constants, have infinite significant figures and don't limit the precision of a calculation. For example, in 2π × 3.14 cm, π is considered exact, so the result's significant figures are determined solely by 3.14 (three sig figs).

10. Why doesn't the number of decimal places always equal the number of significant figures?

The number of decimal places and significant figures are different concepts. Decimal places count digits after the decimal point, while significant figures include all meaningful digits, regardless of the decimal point position. For instance, 0.00100 has 3 significant figures but 5 decimal places, while 1000.0 has 5 significant figures but only 1 decimal place.

11. What are significant figures and why are they important in physics?

Significant figures (sig figs) are the digits in a measurement that carry meaning and reliability. They're important in physics because they indicate the precision of a measurement and help maintain accuracy when performing calculations. Using the correct number of sig figs prevents overstating the precision of results and ensures that calculated values reflect the limitations of the original measurements.

12. How do you determine which digits in a number are significant?

To determine significant figures, follow these rules: 1) All non-zero digits are significant. 2) Zeros between non-zero digits are significant. 3) Leading zeros are never significant. 4) Trailing zeros after a decimal point are significant. 5) Trailing zeros in a whole number are significant only if there's a decimal point. For example, in 0.00305400, there are 5 significant figures (3, 0, 5, 4, 0).

13. How do significant figures relate to uncertainty in measurements?

Significant figures implicitly indicate the uncertainty in a measurement. The last significant figure typically has some uncertainty, while all preceding digits are considered certain. For example, a measurement of 1.23 cm implies an uncertainty in the last digit, suggesting the actual value could be between 1.22 and 1.24 cm.

14. What's the difference between accuracy and precision in terms of significant figures?

Accuracy refers to how close a measurement is to the true value, while precision relates to the reproducibility of measurements and is reflected in the number of significant figures. A measurement can be precise (many sig figs) but inaccurate, or accurate but imprecise. Significant figures primarily indicate precision, not accuracy.

15. How do significant figures apply to very large or very small numbers in scientific notation?

In scientific notation, all digits shown are considered significant, regardless of the exponent. For example, 3.00 × 10^8 has 3 significant figures, while 3.0 × 10^8 has 2. The exponent (10^8) doesn't affect the number of significant figures; it only indicates the number's magnitude.

16. How do you handle significant figures when working with percentages?

When working with percentages, treat the percentage as a two-step process: division by 100 and multiplication by the original number. Apply the multiplication/division rule for sig figs. For example, 13% of 1.54 would be calculated as 0.13 × 1.54 = 0.20 (2 sig figs).

17. What's the relationship between significant figures and error propagation?

Significant figures are a simplified way of handling error propagation. By following sig fig rules, we implicitly account for how uncertainties in measurements affect the uncertainty of the final result, without performing complex error propagation calculations.

18. Can significant figures ever increase during calculations?

Generally, the number of significant figures cannot increase during calculations, as this would imply gaining precision from less precise data. However, in some cases, like when working with exact numbers or when intermediate results are much larger than the uncertainty, it may be appropriate to retain more digits to avoid rounding errors in multi-step calculations.

19. How do you determine significant figures in measured values versus calculated values?

For measured values, significant figures are determined by the precision of the measuring instrument and the care taken in the measurement. For calculated values, sig figs are determined by applying the appropriate rules (addition/subtraction or multiplication/division) based on the sig figs of the input values used in the calculation.

20. Why don't we simply use uncertainties instead of significant figures in all cases?

While uncertainties provide more precise information about measurement precision, significant figures offer a simpler, quicker method for handling precision in everyday calculations. Sig figs are easier to teach and apply, making them useful for introductory physics and quick estimations. However, for more rigorous scientific work, explicit uncertainty calculations are often preferred.

21. How do significant figures relate to the concept of precision in digital versus analog measurements?

Digital measurements often give a false sense of precision due to their discrete readouts. For example, a digital scale showing 10.47 g implies 4 sig figs, but the actual precision might be less. Analog measurements, like reading a meniscus, require judgment and often have fewer sig figs. It's crucial to understand the true precision of the instrument, regardless of its display.

22. What's the difference between absolute and relative uncertainty, and how do they relate to significant figures?

Absolute uncertainty is the actual range of uncertainty (e.g., ±0.1 cm), while relative uncertainty is the ratio of uncertainty to the measured value (e.g., 1%). Significant figures implicitly represent relative uncertainty. A measurement with more sig figs has a smaller relative uncertainty. For instance, 1.00 cm (3 sig figs) implies a smaller relative uncertainty than 1.0 cm (2 sig figs).

23. How do you handle significant figures when dealing with constants like π or e in calculations?

Mathematical constants like π and e are considered to have infinite significant figures. In calculations, they don't limit the number of sig figs in the result. The sig figs in the result are determined by the other measured or calculated values in the equation. However, it's common to use an approximation of these constants (e.g., 3.14 for π) with a specific number of sig figs when appropriate for the calculation's precision.

24. Why is it incorrect to simply count decimal places to determine significant figures?

Counting decimal places is not a reliable method for determining significant figures because sig figs include all meaningful digits, not just those after the decimal point. For example, 1000.0 has 5 sig figs but only one decimal place, while 0.001000 has 4 sig figs but six decimal places. Sig figs depend on the number's precision, not its decimal representation.

25. How do you express the uncertainty of a measurement using significant figures?

Significant figures implicitly express uncertainty. The last significant figure represents the digit with uncertainty. For example, 1.23 cm implies the measurement is certain to 1.2 cm, with some uncertainty in the last digit. To express explicit uncertainty, you might write 1.23 ± 0.01 cm, indicating uncertainty in the second decimal place.

26. What's the importance of significant figures in comparing experimental results to theoretical predictions?

Significant figures are crucial when comparing experimental results to theoretical predictions because they indicate the precision of the measurement. Comparing a theoretical value to an experimental result with more sig figs than justified by the measurement's precision can lead to false conclusions about the agreement between theory and experiment.

27. How do you handle significant figures in exponential decay or growth calculations?

In exponential decay or growth calculations, apply the multiplication/division rule for sig figs. The number of sig figs in the result should match the quantity with the fewest sig figs among the initial value, the base of the exponent (e.g., e), and the exponent itself. Pay special attention to the precision of time measurements in these calculations.

28. Why is it important to consider significant figures in graphing data?

Significant figures are important in graphing because they indicate the precision of each data point. When plotting points or error bars, the number of sig figs should reflect the measurement precision. Similarly, when determining the slope or intercept of a best-fit line, the number of sig figs in these calculated values should be consistent with the precision of the original data.

29. How do you determine significant figures in very small numbers close to zero?

For very small numbers close to zero, count the significant figures starting from the first non-zero digit. Leading zeros are not significant. For example, 0.00034500 has 5 sig figs (3, 4, 5, 0, 0). Scientific notation can help clarify: 3.4500 × 10^-4 clearly shows 5 sig figs.

30. What's the relationship between significant figures and the number of digits displayed on a calculator?

Calculators often display more digits than are significant based on the input values. It's crucial to understand that the extra digits shown by a calculator don't represent increased precision in the result. Always round the calculator output to the appropriate number of significant figures based on the input values and the rules for the operation performed.

31. How do significant figures apply to vector quantities in physics?

When dealing with vector quantities, apply sig fig rules to each component separately. For vector addition or subtraction, use the addition/subtraction rule for each component. For vector magnitude calculations (which involve squares and square roots), use the multiplication/division rule. The final vector should have components with consistent sig figs based on these rules.

32. What's the role of significant figures in dimensional analysis and unit conversions?

In dimensional analysis and unit conversions, significant figures help maintain the appropriate precision throughout the calculation. Conversion factors should be treated as exact numbers unless otherwise specified. The number of sig figs in the final result should be determined by the least precise measurement or conversion factor used in the calculation.

33. How do you handle significant figures when working with logarithmic scales, such as pH or decibels?

For logarithmic scales, the number of decimal places in the logarithmic value corresponds to the number of significant figures in the original measurement. For example, a pH of 4.7 implies two sig figs in the hydrogen ion concentration. When converting back from logarithmic scales, be careful to maintain the appropriate number of sig figs based on the precision of the logarithmic value.

34. Why is it important to distinguish between significant figures and decimal places in scientific writing?

Distinguishing between significant figures and decimal places is crucial in scientific writing because they convey different information. Sig figs indicate the precision of a measurement, while decimal places simply show the position relative to the decimal point. Misinterpreting one for the other can lead to overstating or understating the precision of data, potentially affecting the conclusions drawn from the data.

35. How do you determine significant figures in numbers expressed in scientific notation?

In scientific notation, all digits shown in the coefficient are considered significant. The exponent does not affect the number of sig figs. For example, 3.00 × 10^4 has 3 sig figs, while 3.0 × 10^4 has 2 sig figs. This format makes it easier to identify sig figs compared to standard notation, especially for very large or small numbers.

36. What's the importance of significant figures in reporting experimental results?

Significant figures are crucial in reporting experimental results because they communicate the precision of the measurements and calculations. Using the correct number of sig figs prevents overstating the precision of results, allows for proper comparison with other data or theoretical predictions, and helps other scientists understand the limitations of the experimental setup and methodology.

37. How do you handle significant figures when working with trigonometric functions?

For trigonometric functions, the number of significant figures in the result should match the number of sig figs in the angle measurement. However, be cautious with angles near special values (e.g., 0°, 90°) where small changes can significantly affect the result. In these cases, it may be necessary to carry extra digits in intermediate calculations to avoid rounding errors.

38. What's the relationship between significant figures and the concept of least count in measurements?

The least count of a measuring instrument is the smallest division that can be reliably read, and it often determines the number of significant figures in a measurement. The last significant figure in a measurement typically corresponds to the least count of the instrument. For example, if a ruler has millimeter markings, measurements would typically be reported to the nearest millimeter, with the last sig fig in the millimeter place.

39. How do you apply significant figure rules to numbers in scientific notation during calculations?

When calculating with numbers in scientific notation, apply the standard sig fig rules to the coefficients. For multiplication and division, the result should have the same number of sig figs as the least precise factor. For addition and subtraction, first convert to a common exponent, then apply the rule based on decimal places. After the calculation, express the result in proper scientific notation with the correct number of sig figs.

40. Why is it important to consider significant figures in computer-based data analysis and simulations?

In computer-based data analysis and simulations, it's crucial to consider significant figures because computers can calculate to many decimal places, potentially giving a false sense of precision. Programmers and analysts must ensure that the precision of outputs matches the precision of inputs and that rounding is done appropriately. Failing to do so can lead to misinterpretation of results or false conclusions based on unjustified precision.

41. How do you handle significant figures when working with physical constants in equations?

When working with physical constants in equations, consider the precision needed for your calculation. Many constants are known to high precision and can be treated as exact for most calculations. However, if the constant's precision is relevant to your calculation (e.g., in high-precision experiments), use the constant with the appropriate number of sig figs based on its uncertainty. The final result's sig figs should then be determined by the least precise value used in the calculation.

42. What's the relationship between significant figures and the concept of "guard digits" in complex calculations?

Guard digits are extra digits carried through intermediate steps of a calculation to minimize rounding errors. While the final result should be rounded to the appropriate number of sig figs, using guard digits (usually 1-2 extra digits) in intermediate steps can improve the accuracy of the final result, especially in complex or multi-step calculations. This practice helps balance the principles of significant figures with the need for