Hey Isha! 

First find out the scale of the map 

For example if a topographic map has a scale of 1:50 000. To find out the area we need to find the scale.

If the scale of the map in RF is 1:50 000 then each cm represents 0.5 km.

Length - 2cm×0.5cm = 1.0 km

Width - 2cm × 0.5cm = 1.0km 

Area of in grid square is - 1.0km × 1.0 km = 1.0 sqkm.

Based on  the area of one grid square km you will be able to find out the area of a topographic map.

Wish you all the best.