Estimation can be done by various methods: 
1. Centre line method 
2. Long-wall/short wall method 
3. Partly Centre Line and Partly Cross Wall method/thin wall and thick wall method
Centre line method: In this method, centre line of both long and short wall is determined which is further multiplied by the respective breadth and height.

Long wall and short wall method: In this method, wall lengths are measured separately i.e out to out for Long wall and in to in for short wall which is further multiplied by the respective breadth and height.

Partly Centre Line and Partly Cross Wall Method: This method is adopted when external (i.e., around the building) wall is of one thickness and the internal walls having different thicknesses. In such cases, centre line method is applied to external walls and long wall-short wall method is used to internal walls. It is also known as thin wall and thick wall method.