Calculating the gross floor area requires you to know the area of the floors and the width of the interior walls. For the floors, you will need a tape measure and a calculator. To determine the wall width, you may need to contact the builder. The only rooms that aren't included are plazas or rooftops.

**Measure the floor area**Measure the floor are by going through one room at a time. Measure both the length and width of each room. In the case of rooms that do not have a standard geometry, break the room up into smaller shapes and add the area. For example, consider a room that has a main section that is 10 feet wide and 10 feet long has a narrower section that is five feet wide and long. You would take the two areas, 100 ft^2 and 25 ft^2, and add them. Find the total area by adding all the room areas together at the end.

**Determine the length of the walls**Use the tape measure and measure the length of the walls. Do this for the walls in each room.

**Determine the width of the interior walls**Determine the width of the interior walls and multiply this with the length to determine the area. To do this, you may need to contact your builder or view schematics.

**Add all the areas**Add all the areas up. This final number is the gross area.