To get square footage, multiply the length of the room by the width of the room. This will be measured from the interior of the room. In all of our room scenarios, we will utilize a 10 ft × 15 ft room as an example: This is the straightforward formula: Square Footage = Length (ft) x Width (ft). There are several other methods for calculating room size, but this is the most common one. As you can see, it's very simple to calculate the square footage of a room.