Measuring the overall square footage of a house using the ANSI method is done using the exterior walls. You'll need a long measuring tape -- usually 100 feet will suffice. You'll also need graph paper and a pencil. Pick a corner and make your way around the house moving in either direction. As you make measurements, draw lines accordingly on the graph paper. Each square on the paper will represent one foot. Include any indentations that occur at the floor level only, otherwise disregard them [source: Demesne]. If you measure correctly, your lines should connect and the image on your graph paper should look like the shape of your house. If it doesn't, then your drawing is inaccurate and it's in your best interest to start over.
Each floor of the house should be measured separately at the level of the floor. If the top story of the house is part of an angled roof, the measurements should be taken inside to get an accurate result. Take all measurements to the nearest foot or tenth of a foot [source: Demesne]. There are measuring tapes marked off in tenths of a foot just for this purpose so it might be a good idea to pick one up. Once you've made all the measurements you need, you're almost ready to calculate the total square footage. The only thing left to do is subtract everything that shouldn't be included.
Read on to find out which areas should be included when determining square footage.