How to Calculate the Square Footage of a House

A house that sounds roomy and spacious online may actually have 800 of its total square feet contained in a four-foot-tall dirt-floor basement. What’s up with that? In cities like Washington DC, homebuyers are often confused about the square footage discrepancy between what’s listed and what they see on tour.

So why can square footage information be misleading? It turns out the American National Standards Institute (ANSI) guidelines on how to calculate a home’s square footage may be considered the standard, but there are no official laws that govern this process. Since the rules aren’t governed, some listing agents just post their best guess on a home’s square footage. As a result, buyers and their agents often have to investigate. In addition, some Multiple Listing Services (MLS) report all finished and unfinished square footage of a house as a single number, adding to the confusion. 

If you love a home, does it matter if the 2,000 square foot charmer turns out to be 1,600 square feet? When it comes time for you to sell, 400 fewer feet in measurement can impact the price substantially, since square footage is used to determine a home’s market value. Read on to learn how to calculate the square footage of a house.

Understanding how the square footage of a house is measured

To understand how the square footage of a house is measured, start by referring to your city’s building department records. Many city and county records are now available online, which makes getting this information much easier than it used to be. Some updates – like unpermitted remodeling – may not be reflected in the records, but it will still provide a good baseline estimate.

Then, familiarize yourself with basic ANSI guidelines for calculating square footage for single-family homes. Practices can vary slightly from market to market, but these rules apply to most areas in the country:

• Below-grade spaces (basements, dens, etc.) do not usually count toward a house’s square footage. Even a finished basement oftentimes can’t be counted toward a home’s Gross Living Area (GLA), but it can be noted separately in the listing’s total area. More on this below.
• The ANSI method specifies measuring from the exterior of the house, but the wall width is not usually subtracted to account for actual living space.
• Stairways and closet areas are included in the house’s square footage length.
• Finished attic square footage is included if an area has at least seven minimum feet of clearance.
• Covered, enclosed porches can only be included if heated and using the same system as the rest of the house.
• Garages, pool houses, guest houses, or any rooms that require you to leave the finished area of the main house to gain access are not counted in the square footage of a house.

How to calculate the square footage of a house: Three steps to follow

As a buyer it can be helpful to know how to calculate the square footage of a house yourself – just multiply the length and width of all applicable rooms in the home. All you’ll need to get started is a 100 sq ft tape measure, some graph paper, and a pencil. 

1. Assign a unit of measure

Assign a unit of measure to each square on the paper (ex. 12 inches or a foot) and measure to the nearest tenth of a foot.

2. Pick a wall

Pick a wall and begin measuring the distance, making your way around the interior perimeter of the house in one direction, then drawing lines accordingly on the graph paper. Keep in mind that although ANSI guidelines specify measuring the exterior walls, measuring from the inside will give you a better idea of the actual livable area.

3. Look at your floor plan

Lastly, go back over your floor plan, multiply the rectangular areas, and add them all up to get your final number. If your calculation includes an area that is not permissible, don’t forget to subtract it out.