How to Calculate a Home’s Square Footage

Understanding your home’s square footage requires more than just geometry

Traditional Kitchen, original photo on Houzz

All square feet are not created equal. Even Thomas Jefferson would have agreed. There are basements. There are attics. There are garages remodeled into living spaces, and there are oversized storage rooms locked in remote, poorly accessible corners. 

But when it comes to calculating a home’s square footage, we are left with a number — a simple number often relied upon to estimate a home’s value, cost to construct and overall livability. So it is worth understanding exactly what those simple digits represent. How are they calculated? What rules are used to figure it accurately, and what are the potential flaws in the venerated number known as the square footage of a home? Here’s how to calculate square feet.

The Math

If you’re like me, you have forgotten a thing or two from sophomore geometry. But I still bet you’ve got the chops to figure square footage if necessary. 

For the most part, it’s all about rectangles, and with the help of a few formulas, you can easily figure the square feet of an odd triangle or two as well.

We’ll break it down for you, but first, let’s review the rules used in figuring the official square footage of a home.

First Rule

The first rule is there is no first rule. Although guidelines have been adopted to help unify square footage standards (the American National Standards Institute, or ANSI, is the most widely recognized standard), there are gray areas, malleable guidelines and unique, individual interpretations often resulting in varying calculations for identical properties by real estate agents, appraisers, contractors, building departments and community tax assessors.

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Rely on This

Calculations for the square footage of a home are taken from the outside dimensions of the structure (so exterior and interior wall thickness is included).

If your home exterior is easily accessible, you can do it yourself with a 100-foot tape measure. Walk around the exterior measuring each segment and drawing the shape as you go. If, like most homes, your exterior has a series of square corners, you will be able to take measurements easily at the foundation, rounding measurements to the nearest half foot and sketching the exterior shape of your home. 

If there are overhangs or slopes making exterior measurements difficult, measure from the inside, and add the width of each exterior wall (likely half a foot, but you can check at the door jamb of an exterior wall).

The next step is to sit down with that paper and break the shape into a series of rectangles. (There are plenty of instructional videos available online if you’d like a hands-on lesson.) Then it’s a matter of using simple (relatively speaking) math:

Width x Length = Area 

You might find a triangle or two (Area = 0.5 x Base x Height). For example, a bay window with a pair of angled walls might (geometrically) be a pair of triangles with a rectangle in the middle, as shown in the sketch here. Your calculation for each triangle might look like this:

Area = 0.5 (4 feet base x 3 feet height)
Area = 0.5 (12 feet)
Area = 6 square feet

(Remember, this is floor space we’re talking about, so “height” in this sense is more like the length from where the angle of the triangle begins in the window to the line of where the bay window punches out from the facade of the home, as shown in the sketch.)

Sketch, original photo on Houzz

Add up the spaces of each of your rectangles and triangles, and subtract any areas that should not be included in overall square footage (for instance, partially enclosed patios or exterior storage areas), and you arrive at the total area (square feet).

This formula can be used for specific rooms as well, if you’re calculating for something like installing tile, carpet or wood flooring.

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If Only It Were That Simple

Gabriel Builders Inc., original photo on Houzz

Angled walls are only the beginning of your problems. Some additional, generally accepted rules follow:

  • Below grade, basements are generally not included. They are often noted separately.
  • Attic areas are counted if conditioned in the same manner as the rest of your home, accessible from livable space and with ceilings that are at least 84 inches high.
  • Stairs are included.
  • Garages and other nonconditioned spaces are not included. Some garages have been converted into habitable spaces and are included in the square footage calculations.
  • Condominium square footage is typically calculated using interior dimensions.


The upstairs areas are included, but open spaces, such as the vaulted area seen in the photo here, are not. The foyer area shown is only counted once. In calculating the square footage of homes with second stories, second floor measurements sometimes need to be taken from the interior, investigating the overall shape of the second floor as it pertains to the first. The exterior wall thickness is then added and included in the overall square footage.


This can be one of those gray areas. For an attic to be included in the square footage of a home, it should be finished and conditioned similarly to the rest of the living space. Both of these qualifiers rely on interpretations that might vary, sometimes resulting in conflicts.

Case Design & Remodeling Indy,
original photo on Houzz

Storage Rooms

Storage rooms are included in square footage calculations if the room is conditioned in a similar manner to the rest of the home and is accessible from living space in the home.

Square Footage for Tradespeople

During construction or remodeling, many tradespeople will be more concerned with the actual square footage relating to their specific task. For instance, a hardwood floor installer will typically take measurements from the interior dimensions of each floor space where coverage is required. The intention is to figure area for installation, not the accurate square footage per ANSI guidelines. 

A tile setter will not count the space of walls, cabinets, niches, fireplaces or any area not requiring tile. The same principle relates to carpet installers and so on.

Again, in these cases, the professionals still use the same formula to calculate square footage of specific rooms: Width x Length = Area.

Compare Your Calculations With a Local Building Agency

A Piece in the Puzzle

Square footage is not the only thing to take into account when you value a home, but it’s a factor many do consider when estimating the perceived value of a house. Discrepancies can result in loss of value and lawsuits.

Due to differing individual and geographic interpretations, we still find discrepancies in reported square footage for many homes. Real estate agents typically qualify their advertising with disclaimers such as “square footage to be verified by buyer.” Builders and architects may include areas in their square footage calculations that are later deemed inaccurate by city or county auditors. 

If you have doubts, county tax assessors and local building agencies are two good places to get accurate square footage

Monticello, original photo on Houzz

records. If there are discrepancies, appraisers are a good option for having square footage checked and documented.

If you feel you are up to the challenge yourself, break out the 100-foot tape measure and get to work yourself. In a home like Thomas Jefferson’s Monticello (shown here), you might save yourself quite a headache by having your square footage calculated professionally by an appraiser. But for simpler structures, there is no reason you can’t get your own hands dirty and see how well you recall the lessons from sophomore geometry class.

Categories: Architecture and Interiors, Renovation, Renovation & Restoration