How to Calculate Square Footage

Square footage is just area measured in feet, and the math behind it is something you already learned in grade school. Length times width. The part that trips people up isn't the multiplication, it's the messy stuff around it: rooms that aren't rectangles, measurements taken in inches, and figuring out what actually counts toward a home's size. This guide walks through all of it, with real numbers you can follow along with.

The basic formula

For any rectangle or square, area equals length times width:

Area = length x width

Measure both sides in feet, multiply, and the answer is in square feet. A room that is 12 feet long and 16 feet wide is 12 x 16 = 192 square feet. A 12 by 12 room is 144 square feet. A closet that is 4 feet by 6 feet is 24 square feet. That's the whole engine. Everything else in this guide is a variation on it.

The reason the units work out to "square" feet is worth understanding once. A square foot is a tile that is one foot on each side. When you multiply a 12-foot length by a 16-foot width, you're counting how many of those one-foot tiles fit on the floor, and 192 of them do. If that idea is fuzzy, the short primer on what a square foot is lays it out.

How to measure a rectangular room, step by step

Grab a tape measure or a laser measure. A laser tool is faster and more accurate over long distances, but a 25-foot tape is fine for most rooms.
Measure the length. Run the tape along the longest wall, floor level, from one wall to the opposite wall. Write it down. Say it's 15 feet 6 inches.
Measure the width. Do the same across the room. Say it's 11 feet 3 inches.
Convert inches to feet. 6 inches is 0.5 feet, and 3 inches is 0.25 feet. So you have 15.5 feet by 11.25 feet. (Divide any inch value by 12 to get its share of a foot.)
Multiply. 15.5 x 11.25 = 174.375 square feet. Round at the very end, to about 174 square feet.

If you'd rather not juggle the fractions, enter the raw feet and inches into the feet and inches tool or the room calculator and let it convert for you.

L-shaped and irregular rooms

Very few rooms are one clean rectangle. Bay windows, bump-outs, closets, and open floor plans all break the box. The fix is the same one every contractor and appraiser uses: split the space into rectangles you can measure, find each area, and add them together.

Take an L-shaped living room. Draw it on paper, then cut the L into two rectangles with a single line. Measure the first rectangle at 20 feet by 14 feet, which is 280 square feet. Measure the second at 10 feet by 8 feet, which is 80 square feet. Add them: 280 + 80 = 360 square feet. Done.

This "break it into rectangles" method comes up any time people talk about odd rooms. In a long forum thread on measuring a room with ten different wall lengths, the advice that keeps getting repeated is exactly that: break it up into rectangles, add a triangle or two where the walls angle, and total everything. One responder even worked the poster's dimensions down to about 519 square feet using that approach. There is no magic single formula for an arbitrary shape, only the discipline of chopping it into shapes you know.

The multi-area calculator at the top of this page is built for this. Add each rectangle as its own entry and it keeps a running total, so you don't lose track of the pieces.

Circles and triangles

Some spaces are curved or angled, and length times width won't cut it.

Circles use pi times the radius squared:

Area = pi x r squared (pi is about 3.14159)

The radius is half the diameter, measured from the center to the edge. A round patio 16 feet across has a radius of 8 feet, so the area is 3.14159 x 8 x 8 = about 201 square feet. Use the circle calculator for pools, garden beds, and rounded patios.

Triangles use base times height, divided by two:

Area = (base x height) / 2

Measure the base along the bottom, then the height straight up from that base to the opposite point (the height, not the slanted side). A triangular deck with a 12-foot base and a 9-foot height is (12 x 9) / 2 = 54 square feet. The triangle calculator handles this, including odd triangles where you only know the three side lengths.

For a room that mixes a rectangle and an angled corner, calculate each piece with the right formula and add them. A rectangle plus a triangle is still just addition at the end.

How to measure a whole house

A house is every finished room added together, but the tricky part is what counts as "finished living area." Appraisers and lenders don't guess at this. They follow the ANSI Z765 standard, the American National Standard for measuring single-family homes. Fannie Mae made ANSI-compliant measurement mandatory for its appraisals starting in April 2022, and Freddie Mac adopted the same requirement, so it's now the default language of home size in the United States.

A few of the rules that matter most:

Ceiling height. Finished space only counts if the ceiling is at least 7 feet high. Under a sloped ceiling, at least half the finished floor area must be 7 feet or taller, and nothing below 5 feet counts.
Measure to the exterior walls. For gross living area, appraisers measure the outside footprint of the finished, above-grade space, then round to the nearest whole square foot.
Below-grade space is separate. A basement, even a finished walkout, is reported as below-grade area, not as part of the gross living area above grade. That's why a "2,000 sq ft" house with a finished basement might list more total finished space than its official living area.

This is also why the number on a listing, the number in county tax records, and the number you measure yourself can all disagree. Redfin's guide on wrong square footage notes that tax records are often outdated or were never measured to a standard at all, and that a professional measurement to ANSI is the tiebreaker. If you're checking a home's size for a purchase or a refinance, measure the finished, above-grade rooms and keep the basement in its own bucket. The house calculator is set up around these living-area rules.

Converting from other units

You won't always have measurements in feet. Here's how to get to square feet from the units you're most likely to hit:

Square inches to square feet: divide by 144. A 300-square-inch tile sample is 300 / 144 = about 2.08 square feet. (Converter)
Square yards to square feet: multiply by 9. Carpet quoted at 20 square yards is 20 x 9 = 180 square feet.
Square meters to square feet: multiply by 10.764. A 50-square-meter flat is 50 x 10.764 = about 538 square feet. (Converter)
Acres to square feet: multiply by 43,560. A quarter-acre lot is 0.25 x 43,560 = 10,890 square feet, and to go the other way, divide square feet by 43,560. (Converter)

Price per square foot

Once you have the area, cost is easy:

Total cost = area x price per square foot

Flooring at $4.50 per square foot across a 174-square-foot room is 174 x 4.50 = $783 in materials. Price per square foot also runs the other direction in real estate: divide a home's price by its square footage to compare listings. A $400,000 house at 2,000 square feet is $200 per square foot, and that single number lets you line it up against a $525,000 house at 2,500 square feet ($210 per square foot) even though the sticker prices look far apart. Always add a waste factor for materials, usually 5 to 10 percent for flooring and up to 15 percent for tile with a lot of cuts.

Common mistakes to avoid

Measuring in inches and forgetting to convert. If you multiply inches by inches, you get square inches, not square feet. A 120-inch by 144-inch room is 17,280 square inches, which is 120 square feet, not 17,280. Divide by 144, or convert each side to feet before multiplying.
Not subtracting non-living space. Don't count a garage, an unfinished basement, an attic under 7 feet, or an open two-story foyer twice. These routinely inflate a "square footage" number that shouldn't include them.
Rounding too early. Round every step and the errors stack. Carry the decimals through the whole calculation and round once at the end.
Measuring the roof or wall by its footprint. A pitched roof has more surface than the floor beneath it, and walls have height, not width. Use the roof calculator and wall calculator so the geometry is handled correctly.
Trusting old records. Tax and listing square footage are frequently wrong. When it matters, measure it yourself or get an ANSI measurement.

Frequently asked questions

How do you calculate square footage?

Multiply length by width, with both in feet, for any rectangular space. A 10 by 12 room is 120 square feet. For a shape made of several rectangles, measure and calculate each one, then add the results. Use pi times the radius squared for circles and base times height divided by two for triangles.

How do you figure out square footage of an irregular room?

Break the room into rectangles, and where the walls angle, triangles. Calculate the area of each piece separately, then add them all together. This works for L-shapes, bay windows, and bump-outs, and it's the same method appraisers use on odd floor plans.

How is square footage of a house calculated?

Every finished, above-grade room is measured and added together, following the ANSI Z765 standard that Fannie Mae and Freddie Mac require. Space only counts if the ceiling is at least 7 feet, and basements are reported separately as below-grade area rather than as part of gross living area.

How do you measure square footage without a tape measure?

You can pace it off as a rough estimate. A normal adult stride is close to 2.5 feet, so count steps along each wall, multiply by 2.5 to get feet, then multiply length by width. It's approximate, so use a tape or a laser measure for anything you're buying materials for or listing for sale.

How do you estimate square footage from a floor plan?

Read the dimensions printed on the plan, multiply length by width for each room, and add the rooms together. If only a scale is given, measure on the drawing and multiply by the scale factor. Skip garages, unfinished areas, and anything below the 7-foot ceiling rule when you want living area.

What is the formula for square footage?

Area equals length times width for rectangles. Circles use pi times the radius squared, and triangles use base times height divided by two. If you measured in inches, divide the final result by 144 to convert it to square feet.