Square Footage Calculator: How to Calculate Square Footage

How to measure square footage

1. Sketch the space. Draw a rough outline of the room, including closets, alcoves, and bump-outs. You will write your measurements on it.
2. Divide it into rectangles. Draw lines to split any L-shape, T-shape, or offset into rectangles that do not overlap. Most rooms need one to three.
3. Measure each rectangle. Run the tape wall to wall along the floor for the length, then the width. Measure to the nearest inch.
4. Convert inches to decimal feet. Divide the inches by 12. Ten feet 6 inches is 10.5 ft, not 10.6 ft. The table below does this for you.
5. Multiply and add. Length x width for each rectangle, then add the results. Enter each rectangle as an area in the calculator.
6. Round up for buying materials. The calculator shows whole square feet. When you order material, add the waste allowance the product needs.

Square footage of common room sizes

120 sq ft for a 10 x 12 ft room, 144 sq ft for a 12 x 12, and 400 sq ft for a 20 x 20: multiply the two lengths. Square yards and square meters are rounded to one decimal, as the calculator rounds them when you enter the room as area 1.

The International Residential Code sets the smallest size for a habitable room, such as a bedroom: at least 70 sq ft and at least 7 ft in any horizontal direction, with kitchens the one exception (section R304, as Fine Homebuilding's code column explains). A 7 x 10 ft room meets both; an 8 x 8 ft room (64 sq ft) is too small.

Inches to decimal feet

Area formulas for other shapes

Not every space is a rectangle. These geometry formulas cover most of the rest. Work them out and add the result as its own line in your total (in the calculator, enter the area as a length with a width of 1).

For a room with a bay window or angled corner, measure the main rectangle, then treat the bay as a trapezoid or half circle and add it on.

Heron's formula works for any triangle when you can measure the three sides but not the height: for sides of 10, 12, and 14 ft, s is 18 and the area is the square root of 18 x 8 x 6 x 4 = 3,456, or about 58.8 sq ft. The calculator adds areas but does not subtract them, so for a border enter the strips instead: the two long sides as one area of 38 x 6 ft (two strips 3 ft wide) and the two short sides as another of 16 x 6 ft. It shows "Total area: 324 sq ft."

Unit conversions

NIST defines these area relationships exactly, so you can convert with confidence:

Square feet to square yards, acres, and square meters

1,000 sq ft is 111.1 square yards, 0.023 acres, or 92.9 square meters. Divide square feet by 9 for square yards and by 43,560 for acres, and multiply by 0.09290304 for square meters (the NIST factors above):

Square yards and square meters are rounded to one decimal and acres to four. The calculator multiplies by 0.092903, which gives the same square meters at one decimal. To go back, multiply square meters by 10.76391 (NIST), so about 1,076.4 sq ft make 100 square meters. One acre is 4,840 square yards.

Room square footage vs. home square footage

Measuring a room for flooring is different from measuring a house for a listing or appraisal. Appraisals that follow Fannie Mae's guidelines use the ANSI Z765-2021 standard, and a few of its rules explain why the numbers rarely match:

• Exterior measurements. Sketches use exterior dimensions to the nearest tenth of a foot, so walls are included in the total. The final total is reported to the nearest whole square foot. Condos in multifamily buildings are the exception: Fannie Mae uses interior measurements for those.
• Ceiling height. Finished areas need a ceiling at least 7 ft high. Under a sloped ceiling, at least half of the room must reach 7 ft, and no counted part can be under 5 ft.
• Below-grade space is separate. Any level that is partly or fully below ground is reported as below-grade area, not added to the main total.
• Stairs and open-to-below areas. A staircase counts on the floor it descends from. Openings to the floor below, like a two-story foyer, do not count.

For a project, you want the interior, wall-to-wall measurement of the surface you are covering. For anything that involves the home's official size, ask your appraiser or agent which standard they use.

How to find the square footage of a house

Appraisers use the exterior dimensions of each floor to calculate the gross living area of a house, according to the National Association of Realtors. To get a figure close to theirs:

1. Measure the outside walls of each floor that is above ground, and split any L-shape or bump-out into rectangles.
2. Multiply the length by the width of each rectangle and add them up, floor by floor.
3. Leave out the garage, porches, and patios, which NAR says are excluded from gross living area, and any unfinished space. Report a basement or other below-grade level separately.
4. Add the floors together.

Say both floors of a two-story house measure 40 x 30 ft outside, and the first floor has a 12 x 8 ft bump-out. Enter 40 and 30 as area 1, 12 and 8 as area 2, and 40 and 30 as area 3: the total is 1,200 + 96 + 1,200 = 2,496 sq ft.

What counts toward a home's square footage

7 ft of ceiling height, a level that is entirely above grade, and a finish suitable for year-round use, with walls, floors, and ceilings like the rest of the house: those are the main tests for counting a space in a home's above-grade finished square footage under ANSI Z765-2021, the measuring standard Fannie Mae requires for appraisals. Space by space:

The numbers in the standard's own sample statement add up this way: a two-story house 28.2 x 42.5 ft has 1,198.5 sq ft per level, or 2,397 sq ft on its two above-grade levels. Take off a 100 sq ft two-story space (open to the floor below) and a 96 sq ft unfinished utility room, and you get the 2,201 above-grade finished sq ft it reports. Its basement goes on separate lines, 807 sq ft finished and 392 sq ft unfinished, and the 240 sq ft enclosed porch and the two-car garage are listed apart from all of them.

Measuring more than three areas

The calculator takes up to three rectangles at a time, which covers most single rooms. For a whole floor or an open plan with more pieces:

1. Number every rectangle on your sketch so none gets skipped or counted twice.
2. Run the calculator for the first three, write down the total, then clear the fields and run the next three.
3. Add the subtotals by hand. If you are ordering flooring, apply the waste percentage to the grand total, not to each room, so rounding does not stack up.
4. Keep the sketch with the measurements. It is the fastest way to double-check an order or answer an installer's question later.

The same method works outdoors. A patio, deck, or lawn with curves can be broken into rectangles plus a few half circles or triangles, then added up.

Using square footage for projects

Square footage is the starting point for most material estimates, but each material needs its own adjustment:

• Flooring and tile use the floor area plus a waste allowance for cuts. See the flooring calculator and tile calculator.
• Paint uses wall area, which is the perimeter times the height, not the floor area. The paint calculator handles that.
• Sod and lawn products use the yard's square footage; the sod calculator turns it into pallets.
• Carpet comes off a roll 12 or 15 ft wide, so you order more than the floor area; the carpet calculator lays out the pieces.
• Room air conditioners are sized by floor area; see the AC size calculator.
• Plywood and OSB come in 4 x 8 sheets of 32 sq ft; the plywood calculator turns an area into sheets.
• Gravel and concrete are sold by volume, so they also need a depth; see the gravel calculator and concrete calculator.
• Pool water is a volume too: water surface times average depth; the pool volume calculator turns it into gallons.
• Any other volume, such as fill dirt or a hole you are digging, is square feet times the depth in feet; divide by 27 for cubic yards, or let the cubic yard calculator do it.

Square footage for flooring, tile, carpet, and paint

Here is one 12 x 10 ft room with 8 ft walls, measured for each job, with the figures our calculators give at their default allowances:

For paint, the calculator takes 20 sq ft off for each door and 15 for each window; 302 sq ft at 350 sq ft per gallon is about 1.7 gallons for two coats.

Square feet of walls vs. floor

352 sq ft of wall for a 10 x 12 ft room with 8 ft walls, almost 3 times its 120 sq ft of floor. Wall area is the distance around the room times the wall height; floor area is length times width. For common room sizes, before doors and windows and after the paint calculator's allowance for 1 door and 2 windows (20 sq ft per door, 15 per window):

One wall is its length times its height: a 12 ft wall 8 ft high is 96 sq ft. The square footage calculator can total walls too: enter the distance around the room as the length and the wall height as the width, so 44 and 8 give "Total area: 352 sq ft." A flat ceiling has the same area as the floor.

Common mistakes to avoid

• Mixing inches and decimals. 10 ft 6 in is 10.5 ft. Writing 10.6 ft adds an extra 1.2 inches to every calculation.
• Overlapping rectangles. When you split an L-shaped room, make sure no part of the floor is counted twice.
• Forgetting closets. They often get the same flooring, so measure them as separate areas.
• Using floor area for walls. A 12 x 10 room has 120 sq ft of floor but about 350 sq ft of wall at 8 ft high, before doors and windows.
• Rounding down. If a measurement falls between marks, round up. A slightly high estimate costs a little extra material; a low one costs a second trip.
• Measuring over trim. Measure wall to wall at the floor, not from the face of the baseboard, when the new material will run under or up to the wall.