How to Calculate Square Footage

Square footage is length × width. That one line answers most of the searches that land on this page: a room 12 feet by 10 feet is 120 square feet, and you can stop reading.

The reason this guide is longer than one line is that real measuring jobs are rarely a clean rectangle. Rooms have closets and bump-outs. Houses have sloped ceilings and basements that may or may not count toward the number on the listing. Flooring needs an allowance for offcuts, paint needs walls rather than floors, and the figure your tax assessor holds may have been measured by a different rule than the one an appraiser would use.

Below: the formula, the shapes that are not rectangles, what counts as finished area in a whole house, and how to get the answer out in square meters or acres using the Sq Feet to Sq Meters converter when someone needs it in different units.

For any rectangle or square, measure two sides that meet at a corner and multiply them. Both measurements must be in feet, and the answer comes out in square feet.

square footage = length (ft) × width (ft)

12 ft × 10 ft = 120 sq ft

The part that causes errors is not the multiplication — it is the inches. A tape measure reads 12 ft 6 in, and you cannot multiply feet-and-inches directly. Convert the inches to a decimal by dividing by 12 first.

So a room measuring 12 ft 6 in by 10 ft 3 in is 12.5 × 10.25 = 128.125 sq ft.

There is a second method that avoids decimals entirely, and it is the one to use if you would rather not convert anything: measure everything in inches, multiply, then divide by 144 (because a square foot is 12 × 12 = 144 square inches).

150 in × 123 in = 18,450 sq in
18,450 ÷ 144     = 128.125 sq ft

Same room, same answer to the last decimal place. Use whichever you find easier to do without mistakes — that matters more than which is theoretically tidier.

1. Clear a path along two walls that meet at a corner. You need the tape to lie flat and straight; measuring over a sofa adds inches that are not there.
2. Measure wall to wall, not baseboard to baseboard, and keep the tape at floor level. Hold it taut — a sagging tape reads long.
3. Write down feet and inches as you go. Do not round to the nearest foot yet. Rounding 12 ft 8 in down to 12 ft in a 10 ft wide room loses nearly 7 sq ft.
4. Convert the inches to decimal feet using the table above, or switch everything to inches.
5. Multiply. Length × width, then label the answer sq ft so you do not confuse it with a linear measurement later.
6. Measure closets and bump-outs separately and add them on. A reach-in closet is usually counted as part of the room it opens into.

Measure each side once more before you leave the room. Re-measuring costs thirty seconds; ordering flooring against a wrong number costs considerably more.

Almost every awkward shape is handled by one rule: break it into shapes you can calculate, work them out separately, and add the results together.

L-shaped rooms

Draw the room, then draw one line across it to cut it into two rectangles. Measure each rectangle and add them.

Rectangle A:  12 ft × 10 ft = 120 sq ft
Rectangle B:   8 ft ×  6 ft =  48 sq ft
                     Total  = 168 sq ft

The common mistake is measuring the full outside length of both legs, which counts the overlapping corner twice. Cut the shape first, then measure each piece as its own room.

Triangles

Half the base times the height, where height is the perpendicular distance from the base to the opposite point — not the length of a sloping side.

½ × base × height
½ × 10 ft × 8 ft = 40 sq ft

Circles and curved bays

Measure from the centre to the edge to get the radius, then multiply pi by the radius squared. A circular room with a 6 ft radius is 3.1416 × 36 = 113.1 sq ft. For a semicircular bay window, work out the full circle and halve it.

Anything genuinely irregular

Cover the shape with the largest rectangles that fit, then handle the leftover edges as triangles. For a curved or angled wall you cannot reduce to either, measure a rectangle that contains it and subtract the piece outside the wall. The arithmetic stays simple; only the sketch gets busy.

Adding up every room gives you a usable number for furnishing or flooring. It does not give you the number that appears on a listing or an appraisal, and the gap between the two is where most arguments about square footage start.

For residential property in the United States, appraisers work to the ANSI Z765 standard, which Fannie Mae has required on conventional appraisals since 2022. Its rules differ from room-by-room addition in ways that matter:

• Measured from the outside. Finished area is taken to the exterior face of the walls, so the thickness of every exterior wall is included — which is why the ANSI figure comes out larger than the sum of the rooms.
• Below-grade space is reported separately. A finished basement is not added into above-grade living area, however good it looks. This single rule explains most listing-versus-assessor discrepancies.
• Ceiling height gates the count. Finished area generally needs a ceiling of at least 7 feet. Under a sloped ceiling, at least half the floor area has to meet that height, and floor area under a ceiling below 5 feet does not count at all.
• Garages, unfinished attics and open porches are excluded, regardless of how they are used day to day.

Practical consequence: if you are checking a listing, compare like with like. A 1,800 sq ft listing and a 2,400 sq ft owner's estimate are often the same house, with the basement counted once and not the other time. If the number is going to affect a price, get it measured by an appraiser to the standard — an informal figure is fine for buying rugs and is not fine for pricing a sale.

⚠️ Assessor and MLS practices vary by state and by board, and this section describes the US standard. Elsewhere, and in much of the world, floor area is quoted in square meters on a different basis again — the Sq Feet to Sq Meters converter handles the unit change, but the counting rules are local.

Once you have the floor area, most jobs need one more step.

Flooring: add a waste allowance

Never order exactly your square footage. Planks get cut to fit at walls, and the offcuts are rarely reusable. The standard allowances:

So a 168 sq ft room laid straight needs 168 × 1.10 = 184.8 sq ft, and you order 185. Buy the whole allowance in the first order: dye lots and production batches shift, and a top-up box bought three weeks later can be visibly the wrong shade.

Paint: you need wall area, not floor area

Floor square footage tells you nothing about how much paint to buy. Wall area is the room's perimeter multiplied by the ceiling height, minus the openings.

One wall, 12 ft wide × 9 ft high   = 108 sq ft
Less a door        (3 ft × 7 ft)  = − 21 sq ft
Less two windows (2 × 3 ft × 5 ft) = − 30 sq ft
                          Paintable = 57 sq ft

A US gallon covers roughly 350–400 sq ft per coat, and most colour changes need two coats. Subtract only openings bigger than about 15 sq ft when estimating a whole room — deducting every small window tends to leave you short.

Linear feet are not square feet

Trim, baseboard, worktop edging and fencing are sold by linear feet — a straight-line distance, with no width in it. Measure the run and buy that many feet. Multiplying by anything here gives you a number in the wrong unit.

Once the area is in square feet, every other unit is one multiplication away. These factors are exact by definition, not rounded approximations.

Worth committing to memory: an acre is exactly 43,560 square feet and a square mile is exactly 640 acres. Both are exact whole numbers, not roundings, which makes them useful for sanity-checking any lot calculation.

Some worked conversions:

• A 1,500 sq ft home is 139.35 square meters.
• A 2,000 sq ft lot is 0.0459 acres.
• A quarter-acre lot is 10,890 sq ft.
• A 100 square meter apartment is 1,076.4 sq ft.

To do these without arithmetic, use Sq Feet to Sq Meters, Sq Meters to Sq Feet or Sq Feet to Acres. For hectares and square miles, the Unit Converter covers every area unit in one place, and Sq Meters to Acres and Acres to Hectares handle land measurements directly. All of them run in your browser.

• Rounding while you measure. Round at the end, once. Rounding each wall to the nearest foot in a six-room house can move the total by 50 sq ft or more.
• Mixing units mid-calculation. Multiplying feet by inches gives a number that means nothing. Put everything in one unit before you multiply — and check the answer looks plausible for a room.
• Double-counting the corner of an L-shaped room. Cut the shape into rectangles on paper first, then measure the pieces, rather than measuring the outside edges and hoping.
• Using floor area to buy paint. Paint covers walls. Perimeter × ceiling height, minus the large openings.
• Assuming every published figure was measured the same way. Listing, tax record and owner estimate routinely disagree because they count basements, garages and low-ceilinged space differently — not because someone made an arithmetic error.

If you are working through several rooms and want to check your percentages and allowances as you go, Percentage Calculator handles the waste-factor step, and the percentage guide covers the method.