How to calculate square footage of a room, a floor or a house

Measure in feet and inches, then convert

A tape measure reads in feet and inches, and the mistake that spoils most square footage is treating 11 ft 8 in as 11.8. Inches are twelfths, so 8 inches is 0.67 of a foot and the measurement is 11.67 ft. Write each measurement as feet plus inches divided by 12, and only then multiply. For a 13 ft 4 in wall, that is 13.33 ft. The room is 11.67 × 13.33 = 155.6 square feet. An easier route when the numbers are awkward is to work entirely in inches: 140 × 160 = 22,400 square inches. Divide that by 144, which gives the same 155.6. For depth as well as area, how to calculate cubic yards takes the square feet on to a volume.

Measure along the floor, at the baseboard, on the longest run of each wall. Measure the width in two places. Walls bow and rooms are rarely true rectangles, and for buying material the larger figure is the safe one.

A rectangle is length times width

Nearly every room is a rectangle or can be cut into rectangles. Multiply the two sides in feet. A 12 by 15 ft living room is 180 square feet. That is 20 square yards (divide by 9) or 16.7 square meters (multiply by 0.0929). Carpet is often priced by the square yard and tile by the square foot, so keep both numbers.

Odd rooms: split, measure, add, subtract

An L-shaped room is two rectangles. Measure the long leg and the short leg separately, or measure the full outside rectangle and subtract the missing corner. A family room 22 ft by 14 ft has a 7 by 6 ft corner taken by a stair. That leaves 308 − 42 = 266 square feet. A room with a bay window is a rectangle plus a trapezoid. Measure the bay's back wall, its front opening and its depth. Average the two widths and multiply by the depth. A closet that gets a different floor comes out of the total. So does a fireplace hearth, a kitchen island set on the subfloor, or a built-in that will not be floored under.

Circles, triangles and trapezoids

A circle is pi (3.1416) times the radius squared, and the radius is half the measurement across the widest point. A 10 ft round patio is 3.1416 × 5 × 5 = 78.5 square feet. A triangle is half the base times the height, where the height is measured square to the base, not along the slanted side. A corner bed 8 ft along one wall and 6 ft along the other is 24 square feet. When all you have is the three sides, Heron's formula works from half the perimeter, and the calculator does it for you. A trapezoid is what most bay windows and tapered lots are. Its area is the average of the two parallel sides times the distance between them. Sides of 10 and 6 ft set 4 ft apart are 32 square feet.

Walls, for paint and paneling

Wall area is the perimeter of the room times the ceiling height, less the openings. A 14 by 11 ft room has 50 ft of wall. At 9 ft high that is 450 square feet. A 3 by 7 ft door is 21 square feet. A 3 by 4 ft window is 12, so with one door and two windows the room has 405 square feet of wall to paint. Divide that by the coverage printed on the paint can for the gallons per coat.

A whole house, the way the listing measures it

Listings and appraisals in the US follow ANSI Z765, the standard Fannie Mae requires of appraisers; the figures here are from the standard's 2021 public-comment draft. It measures finished areas at floor level to the outside face of the exterior walls, floor by floor. It counts only space with a ceiling of at least 7 ft (6 ft 4 in under beams and ducts). Under a sloped ceiling it counts nothing below 5 ft and requires half the room to clear 7 ft. Measurements are taken to the nearest inch or tenth of a foot.

The total is reported to the nearest whole square foot, with above-grade and below-grade areas reported separately. A 40 by 28 ft two-story house is 1,120 square feet per floor, 2,240 in all, before any bump-outs or cut-outs. A garage and any unfinished area are left out. Measuring room by room from the inside gives a smaller total, because the walls themselves are left out. That inside figure is the one to buy flooring with. The outside figure is the one on the listing.

Turning the area into material

Flooring, tile and carpet are bought by area plus the cutting allowance the maker prints on the box. The allowance is more for diagonal or patterned work and for rooms with many angles. It is bought in whole boxes, and the leftover box is the repair stock. Paint goes by wall area divided by the coverage on the can. Concrete, gravel and mulch go a step further, area times depth, which is what the material calculators do once the square footage is in.