How the calculator works
Square footage is area: how many 1-foot-by-1-foot squares fit on a floor. The calculator finds the area of each shape you enter, multiplies it by how many identical copies there are, adds every area, then subtracts anything you mark as a cutout, such as a kitchen island you will not floor or a closet you are not painting. It converts your units to feet first, so you can measure in inches, yards, meters or centimeters.
| Shape | What to measure | Formula |
|---|---|---|
| Rectangle or square | Length and width | length × width |
| L-shape | Two non-overlapping rectangles | (length A × width A) + (length B × width B) |
| Triangle | Base and the height at a right angle to it | base × height ÷ 2 |
| Circle | Diameter (widest point) | π × (diameter ÷ 2)² |
| Trapezoid | The two parallel sides and the distance between them | (side a + side b) ÷ 2 × height |
Unit conversions
The conversions come from the National Institute of Standards and Technology. These are exact by definition: 1 foot = 0.3048 meter, 1 inch = 0.0254 meter, and 1 square foot = 0.09290304 square meter. So one square meter is about 10.764 square feet, and one square yard is 9 square feet (3 ft × 3 ft). Carpet is often priced per square yard, so divide square feet by 9 to compare.
Worked example: a small two-bedroom
- Living room, L-shaped: main part 18 × 12 ft = 216 sq ft, plus a dining nook 8 × 6 ft = 48 sq ft. Total 264 sq ft.
- Two matching bedrooms, 12 × 11 ft each: 132 × 2 = 264 sq ft.
- Subtract a 6 × 3 ft kitchen island you will not put flooring under: −18 sq ft.
- Total: 264 + 264 − 18 = 510 sq ft, or 510 × 0.09290304 = 47.38 m².
- At $6.00 per sq ft for installed material, the project is 510 × $6 = $3,060 before waste and extras.
These are the defaults the calculator loads with, so you can see each step in the table on the right and then replace them with your own rooms.
Measuring tips
- Measure wall to wall at floor level, at the longest and widest points. Old buildings are rarely square, so measure both ends of a long wall and use the larger number when you are buying material.
- Convert inches to decimal feet before you enter feet: 3 in = 0.25 ft, 6 in = 0.5 ft, 9 in = 0.75 ft. Or switch the unit to inches and enter the raw numbers.
- Break odd rooms into rectangles and triangles. A bay window is often a rectangle plus two triangles, or one trapezoid.
- Count closets if you are flooring them, and subtract them if you are not.
Why a listing can show a different number
Room-by-room math gives you net floor area for buying materials. A home's official square footage is usually measured another way. For houses, including townhouses and rowhouses, Fannie Mae requires appraisers to use the ANSI Z765-2021 standard on appraisals dated April 1, 2022 or later for loans it buys. Under it, measurements are taken to the nearest inch or tenth of a foot and reported to the nearest whole square foot; openings to the floor below, such as a two-story foyer, are not counted; and finished areas need a ceiling height of at least 7 feet (with special rules for sloped ceilings). Space partly or fully below grade is reported separately.
What to do next
Use your total in a project calculator: flooring adds a waste factor and counts boxes, paint uses wall area rather than floor area, and drywall counts sheets. To size a window unit for a room, try the room air conditioner calculator. Touring apartments? Bring a tape measure and the apartment viewing checklist.