How to use the square footage calculator
- Measure the length and width of the space in feet. Use a tape for rooms and a measuring wheel for yards and lots.
- Convert any inches to decimal feet before entering them: divide by 12, so 6 in = 0.5 ft and 3 in = 0.25 ft.
- Set the quantity if you have several identical areas, such as three 10 × 12 bedrooms.
- Read all four units. Square yards are used for carpet and sod. Square meters suit metric product labels. Acres are for land.
- For other shapes, switch to the L-shaped room, circle or triangle calculators. Use the flooring calculator to turn an area into boxes.
Square footage formulas
| Shape | Area formula |
|---|---|
| Rectangle or square | length × width |
| Triangle | ½ × base × height (height measured square to the base) |
| Circle | π × (diameter ÷ 2)² |
| L-shape or irregular room | split into rectangles, then add them |
Unit conversions
| From square feet to | Do this | Why |
|---|---|---|
| Square yards | ÷ 9 | 1 yd² = 3 ft × 3 ft |
| Square meters | × 0.09290304 | 1 ft = 0.3048 m exactly (NIST), and 0.3048² = 0.09290304 |
| Square inches | × 144 | 12 in × 12 in |
| Acres | ÷ 43,560 | 1 acre = 43,560 ft² (NIST Handbook 44, Appendix C) |
Worked examples
A 12 × 10 ft bedroom
- 12 × 10 = 120 ft².
- In square yards: 120 ÷ 9 = 13.33 yd².
- In square meters: 120 × 0.09290304 = 11.15 m².
A room measured in feet and inches
Take a room that is 12 ft 6 in by 10 ft 3 in.
- Convert: 12 + 6/12 = 12.5 ft, and 10 + 3/12 = 10.25 ft.
- Area: 12.5 × 10.25 = 128.13 ft².
A common mistake is to type 12.6 and 10.3 instead. That gives 129.78 ft², because 6 inches is half a foot, not 0.6 of one.
An L-shaped room
Split it into a 14 × 12 ft main area and a 6 × 4 ft nook that doesn't overlap it. 168 + 24 = 192 ft².
A round patio, 10 ft across
π × 5² = 78.54 ft².
A gable wall
The wall is 24 ft wide, and the peak is 6 ft above the top of the wall. ½ × 24 × 6 = 72 ft². Add this to the rectangular wall below it if you are painting or siding the whole end.
A 10,000 ft² lot
10,000 ÷ 43,560 = 0.23 acres.
How to measure a room accurately
- Measure at floor level, wall to wall. Baseboards can hide a few inches, so press the tape against the drywall above them, or measure baseboard to baseboard and add their thickness.
- Measure both sides. Rooms are rarely perfectly square. If opposite walls differ, use the longer measurement for buying materials.
- Sketch first. Draw the room, note every measurement on the sketch, and mark doorways, closets and bump-outs.
- Decide what to include. For flooring, include closets and the area under appliances that will sit on the new floor. For paint, measure walls instead (see the paint calculator).
What square footage is used for
| Project | What to measure | Next step |
|---|---|---|
| Flooring, tile | Floor area, including closets | Add a cutting allowance and divide by box coverage (flooring calculator) |
| Paint | Wall area (perimeter × height), minus openings | Paint calculator |
| Mulch, gravel, topsoil | Bed or area size | Multiply by depth (mulch calculator) |
| Concrete | Slab area | Multiply by thickness (concrete calculator) |
| Sod, seed, fertilizer | Lawn area | Use the coverage on the product label |
Tips and common mistakes
- Don't mix feet and inches in one multiplication. Convert everything to feet first.
- Don't double-count overlaps. When you split a room into rectangles, make sure they share an edge, not an area.
- Use the perpendicular height for triangles, not the sloped side. The sloped edge always overstates the area.
- Round measurements up for materials, and keep the unrounded area for price comparisons.
- Square footage on a home listing is often measured to a specific standard and may exclude garages or unfinished space. A calculator measures what you measure, so it won't necessarily match a listing.
Frequently asked questions
How do I calculate square feet?
Multiply length by width, both in feet. A 12 × 10 ft room is 120 ft².
How do I calculate square feet from inches?
Multiply the two sides in inches and divide by 144. A 30 × 48 in rug is 1,440 ÷ 144 = 10 ft². You can also convert each side to feet first: 2.5 × 4 = 10 ft².
How many square feet is a 12 × 12 room?
144 ft², which is 16 yd² or about 13.38 m².
How do I convert square feet to square yards?
Divide by 9. For example, 120 ft² ÷ 9 = 13.33 yd².
How many square feet are in an acre?
Exactly 43,560 ft², according to NIST Handbook 44. That's about 4,046.86 m².
How do I find the square footage of an irregular room?
Split it into rectangles and triangles, calculate each one, and add them. For curved walls, approximate the curve with a few short straight sections, or treat a semicircular bay as half a circle.
Does square footage include walls?
Floor square footage is the floor area only. For painting, you need the wall area: the room's perimeter × wall height, minus doors and windows.
Sources
- NIST Handbook 44 (2026), Appendix C, General Tables of Units of Measurement: 1 acre = 43,560 ft²; 1 yd² = 9 ft². https://www.nist.gov/document/2026-nist-handbook-44-appendix-c
- NIST, U.S. survey foot: the international foot (1 ft = 0.3048 m exactly) applies in all applications. https://www.nist.gov/pml/us-surveyfoot
- NIST, SI units of length: 1 in = 25.4 mm exactly. https://www.nist.gov/pml/owm/si-units-length