How to Calculate Square Feet: Rooms, Houses and Irregular Shapes
To calculate square feet, measure the length and width in feet and multiply them. A room 12 ft by 15 ft is 12 × 15 = 180 square feet.
For an L-shaped or oddly shaped space, split it into rectangles, triangles and circles, find each area and add the results.
What is the formula for square feet?
area (sq ft) = length (ft) × width (ft)- Measure wall to wall along the floor in feet.
- Multiply the two measurements.
- Write the result in square feet (sq ft, or ft²).
Rooms are rarely perfectly square. Measure each side at both ends. If the two readings differ by more than an inch or two, use the larger numbers when you are ordering material.
How do you calculate square feet with feet and inches?
Divide the inches by 12 to turn them into a decimal part of a foot, then multiply as usual. Do not round until the end.
Example. A room measures 14 ft 8 in by 11 ft 3 in.
- 14 ft 8 in = 14 + 8 ÷ 12 = 14.6667 ft
- 11 ft 3 in = 11 + 3 ÷ 12 = 11.25 ft
- 14.6667 × 11.25 = 165 sq ft
Check it in inches: 176 in × 135 in = 23,760 sq in, and 23,760 ÷ 144 = 165 sq ft. There are 144 square inches in a square foot, because 12 × 12 = 144. Typing 12 ft 6 in as 12.6 is a common slip: 6 inches is 0.5 ft, so it is 12.5.
How do you measure an L-shaped or irregular room?
Cut the shape into rectangles that do not overlap, find each area and add. An L-shaped room can be split two ways, and both must give the same total.
The second method is to measure the whole bounding box (18 × 12 = 216 sq ft) and subtract the missing corner (8 × 5 = 40 sq ft). Use whichever needs fewer measurements. Open-plan spaces work the same way: treat each zone as its own rectangle and add them.
Triangles, circles and cutouts
| Shape | Formula | Example |
|---|---|---|
| Rectangle | L × W | 12 × 15 = 180 sq ft |
| Triangle | ½ × base × height | ½ × 8 × 6 = 24 sq ft |
| Circle | π × (diameter ÷ 2)² | π × 5² = 78.54 sq ft for a 10 ft circle |
For a triangle, the height is measured at a right angle to the base, not along the slanted side. An angled bay or a gable end is usually a rectangle plus a triangle. For a circle, use the full value of pi from your calculator, not 3.14.
Cutouts. Subtract areas you will not cover. A fixed kitchen island you will not floor under comes off the total. Closets count if you are flooring them. Stairs are not floor area: measure each tread separately.
Wall area
For paint or drywall, find the perimeter, multiply by the ceiling height and subtract openings. A 12 ft × 10 ft room with 8 ft ceilings has a perimeter of 2 × (12 + 10) = 44 ft, so the gross wall area is 44 × 8 = 352 sq ft. A 3 ft × 7 ft door (21 sq ft) and a 3 ft × 5 ft window (15 sq ft) leave 352 − 21 − 15 = 316 sq ft. To turn wall area into sheets, see how many sheets of drywall you need, or into gallons, how many square feet a gallon of paint covers. Trim is measured by length rather than area: see how to calculate linear feet. For baseboard around a room, how much baseboard you need subtracts the doorways and adds waste.
A whole house
Calculate each finished room or zone, then add them level by level. Two measuring methods give different totals for the same house. Outside dimensions include the wall thickness. Illustration only: a 40 ft × 28 ft footprint is 1,120 sq ft measured outside, while 6 in walls leave 39 ft × 27 ft, or 1,053 sq ft, inside.
Listings, appraisals and tax records each follow their own rules about what counts, such as garages, basements and stairwells. Ask which rule applies before you compare two numbers. For how a house compares with the national figures, see the guide on the average square footage of a house.
Enter feet and inches, add or subtract several shapes and see the total in square feet, square yards, square meters and acres.
Common room sizes
| Room (ft) | Sq ft | Sq yd | Sq m |
|---|---|---|---|
| 10 × 10 | 100 | 11.11 | 9.29 |
| 10 × 12 | 120 | 13.33 | 11.15 |
| 12 × 12 | 144 | 16.00 | 13.38 |
| 12 × 14 | 168 | 18.67 | 15.61 |
| 12 × 16 | 192 | 21.33 | 17.84 |
| 14 × 16 | 224 | 24.89 | 20.81 |
| 16 × 16 | 256 | 28.44 | 23.78 |
| 16 × 20 | 320 | 35.56 | 29.73 |
| 20 × 20 | 400 | 44.44 | 37.16 |
To convert, divide square feet by 9 for square yards, multiply by 0.09290304 for square meters (exact) and divide by 43,560 for acres. For big lots, see how many square feet are in an acre.
Common mistakes
- Mixing units. Multiplying feet by inches gives a meaningless number. Convert first.
- Rounding early. Round only the final area.
- Counting the overlap twice. When you split an L, the pieces must share an edge but not an area.
- Forgetting the allowance. Square feet is the net area. Flooring suppliers commonly suggest adding about 7% to 10% for a straight layout and more for diagonal or herringbone patterns. The product label and installer instructions come first, and the tile guide shows how grout joints change the tile count. The Flooring Calculator adds the waste and rounds up to whole boxes.
- Treating an estimate as a survey. A tape-measured sketch is fine for ordering materials, not for legal or lending decisions.
Questions people ask
How many square feet is a 12×12 room?
12 × 12 = 144 sq ft, which is 16 square yards.
How many square feet is 30×40?
30 × 40 = 1,200 sq ft. If that is a house footprint measured outside the walls, the inside floor area is smaller.
How do I convert square feet to square yards or square meters?
Divide square feet by 9 for square yards, or multiply by 0.09290304 (exact) for square meters. 144 sq ft is 16 square yards and 13.38 square meters. The square feet to square yards guide has a table.
What size is 4 square feet?
Any length times width that gives 4: 2 ft × 2 ft, or 1 ft × 4 ft. In square inches it is 4 × 144 = 576.
Should I measure inside or outside the walls?
For flooring, paint and drywall, measure inside, wall to wall. Outside dimensions include the wall thickness: a 40 ft × 28 ft footprint is 1,120 sq ft outside, but with 6 in walls the inside is 39 ft × 27 ft, or 1,053 sq ft.
Sources
- NIST, Handbook 44 (2026), Appendix C: General Tables of Units of Measurement. Square foot, square yard and acre definitions, and the exact foot-to-meter factor. Accessed October 16, 2026.
- Lumber Liquidators, Estimating Your DIY Flooring Project. Typical flooring waste allowances by layout. Accessed October 16, 2026.