Envelope · Measuring
Square footage calculator
Square feet = length × width, both in feet. A room 12 ft × 10 ft is 120 ft² (11.15 m²); 10 ft 6 in is 10.5 ft. An L-shaped room is the whole rectangle less the missing corner; a plot is split into triangles measured by their three sides. Add the waste and round up to whole boxes before you buy.
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How this is calculated
The example, by hand: living room 20 × 15 − 6 × 5 = 270 ft²; kitchen 12 × 10 = 120, less the island 6 × 3 = 18, leaves 102; hall 3.5 × 10 = 35. Total 407 ft², which is 37.81 m². With 10 % waste that is 447.70 ft², and in boxes of 20 ft² it is 22.39, so 23 boxes.
Money is worked with exact decimals and rounded to the cent once, at the end, so the cost matches the same sum done by hand.
How to calculate square footage, step by step
- Measure the length and the width of the room at floor level, wall to wall.
- Turn inches into feet: divide them by 12 (6 in is 0.5 ft, 3 in is 0.25 ft).
- Multiply length by width. That is the square footage of a rectangle.
- Split any other shape into rectangles and triangles, work out each and add them; take off what will not be covered, like a kitchen island or a fireplace hearth.
- Add the waste allowance and round up to whole boxes or packs.
The square footage of a lot
Land is rarely square. Walk it into triangles: for each one measure the three sides, and Heron’s formula gives the area without any angle. A four-sided lot is two triangles that share a diagonal, so it needs the four sides and that diagonal; with the sides alone the same four lengths can enclose many different areas. The plot 30, 25, 28 and 22 ft with a 38 ft diagonal comes to 679.29 ft². For a deed or a sale, the figure that counts is the surveyor’s.
Square feet of common room sizes
Floor area of a rectangular room, in square feet, square metres and square yards (carpet).
| Room | ft² | m² | yd² |
|---|---|---|---|
| 10 × 10 ft | 100 | 9.29 | 11.11 |
| 10 × 12 ft | 120 | 11.15 | 13.33 |
| 12 × 12 ft | 144 | 13.38 | 16.00 |
| 12 × 14 ft | 168 | 15.61 | 18.67 |
| 12 × 16 ft | 192 | 17.84 | 21.33 |
| 14 × 16 ft | 224 | 20.81 | 24.89 |
| 15 × 20 ft | 300 | 27.87 | 33.33 |
| 20 × 20 ft | 400 | 37.16 | 44.44 |
Questions people ask
- How do you calculate square footage?
- Measure the length and the width in feet and multiply them: a room 12 ft by 10 ft is 120 square feet. Turn inches into feet first by dividing them by 12, so 10 ft 6 in is 10.5 ft. If the room is not a rectangle, split it into rectangles and triangles, work each one out and add them up, and take off any area that will not be covered.
- How do I calculate the square footage of a room?
- Measure wall to wall at floor level, the longest way and across, and multiply. For an L-shaped room, measure the whole rectangle and take off the missing corner: 20 × 15 ft less 6 × 5 ft is 270 ft². For a bay or a curved wall, add a half circle: π × diameter² / 8.
- How do you calculate the square footage of a lot or land?
- Split it into triangles and measure the three sides of each. With Heron’s formula, s = (a + b + c) / 2 and the area is √(s(s − a)(s − b)(s − c)): sides of 30, 40 and 50 ft give 600 ft². A four-sided lot needs its four sides and one diagonal, which split it into two triangles; the four sides alone do not fix its area. One acre is 43,560 ft².
- How many square feet are in a square metre?
- 10.7639. One foot is exactly 0.3048 m, so one square foot is exactly 0.09290304 m², and a square yard, 9 ft², is 0.83612736 m².
- How much extra flooring or tile should I buy?
- Enough for the cuts and a few spares. Installers usually allow about 10 % for a straight lay and more, around 15 %, for diagonal or herringbone patterns; the product’s installation guide is the figure to follow. Then round up to whole boxes: 447.7 ft² in boxes of 20 ft² is 23 boxes.
- How do I work out how much paint I need?
- Wall area is the perimeter of the room times its height, less the doors and windows. Multiply by the number of coats and divide by the coverage per coat printed on the tin, in square feet per gallon or square metres per litre.
What this page does not do
- Plan area, not slope. A plot on a hillside is measured on the level, as on a map; a pitched roof needs its slope length, not its plan.
- No survey. For a deed, a sale or a planning application, the area that counts is the one a surveyor signs.
- Waste and coverage are your figures. They depend on the product, the pattern and the wall; the installation guide and the tin say which.