On this page (11)
How to use this calculator#
- Measure at floor level, wall to wallRun the tape along the skirting rather than at waist height, where it sags and catches furniture. Record to the nearest inch or centimetre — a two-inch error across a 15 ft wall is 2.5 sq ft, which is a whole box of tile on a large floor.
- Split the space into simple shapesAn L-shaped room is two rectangles divided at the inside corner. A bay is a rectangle plus a trapezoid. Give closets, alcoves and hallways their own line rather than averaging an irregular wall — averaging is where the error compounds.
- Measure both directions in the same unitsMixing 12 ft by 350 cm produces nonsense. Convert everything to feet or everything to metres before multiplying, and convert the finished area only once at the end.
- Decide what to subtract, and what not toFor flooring, subtract fixed islands, hearths and built-in units that stay in place; do not subtract freestanding furniture. For a paint or plaster take-off, subtract doors and windows from the wall area instead.
- Add a waste allowance before you orderThe measured figure is the finished surface, not the amount to buy. Add 5–10% for straight-laid flooring, 10% for tile in a square room and 15% for diagonal or herringbone layouts.
The formula#
Area by shape
Rectangle: A = L × W • Triangle: A = ½ × b × h • Circle: A = π × r² • Trapezoid: A = ½ × (a + b) × h
- L, W
- Length and width of a rectangle, in the same unit
- b, h
- Base and perpendicular height of a triangle (h is measured square to the base, not along a sloping side)
- r
- Radius of a circle — half the diameter, which is the measurement people actually take
- a, b
- The two parallel sides of a trapezoid
- π
- 3.14159
Area units are length units squared, so their conversion factors are the length factors squared. 1 ft = 0.3048 m, therefore 1 sq ft = 0.3048² = 0.09290304 m². Converting a room's length instead of its area is the standard error: 3 m is 9.84 ft, but 3 m² is 32.29 sq ft — the two answers differ by a factor of 3.28.
How to measure square footage accurately#
Measure wall to wall at floor level rather than at eye height, and record to the nearest inch or centimetre — a two-inch error across a 15-foot wall throws the total out by 2.5 sq ft. For a rectangle multiply length by width; for a circle use pi times the radius squared; for a triangle use half the base times the height.
Break irregular spaces into sections instead of averaging. An L-shaped room is two rectangles split at the inside corner. A bay window is a rectangle plus a trapezoid. Closets, alcoves and hallways each get their own line, then everything adds to a gross figure you can order materials against.
Converting between square feet and square metres#
One square metre is 10.7639 square feet, and one square yard is exactly 9 square feet. The classic mistake is converting the linear measurement instead of the area: 3 metres is 9.84 feet, but 3 square metres is 32.29 square feet, not 9.84. Carpet is quoted per square yard in the US and per square metre almost everywhere else, so convert before you compare two quotes.
Costing a project by the square foot#
Multiply the total area by the price per square foot for a materials budget, then add 5-10% for offcuts. A 12 by 14 ft bedroom is 168 sq ft; at $3.20 per sq ft for laminate that is $537.60, or about $565 once waste is covered. Installation is normally quoted separately at $2-5 per sq ft, so always ask whether a price is supply-only or supply-and-fit.
Worked examples#
An L-shaped living room
A room 20 ft by 14 ft with a 6 ft by 5 ft section missing at one corner.
- Treat it as the full rectangle minus the missing corner
- Full rectangle: 20 × 14 = 280 sq ft
- Missing corner: 6 × 5 = 30 sq ft
- Net area: 280 − 30 = 250 sq ft
- In metric: 250 × 0.09290304 = 23.23 m²
- Flooring to order with 10% waste: 250 × 1.10 = 275 sq ft
250 sq ft (23.23 m², 27.78 sq yd). Order 275 sq ft of material.
Costing a laminate floor
A 12 ft by 14 ft bedroom, laminate at $3.20 per sq ft supply-only, plus $2.50 per sq ft to fit.
- Area = 12 × 14 = 168 sq ft
- With a 10% waste allowance: 168 × 1.10 = 184.8, so buy 185 sq ft
- Material: 185 × $3.20 = $592.00
- Installation is charged on the finished area, not the ordered area: 168 × $2.50 = $420.00
- Total = $592.00 + $420.00 = $1,012.00
$1,012 all in — and note that waste is paid for on the material but not on the labour.
Reference tables#
| Unit | Square feet | Square metres | Square yards |
|---|---|---|---|
| 1 square foot | 1 | 0.092903 | 0.1111 |
| 1 square yard | 9 | 0.836127 | 1 |
| 1 square metre | 10.7639 | 1 | 1.19599 |
| 100 sq ft (1 roofing square) | 100 | 9.2903 | 11.111 |
| 1 acre | 43,560 | 4,046.856 | 4,840 |
| 1 hectare | 107,639 | 10,000 | 11,959.9 |
One square inch is 0.00064516 m² and there are 144 square inches in a square foot. Carpet is priced per square yard in the US and per square metre nearly everywhere else, so convert before comparing quotes.
| Room size | Square feet | Square metres | Square yards |
|---|---|---|---|
| 8 × 10 ft | 80 | 7.43 | 8.89 |
| 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 |
These are floor areas. Wall area is the perimeter times the ceiling height, which is a completely different sum — a 12 × 14 ft room has 168 sq ft of floor but 416 sq ft of wall at 8 ft.
| Shape | Formula | Worked figure |
|---|---|---|
| Rectangle or square | length × width | 12 × 14 = 168 sq ft |
| Triangle | ½ × base × perpendicular height | ½ × 10 × 6 = 30 sq ft |
| Circle | π × radius² | π × 5² = 78.54 sq ft |
| Half circle (bay window) | ½ × π × radius² | ½ × π × 3² = 14.14 sq ft |
| Trapezoid | ½ × (side a + side b) × height | ½ × (10 + 14) × 8 = 96 sq ft |
| Ring or border | π × (outer radius² − inner radius²) | π × (6² − 5²) = 34.56 sq ft |
| L-shape | big rectangle − missing rectangle | 280 − 30 = 250 sq ft |
For a circle, measure the diameter across the widest point and halve it — measuring a radius directly means guessing where the centre is.
Common mistakes#
- Converting the length instead of the area3 metres is 9.84 feet, but 3 square metres is 32.29 square feet. Applying the length factor to an area under-reports it by a factor of 3.28 — order flooring that way and you will receive roughly a third of what you need.
- Measuring in feet and inches, then multiplying as decimalsA wall of 12 ft 6 in is 12.5 ft, not 12.6. Multiplying 12.6 by 14 gives 176.4 sq ft against a true 175 sq ft. Divide inches by 12 before multiplying, or work entirely in inches and divide the result by 144.
- Ordering exactly the measured areaEvery cut leaves an offcut, planks must be staggered, and tiles at the perimeter are trimmed. Buying exactly 168 sq ft for a 168 sq ft room guarantees a shortfall, and the replacement pack will be a different batch with a visible shade difference.
- Comparing an estate agent's floor area with your ownReal-estate gross living area is normally measured from the exterior wall faces and includes internal partitions, so a listing figure runs 5–15% larger than the interior floor you will actually cover. Always measure inside the room before ordering materials.
Frequently asked questions#
How do I calculate the square footage of an L-shaped room?
Split it into two rectangles at the inside corner, work out length times width for each, and add them. This calculator lets you add sections so the running total is done for you.
Is square footage measured inside or outside the walls?
For flooring, paint and most DIY work, measure the interior floor area wall to wall. Real estate gross living area is normally measured from the exterior wall faces, so listing figures run noticeably larger.
How many square feet are in a square metre?
One square metre equals 10.764 square feet. Going the other way, divide square feet by 10.764 to get square metres.
Should I include closets in the square footage?
For flooring and paint, yes — include any surface you will actually cover. For property listings, closets inside the heated envelope usually count, but garages and unfinished basements do not.
Key terms#
- Gross versus net area
- Gross is the full footprint before any deductions. Net subtracts fixed obstructions such as islands, hearths and built-in units. Order materials against the gross figure plus waste, and price labour against the net.
- Square (roofing)
- A trade unit equal to 100 square feet (9.29 m²), used to quote roofing and siding. A 2,000 sq ft roof is 20 squares.
- Waste allowance
- The percentage added to the measured area to cover offcuts, breakages and pattern matching. Typically 5–10% for straight-laid flooring, 10% for square-set tile and 15% or more for diagonal, herringbone or heavily patterned layouts.
- Linear versus square measure
- Linear measure is a single dimension — skirting board, coving, worktop edging. Square measure is two dimensions multiplied. Buying skirting by the square foot, or flooring by the linear foot, is a costly category error.
- Acre
- Exactly 43,560 square feet or 4,046.8564 m², historically the area one team of oxen could plough in a day. A hectare is 10,000 m², or 2.4711 acres.
Sources#
Put this calculator on your site
Free, no sign-up, and no attribution fee — just keep the credit line that comes with the code. The calculator stays up to date automatically.