The units in one table
| Unit | Square metres | Where it is used |
|---|---|---|
| Square inch | 6.4516e-4 | Paper, screens, small parts |
| Square foot | 0.0929 | Floor plans, rent, flooring, paint in the US |
| Square yard | 0.8361 | Carpet and fabric in the US |
| Square metre | 1 | Floor plans and land almost everywhere else |
| Acre | 4,046.873 | Farmland and lots in the US and UK |
| Hectare | 10,000 | Farmland and land registers in metric countries |
| Square kilometre | 1,000,000 | Cities, parks, countries |
| Square mile | 2,589,988 | Counties, cities and countries in the US |
NIST marks three of these as exact: the square inch, the square foot and the hectare. The acre and square mile in its table are based on the US survey foot, which differs from the international foot by two parts in a million, a distinction that matters to surveyors and to nobody else. The area converter applies these factors between any pair.
Why area factors are squared
A foot is 0.3048 metres, so a square foot is 0.3048 × 0.3048 = 0.09290304 square metres. Anyone who converts an area by the length factor alone gets an answer three times too large or too small. The safe habits are to convert the sides first and multiply, or to use the squared factor:
Square feet × 0.09290304 = square metres. Square metres × 10.7639 = square feet.
A 12 by 15 foot room: 180 square feet, or 16.72 square metres, either way. The square footage calculator takes the sides and does the multiplication for rooms and L-shaped spaces, and the area calculator handles circles, triangles and other shapes; the room measuring guide covers the tape-measure part.
Land: acres, hectares and sections
| Relationship | Value |
|---|---|
| Square feet in an acre | 43,560 |
| Side of a square acre | 208.7 ft (63.6 m) |
| Acres in a hectare | 2.4710 |
| Hectares in an acre | 0.4047 |
| Acres in a square mile | 640 |
| Hectares in a square kilometre | 100 |
| Square miles in a square kilometre | 0.3861 |
A suburban lot of 100 by 150 feet is 15,000 square feet, 0.344 acres, 1394 square metres or 0.139 hectares. The hectare's convenience is that it is a 100-metre square, so a field's hectares are its length times its width in metres divided by 10,000, with no odd constant anywhere. The length converter turns feet into metres for the sides when a plan is in one system and the register in the other.
Rules of thumb that are close enough
- Square metres to square feet: multiply by 10.76, or roughly by 11 and take a little off.
- An acre is about 0.4 hectare; a hectare about 2.5 acres.
- A square mile is 640 acres or 2.59 square kilometres.
- An American football field between the goal lines, 100 by 53⅓ yards, is 1.10 acres; a soccer pitch of 105 by 68 metres is 0.71 hectares.
For anything that will be written into a contract or a permit, use the exact factors and the unit converter, not the rules of thumb.
Frequently asked questions
How many square feet are in an acre?
43,560. NIST gives the acre as 4,046.873 square metres and the square foot as 0.09290304 square metres exactly, and the ratio is the familiar 43,560. A square lot of one acre is about 209 feet on each side.
How many acres are in a hectare?
2.471. A hectare is exactly 10,000 square metres, a square 100 metres on each side; an acre is 4,046.873 square metres, so a hectare is just under two and a half acres. The reverse factor is 0.4047 hectares per acre.
How do I convert square feet to square metres?
Multiply by 0.09290304 (exact), or divide by 10.764. A 12 by 15 foot room is 180 square feet, which is 16.7 square metres. Converting the sides first and multiplying gives the same answer; converting the area with a length factor does not, because area scales with the square of length.
How many square metres are in an acre?
4,046.873, by NIST's factor, which is about 0.405 hectares. The hectare was defined from the metre, which is why it is a round 10,000 square metres, while the acre is an older land unit that only happens to convert to an odd number.
Is a square mile 640 acres?
Yes: 640 acres, from NIST's 2,589,988 square metres per square mile. A quarter section of farmland, a quarter of a square mile, is 160 acres.