Why area conversions surprise people
Area units are squares of length units, so the conversion factor is squared too. One foot is 0.3048 m, so one square foot is 0.3048² = 0.09290304 m², exactly. The common slip is to use the length factor: 1 m is 3.281 ft, but 1 m² is 10.7639 ft², not 3.281 ft². The converter stores every unit against the square metre, from mm² up to square miles, and the factors are the exact international ones.
Worked examples
- A 1,500 ft² flat is 1,500 × 0.09290304 = 139.35 m².
- 1 m² = 1 / 0.09290304 = 10.7639 ft².
- 1 hectare = 10,000 m², which is 2.4711 acres. A 5 ha plot is therefore 12.36 acres.
- An acre is 4,046.8564224 m², or 43,560 ft², which is 4,840 yd².
Hectares and acres in practice
The hectare (symbol ha) is a non-SI unit accepted for use with SI: a square 100 m on each side. The acre traditionally meant a day's ploughing for a team of oxen, and today it is 1/640 of a square mile. Land listings in the US, UK and India often quote acres, while agricultural statistics in most of the world use hectares. Square miles convert to 2.59 km² and 640 acres.
| Unit | In square metres | Easy comparison |
|---|---|---|
| 1 cm² | 0.0001 | 100 mm² |
| 1 in² | 0.00064516 | 6.4516 cm² |
| 1 yd² | 0.83612736 | 9 ft² |
| 1 acre | 4,046.8564 | 0.4047 ha |
| 1 ha | 10,000 | 2.471 acres |
| 1 km² | 1,000,000 | 100 ha, 247 acres |
Mistakes and limits
- Linear versus square. A room "10 by 12 feet" is 120 ft², and the 12 ft side is not interchangeable with the 12 ft² result. Multiply the two sides first, then convert once.
- Survey acres. The older US survey acre differs by about four parts per million from the international acre used here. That is invisible in a garden and noticeable only in a large-scale legal description.
- Regional units. Bigha, cent, guntha, tsubo and similar local units vary by region and are not in this list, so convert them to square metres from the source document first.
- Display rounding. Results are shown to seven significant digits, and values below 0.000001 appear in scientific notation, which is what a few cm² expressed in km² will do.
To convert a single side before multiplying, use the length converter, and see metric vs imperial units for the wider picture.