Distance units
| Unit | In metres | Notes |
|---|---|---|
| Metre (m) | 1 | The SI base unit. |
| Foot (ft) | 0.3048 | Defined as exactly 0.3048 m since 1959. |
| Yard (yd) | 0.9144 | Three feet, exactly. |
| Kilometre (km) | 1,000 | About 0.621 miles. |
| Mile (statute) | 1,609.344 | Exactly 1,760 yards. The one on road signs. |
| Nautical mile (NM) | 1,852 | One arc-minute of latitude. Exact by definition since 1929. |
The two conversions worth committing to memory are 1 km = 0.621 miles and 1 mile = 1.609 km. For mental arithmetic, multiplying kilometres by 0.6 and adding a little gets you within a couple of percent, which is usually closer than the measurement deserves anyway.
Area units
| Unit | In square metres | Notes |
|---|---|---|
| Square metre (m²) | 1 | A square 1 m on a side. |
| Square foot (ft²) | 0.092903 | About one-eleventh of a square metre. |
| Square yard (yd²) | 0.836127 | Nine square feet. |
| Acre | 4,046.856 | 4,840 square yards. One furlong by one chain. |
| Hectare (ha) | 10,000 | A square 100 m on a side. 2.471 acres. |
| Square kilometre (km²) | 1,000,000 | 100 hectares, or 247.1 acres. |
| Square mile (mi²) | 2,589,988 | 640 acres, or 2.590 km². |
Note the trap in the second column: area units do not convert with the same factor as their linear counterparts. A mile is 1.609 km, but a square mile is 2.590 km² — 1.609 squared. Halving a length quarters the area, and this catches people out constantly.
Why acres are such an awkward number
An acre is 4,046.856 m², which looks arbitrary because it is inherited rather than designed. It was defined as one chain by one furlong — a strip 22 yards wide and 220 yards long, being roughly the area a yoke of oxen could plough in a day. The shape mattered: long and thin suited ploughing, because turning the team was the slow part.
The hectare was invented to fix exactly this. It is a square 100 m on a side, 10,000 m², and it converts to square kilometres by moving a decimal point. That 1 hectare ≈ 2.471 acres is the bridge between the two systems, and rounding it to “two and a half” is accurate to about 1%.
Which unit to ask for
- Square metres for anything building-sized — rooms, roofs, gardens, car parks.
- Hectares or acres for land: fields, plots, parks, smallholdings. Which of the two depends entirely on where you are.
- Square kilometres or square miles for anything at the scale of a town or larger, where hectares stop being readable.
- Nautical miles only at sea or in the air, where they align with the latitude grid on the chart.
Put these to use in the area calculator or measure distance. Related: glossary and how long is a degree of latitude.