Length is the most frequently converted quantity in everyday life, largely because the United States, the United Kingdom and everyone else settled on different answers. The metric system scales by powers of ten; imperial units carry factors of 12, 3, 1,760 and 5,280 that have to be memorised rather than derived.
One useful anchor: the inch has been defined as exactly 25.4 mm since 1959, by international agreement. Every other imperial length follows from that, so all conversions here are exact rather than approximate.
Conversion factors
| From | To | Multiply by |
|---|---|---|
| Inches | Centimetres | 2.54 (exact) |
| Feet | Metres | 0.3048 (exact) |
| Yards | Metres | 0.9144 (exact) |
| Miles | Kilometres | 1.609344 (exact) |
| Centimetres | Inches | 0.393701 |
| Metres | Feet | 3.28084 |
| Kilometres | Miles | 0.621371 |
| Nautical miles | Kilometres | 1.852 (exact) |
Approximations worth memorising
- A metre is a little over three feet — close to a long walking pace.
- Kilometres to miles: multiply by 0.6, or take five-eighths. A 10 km run is about 6.2 miles.
- Miles to kilometres: add 60%. 60 mph is roughly 96 km/h, which is why 100 km/h limits feel familiar.
- A centimetre is about the width of a fingernail; an inch is about the top joint of a thumb.
- Consecutive Fibonacci numbers approximate the mile-kilometre ratio: 5 miles ≈ 8 km, 8 miles ≈ 13 km.
Height in feet and inches
Converting a height needs two steps, and skipping the first is where errors creep in. Convert the whole thing to inches, then to centimetres: 5 ft 10 in is (5 × 12) + 10 = 70 in, and 70 × 2.54 = 177.8 cm. Going the other way, divide centimetres by 2.54 for total inches, then divide by twelve for feet with the remainder as inches. 180 cm gives 70.9 in, which is 5 ft 10.9 in.
Specialist length units
- Nautical mile: 1,852 m, defined as one minute of latitude — the reason it exists at all.
- Light-year: about 9.46 trillion km, a distance rather than a duration.
- Micron (µm): one millionth of a metre, standard in manufacturing tolerances.
- Hand: exactly 4 inches, still used for horse height.
- Furlong: one-eighth of a mile, surviving almost solely in horse racing.
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