How length units are defined
The metre is the SI base unit: since 1983 it has been defined as the distance light travels in a vacuum in 1/299,792,458 of a second. Every metric length here is a decimal multiple of it — a kilometre is 1,000 m, a centimetre 0.01 m, a micrometre 10⁻⁶ m, a nanometre 10⁻⁹ m.
The imperial units are defined against the metre, not the other way round. The 1959 international yard and pound agreement fixed the yard at exactly 0.9144 m, which makes the foot 0.3048 m, the inch 0.0254 m and the mile 1,609.344 m — all exact, so cm-to-inch conversions never involve a rounded constant.
The remaining units belong to specific trades. The nautical mile is exactly 1,852 m (roughly one minute of latitude), the ångström is 10⁻¹⁰ m and is still used for atomic spacing, the astronomical unit is exactly 149,597,870,700 m, and a light-year is 9,460,730,472,580,800 m — a distance, never a length of time.
Converting by hand
Centimetres and inches: divide by 2.54 to go cm → in, multiply by 2.54 for in → cm. 175 cm ÷ 2.54 = 68.898 in, and 68.898 ÷ 12 = 5 ft with 8.9 in left, so 175 cm is 5 ft 8.9 in. Going the other way, 5 ft 10 in = 70 in × 2.54 = 177.8 cm exactly.
Miles and kilometres: multiply miles by 1.609344, or divide kilometres by the same number. A marathon is 42.195 km ÷ 1.609344 = 26.219 miles; a 5K is 3.107 miles; 100 yards on an American football field is 91.44 m, which is why a “100 m dash” and a “100 yard dash” are different races.
Millimetres and fractions of an inch: 1 mm = 0.03937 in, and 1/16 in = 1.5875 mm. Hardware sizes cross this line constantly — a 1/2 in pipe is 12.7 mm, a 3/8 in socket is 9.525 mm, and an M6 bolt (6 mm) is close to but not the same as a 1/4 in one (6.35 mm).
Quick reference
- 1 in = 2.54 cm · 1 ft = 30.48 cm · 1 yd = 0.9144 m
- 5 ft 0 in = 152.4 cm · 5 ft 6 in = 167.64 cm · 5 ft 10 in = 177.8 cm · 6 ft 2 in = 187.96 cm
- 160 cm = 5 ft 3.0 in · 170 cm = 5 ft 6.9 in · 180 cm = 5 ft 10.9 in · 190 cm = 6 ft 2.8 in
- 1 mi = 1.609344 km · 1 km = 0.621371 mi · 5 km = 3.107 mi · 10 km = 6.214 mi
- 1 nmi = 1.852 km = 1.15078 mi · 100 m = 109.361 yd · 1 mm = 0.03937 in
- Screen sizes: 15.6 in = 39.6 cm · 27 in = 68.6 cm · 6.1 in = 15.5 cm (all diagonals)
Statute mile, nautical mile and kilometre
The statute mile (1,609.344 m) is the one on road signs in the US and UK. The nautical mile (1,852 m) is 15% longer and is used at sea and in the air because it maps onto the earth: one minute of latitude is one nautical mile, so a chart position converts to distance without a scale factor.
Speeds follow the same split: a knot is one nautical mile per hour (0.514444 m/s), so 10 knots is 18.52 km/h, not 16.09 km/h. Aviation altitudes are in feet, runway lengths in metres or feet depending on the country, and visibility in statute miles in the US but metres almost everywhere else.
There is also a US survey foot, defined as 1200/3937 m, which was used for land surveying and state plane coordinates. NIST and the NGS retired it on 31 December 2022; it differs from the international foot by about 2 parts per million, which is 3.2 mm over a mile but several centimetres across a county boundary.
Common mistakes
Mixing up the prime marks. One prime means feet and a double prime means inches: 5′10″ is five feet ten inches, not five feet ten feet. Written with straight quotes it becomes 5'10", which spreadsheets and forms often mangle — enter the value in inches (70) if a field refuses the notation.
Rounding 2.54 to 2.5. It looks harmless and costs 1.6% — over a 2 m span that is 3 cm. The same applies to using 1.6 instead of 1.609344 for miles: a 26.2-mile marathon becomes 41.9 km instead of 42.2 km, a difference of 300 m.
Assuming a metre is “about a yard”. It is 1.0936 yd, or 3 ft 3.37 in — nearly 9% longer than 3 feet. For fabric, rope, cable runs and shipping dimensions that 9% is the difference between enough and not enough.
Traditional units are not in the table. Convert them first: 1 Korean ja (자) ≈ 30.3 cm, 1 chi (치) ≈ 3.03 cm, 1 ri (리) ≈ 392.7 m; 1 Japanese shaku ≈ 30.3 cm. They are historical approximations rather than defined SI values, which is why they are kept out of an exact conversion table.
Where you need these conversions
Height and body measurements. Medical forms, visa applications and dating profiles ask for whichever system the country uses, and gym equipment is often labelled in the other. Height in centimetres also feeds directly into a BMI calculation, where an error of a few centimetres shifts the result by a full point.
Running, cycling and travel. Race distances, treadmill readouts and GPS watches mix miles and kilometres, and road signs change at the border — Ireland in kilometres, Northern Ireland in miles, on the same island.
Buying things sized abroad. Screens, tyres, bike frames, lumber and luggage are quoted in inches or feet even in metric countries. A 27-inch monitor is 68.6 cm across the diagonal, and a carry-on limit of 22 × 14 × 9 in is 55.9 × 35.6 × 22.9 cm.