Metric and imperial length
The metric system scales in powers of ten from the meter, which makes conversion within it a matter of moving the decimal point: a kilometer is 1,000 meters, a centimeter is one hundredth, a millimeter one thousandth. Imperial units come from historical body-based and agricultural measures, which is why they use irregular factors — 12 inches to a foot, 3 feet to a yard, 1,760 yards to a mile.
The two systems have been formally linked since 1959, when the international yard and pound agreement fixed the inch at exactly 25.4 millimeters. Every metric-imperial length conversion derives from that single exact definition, so these conversions are exact rather than approximate.
| Unit | In meters | Notes |
|---|---|---|
| Millimeter (mm) | 0.001 m | 1/1000 m |
| Centimeter (cm) | 0.01 m | 1/100 m; 2.54 cm = 1 in exactly |
| Meter (m) | 1 m | SI base unit |
| Kilometer (km) | 1,000 m | 0.621371 mi |
| Inch (in) | 0.0254 m | Exact since 1959 |
| Foot (ft) | 0.3048 m | 12 in |
| Yard (yd) | 0.9144 m | 3 ft |
| Mile (mi) | 1,609.344 m | 5,280 ft; 1.609344 km |
| Nautical mile | 1,852 m | One minute of latitude |
Conversions worth memorizing
A handful of factors cover most everyday needs.
- 1 inch = 2.54 cm exactly
- 1 foot = 30.48 cm exactly
- 1 meter ≈ 3.281 feet or 39.37 inches
- 1 mile = 1.609344 km exactly
- 1 km ≈ 0.621 miles
- 1 yard = 0.9144 m exactly
- 1 nautical mile = 1,852 m exactly (one minute of latitude)
| Value | cm → in | in → cm | m → ft | ft → m | km → mi | mi → km |
|---|---|---|---|---|---|---|
| 1 | 0.394 | 2.54 | 3.281 | 0.305 | 0.621 | 1.609 |
| 5 | 1.969 | 12.7 | 16.404 | 1.524 | 3.107 | 8.047 |
| 10 | 3.937 | 25.4 | 32.808 | 3.048 | 6.214 | 16.093 |
| 25 | 9.843 | 63.5 | 82.021 | 7.62 | 15.534 | 40.234 |
| 50 | 19.685 | 127 | 164.042 | 15.24 | 31.069 | 80.467 |
| 100 | 39.37 | 254 | 328.084 | 30.48 | 62.137 | 160.934 |
Why the nautical mile is different
The nautical mile is not an arbitrary length: it was defined as one minute of arc of latitude along a meridian, which ties it directly to the Earth's geometry. That property makes it uniquely convenient for navigation, since a degree of latitude is 60 nautical miles anywhere on the globe. It is now fixed at exactly 1,852 meters by international agreement, and the knot — one nautical mile per hour — remains the standard speed unit in aviation and at sea.
When conversion errors matter
Unit confusion has caused real and expensive failures. NASA lost the Mars Climate Orbiter in 1999 because one team supplied thrust data in pound-force seconds while the receiving software expected newton-seconds. In 1983, an Air Canada 767 ran out of fuel mid-flight after a loading calculation mixed pounds and kilograms.
The practical lesson is to carry units through a calculation rather than working with bare numbers, and to sanity-check the magnitude of a result. If converting meters to feet produces a smaller number, the factor was applied in the wrong direction.
Height conversion chart
Height is the everyday length conversion most people actually need. Feet and inches to centimetres: multiply the total inches by 2.54. The chart runs from 4′10″ to 6′6″ in two-inch steps.
| Height | Inches | cm | m |
|---|---|---|---|
| 4′10″ | 58 | 147.3 | 1.473 |
| 5′0″ | 60 | 152.4 | 1.524 |
| 5′2″ | 62 | 157.5 | 1.575 |
| 5′4″ | 64 | 162.6 | 1.626 |
| 5′6″ | 66 | 167.6 | 1.676 |
| 5′8″ | 68 | 172.7 | 1.727 |
| 5′10″ | 70 | 177.8 | 1.778 |
| 6′0″ | 72 | 182.9 | 1.829 |
| 6′2″ | 74 | 188 | 1.88 |
| 6′4″ | 76 | 193 | 1.93 |
| 6′6″ | 78 | 198.1 | 1.981 |