Convert any length value between ten different units: millimetres, centimetres, metres, kilometres, inches, feet, yards, miles, nautical miles, and light-years. Both sides are editable — type into the source or the target and the other updates. Swap button flips the direction. Five quick-pick buttons jump to common pairs (km ↔ mi, ft ↔ m, in ↔ cm).
How it works
All values route through metres as the canonical base. Converting from source to target is a single two-multiplication operation: multiply by metres-per-source, then divide by metres-per-target. The factor table holds exact conversion constants derived from the 1959 international yard-and-pound agreement (inch = 0.0254 m exactly) and international standards for nautical miles (1852 m exactly) and light-years (Julian year × c).
Example: highway speed limits
A 60 mph speed limit in km/h: enter 60 in the source (mi), target shows 96.56 (km). Or set up the converter in mi → km permanent-style with the swap button. Useful for anyone driving across the US-Canadian or US-Mexican border, where the speed-limit signs change units. (Speed is a different quantity — for mph ↔ km/h directly, use the speed converter.)
Example: metric-to-imperial translation
Height 1.75 metres in feet and inches: 1.75 m → 5.7415 ft → 5 ft + 0.7415 × 12 = 5 ft 8.9 in. Or directly: 1.75 m = 68.898 in ≈ 5 ft 8.9 in. The tool gives you the single target unit cleanly; mixed units (feet and inches) need a bit of manual arithmetic on the feet-decimal.
Example: interstellar distances
The nearest star other than the Sun is Proxima Centauri at about 4.246 light-years. In kilometres: 4.246 × 9.46 × 10¹² ≈ 4.01 × 10¹³ km — 40 trillion km. The scientific-notation display handles values of this scale without overflowing; the precision stays at about 15 significant digits regardless of magnitude.
Exact vs. approximate factors
| Conversion | Value | Exact? |
|---|---|---|
| 1 in → m | 0.0254 | yes (1959 agreement) |
| 1 ft → m | 0.3048 | yes (= 12 × 0.0254) |
| 1 yd → m | 0.9144 | yes (= 3 × 0.3048) |
| 1 mi → m | 1609.344 | yes (= 1760 × 0.9144) |
| 1 nmi → m | 1852 | yes (international convention) |
| 1 ly → m | 9.4607304725808 × 10¹⁵ | yes (Julian year × c) |
| 1 km → m | 1000 | yes (metric) |
The light-year uses the Julian year (365.25 days × 86400 seconds = 31,557,600 seconds), multiplied by the speed of light 299,792,458 m/s. This is the IAU-recommended convention. The “solar year” light-year or “Gregorian year” light-year would differ by a few tens of thousands of km over that astronomical distance — close to rounding error for all practical purposes.
What this tool does not do
It doesn’t handle mixed units like “5 feet 8 inches”. You’d need to convert each component separately and sum. A dedicated imperial-height converter handles the composite case.
It doesn’t support surveying units — rods, chains, furlongs, and so on. Those have exact relationships to the inch (e.g., 1 furlong = 660 ft) but aren’t common enough to include in the main table.
It doesn’t include astronomical units (au), parsecs, or Hubble-length units. Astronomy-specific conversions belong in a dedicated astro calculator.
It doesn’t handle geographic coordinates. Converting between degrees of latitude and metres requires spherical trigonometry and varies with latitude — a different tool entirely. For flat Cartesian distances instead, the distance between points calculator does the Pythagorean math.
It doesn’t display mixed units in the output. The tool gives you a single decimal value in the target unit; for “5 ft 8.9 in” style output, you’d need a dedicated imperial-height display.