Exact definitions, not approximations
Since 1959 the English units have been defined in metric terms, so these conversions are exact, not rounded: 1 inch = 25.4 mm, 1 pound = 453.59237 g, 1 US gallon = 3.785411784 L, 1 mile = 1,609.344 m. This converter uses those legal definitions throughout, so round-trips come back to exactly the number you started with.
Conversions worth memorizing
| Rule of thumb | Exact |
|---|---|
| 1 inch ≈ 2.5 cm | 25.4 mm |
| 1 meter ≈ 3.3 feet | 3.2808 ft |
| 1 km ≈ 0.6 mile | 0.62137 mi |
| 1 kg ≈ 2.2 pounds | 2.20462 lb |
| 1 liter ≈ 1 quart | 1.0567 qt (US) |
| 1 mil = 1 thou | 0.0254 mm exactly |
| °F ≈ double °C plus 30 | °F = °C × 9/5 + 32 |
Frequently asked questions
- US or Imperial (UK) volumes?
- US customary. The UK pint and gallon are about 20% larger (an Imperial gallon is 4.546 L vs the US 3.785 L) — a genuine trap in recipes and fuel economy figures. Weights and lengths are the same in both systems.
- Why does temperature behave differently?
- It's the one category where zero doesn't line up: 0 °C = 32 °F = 273.15 K. Conversions are shift-and-scale, not just scale — which is why "double it and add 30" only approximates °C → °F.
- What's a mil?
- A thousandth of an inch (0.0254 mm exactly), also called a thou. PCB trace widths, wire insulation, and machining tolerances live in mils — not to be confused with millimeters.
- Is a metric tonne the same as a US ton?
- No — the metric tonne is 1,000 kg; the US ("short") ton is 2,000 lb ≈ 907.2 kg. About a 10% difference that matters in freight.