Unit converter — metric and imperial, both directions
Convert between metric and imperial units across six categories: length, weight/mass, temperature, volume, area, and speed. Built for recipes, travel, schoolwork, DIY and engineering — any time you need to move between systems quickly and accurately.
Exact definitions, not approximations
Every category defines a single base unit and converts through it. Lengths route through metres, weights through kilograms, temperatures through Celsius, volumes through litres, areas through square metres, and speeds through metres per second. Your input is first converted to the base unit, then from the base unit to your target, so any pair within a category works without a special-case formula. Constants are exact where a definition exists (1 inch = 0.0254 m, 1 mile = 1609.344 m, 1 lb = 0.453592 kg). Both fields are live — edit either side — and the ⇄ button swaps the pair. Results are shown to 8 significant figures with trailing zeros removed.
Category reference and useful examples
Length
| From | To | Factor | Example |
|---|---|---|---|
| 1 inch | cm | × 2.54 | 6 in = 15.24 cm |
| 1 foot | metres | × 0.3048 | 6 ft = 1.8288 m |
| 1 mile | km | × 1.609344 | 5 mi = 8.047 km |
| 1 metre | inches | × 39.3701 | 2 m = 78.74 in |
Common real-world need: a recipe from a US cookbook calls for 9-inch pan — that is 22.86 cm.
Weight / mass
| From | To | Factor | Example |
|---|---|---|---|
| 1 kg | lb | ÷ 0.453592 | 70 kg = 154.3 lb |
| 1 lb | kg | × 0.453592 | 150 lb = 68.04 kg |
| 1 oz | grams | × 28.3495 | 4 oz = 113.4 g |
Common real-world need: a US recipe calls for 8 oz of flour — that is 227 g.
Temperature
| Conversion | Formula | Example |
|---|---|---|
| °C to °F | × 9/5 + 32 | 20°C = 68°F |
| °F to °C | (F − 32) × 5/9 | 98.6°F = 37°C |
Quick tip: body temperature is 37°C / 98.6°F; a summer day of 30°C is 86°F.
Volume
| From | To | Factor | Example |
|---|---|---|---|
| 1 litre | US cups | × 4.22675 | 0.5 L = 2.11 cups |
| 1 US gallon | litres | × 3.78541 | 5 gal = 18.93 L |
| 1 UK gallon | litres | × 4.54609 | 5 gal = 22.73 L |
The UK and US gallon differ by about 20% — a critical distinction for fuel economy comparisons.
Speed
| From | To | Factor | Example |
|---|---|---|---|
| 1 mph | km/h | × 1.60934 | 60 mph = 96.56 km/h |
| 1 km/h | mph | × 0.62137 | 100 km/h = 62.14 mph |
| 1 m/s | mph | × 2.23694 | 10 m/s = 22.37 mph |
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Every conversion here is a defined constant, not a measurement
Modern unit conversion is exact by international agreement. One inch is exactly 25.4 mm — fixed by the 1959 international yard and pound agreement, not measured to be so — and one pound is exactly 0.45359237 kg. Since the 2019 SI redefinition, even the base units themselves are defined by fixing physical constants (the speed of light for the metre, the Boltzmann constant for the kelvin), as documented by the BIPM’s SI Brochure. The practical consequence: conversion error in real work comes from rounding and from choosing the wrong unit, never from the conversion factors themselves. NIST Special Publication 811 is the standard reference for the factors and for unit-style rules.
When unit mistakes actually bite
The famous cautionary tale is NASA’s Mars Climate Orbiter (1999), lost because one team supplied thruster impulse in pound-force seconds while the receiving software expected newton-seconds. The everyday versions are less dramatic but constant: US recipes measuring flour in volume (cups) versus European scales in grams; “ton” meaning 907 kg (US short), 1,016 kg (UK long) or 1,000 kg (metric tonne) depending on the document; and fluid ounces differing between US (29.57 ml) and imperial (28.41 ml) measures. The defence is always the same — carry the unit name with the number through every calculation, and convert once, explicitly, at a known point rather than repeatedly and implicitly.