Metric Converter
Choose a measurement category, enter your value, then pick the units you are converting from and to. The result updates instantly. Tap "Show your work" for the exact formula and factor used.
How this converter works
Every unit in each category is stored against a single SI base unit (metres for length, kilograms for mass, litres for volume, and so on). Converting from any unit to any other is a two-step process: first multiply the input by the factor that expresses the source unit in terms of the SI base, then divide by the factor for the target unit. The combined ratio - the factor you see in the results - gives you how many target units equal one source unit. Multiplying your value by that ratio gives the answer. Temperature is the one exception: Celsius, Fahrenheit, Kelvin and Rankine do not share a common zero point, so the conversion involves an offset step as well as scaling.
Metric vs. imperial - what is the difference?
The metric system (formally the International System of Units, or SI) is built on powers of ten, so converting between millimetres, centimetres, metres and kilometres is simply a matter of moving the decimal point. It is the global scientific standard and the everyday system in most of the world. The imperial system - still in everyday use in the United States (and partially in the UK for road distances and body weight) - uses arbitrary historical ratios: 12 inches to a foot, 3 feet to a yard, 1,760 yards to a mile, and so on. Because those ratios are not based on ten, converting between imperial units often requires remembering specific numbers. This calculator handles all the arithmetic for you, including US and UK variants where they differ (such as gallons).
Coverage - 9 categories and over 70 units
The converter covers length (millimetres to light-years), mass/weight (milligrams to long tons), temperature (Celsius, Fahrenheit, Kelvin and Rankine), volume (millilitres to UK gallons), area (square millimetres to square miles and acres), speed (m/s, km/h, mph, knots and Mach), time (nanoseconds to years), pressure (pascals to PSI and inches of mercury), and energy (joules to kilowatt-hours and BTU). For each conversion, the page also shows the exact factor and its inverse so you can cross-check the reverse direction instantly.
Worked examples
1 metre to feet: 1 m × 3.28084 ft/m = 3.281 ft. -- 100 km/h to mph: 100 × 0.62137 = 62.14 mph. -- 20 Celsius to Fahrenheit: (20 × 9/5) + 32 = 68 °F. -- 5 litres to US gallons: 5 × 0.26417 = 1.321 US gal. -- 1 atmosphere to PSI: 101,325 Pa ÷ 6,894.757 = 14.696 psi.
Common metric-to-imperial quick reference
| Metric | Imperial | Factor |
|---|---|---|
| 1 km | 0.621 miles | × 0.6214 |
| 1 m | 3.281 feet | × 3.2808 |
| 1 cm | 0.394 inches | × 0.3937 |
| 1 kg | 2.205 pounds | × 2.2046 |
| 1 g | 0.035 ounces | × 0.0353 |
| 1 litre | 0.264 US gal | × 0.2642 |
| 1 litre | 1.057 US qt | × 1.0567 |
| 0 °C | 32 °F | (°C × 9/5) + 32 |
| 100 °C | 212 °F | (°C × 9/5) + 32 |
| 1 kPa | 0.145 psi | × 0.1450 |
| 1 kJ | 0.948 BTU | × 0.9478 |
| 1 km/h | 0.621 mph | × 0.6214 |
Rounded values for everyday use. Exact factors are used in calculations above.
Frequently asked questions
Why does the converter show six decimal places?
Most conversion factors are irrational or have many significant digits, so rounding early introduces error. Displaying six decimal places lets you use the result at whatever precision your task needs. For everyday purposes, two or three decimal places are usually enough.
What is the difference between a US gallon and a UK gallon?
1 US liquid gallon = 3.785 litres; 1 UK (imperial) gallon = 4.546 litres. They differ by about 20 percent, so it matters which you choose. This converter offers both as separate options in the Volume category.
How does the calculator handle temperature - it is not a simple multiplication?
Temperature scales differ in both their zero points and their step sizes. To convert, the calculator first maps the input to Kelvin (the absolute zero-based scale), then maps from Kelvin to the target unit. For example, converting Celsius to Fahrenheit: K = °C + 273.15, then °F = K × 9/5 - 459.67. This simplifies to °F = (°C × 9/5) + 32.
What does the "conversion factor" output mean?
The conversion factor tells you how many target units equal exactly one source unit. If you are converting metres to feet, the factor is 3.28084 - meaning 1 metre equals 3.28084 feet. You can then multiply any number of metres by 3.28084 to get feet without using the calculator again. The inverse factor works in the opposite direction.
How accurate are the conversion factors?
All factors are exact or to at least 9 significant figures, matching international standards. The inch is defined as exactly 0.0254 metres; the pound as exactly 0.45359237 kilograms; the nautical mile as exactly 1,852 metres. For practical engineering or scientific work, these are more than sufficient.
Can I convert units within the same system - for example miles to feet?
Yes. Both miles and feet are in the length category, so you can select both from the same dropdown. The converter works between any two units in a category, regardless of whether they belong to the metric or imperial system.