Unit Converter
Convert length, weight, temperature, volume, and speed between metric and imperial units. Results update instantly as you type.
Last updated: October 2026
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How the conversions work
Every unit in a category is defined by its relationship to one base unit: meters for length, kilograms for weight, liters for volume, and meters per second for speed. Converting is a two-step trip through the base:
Converting 5 miles to kilometers: 5 x 1609.344 / 1000 = 8.04672 km. The factors are exact definitions, not measurements: the international mile is defined as exactly 1,609.344 meters, the pound as exactly 0.45359237 kilograms, and the inch as exactly 2.54 centimeters. Because the factors are exact, the conversions are exact too, apart from display rounding.
Temperature is the special case
Length, weight, volume, and speed all convert with a single multiplication. Temperature cannot, because Celsius, Fahrenheit, and Kelvin put zero in different places. Conversion needs a scale factor and an offset:
- Fahrenheit = Celsius x 9/5 + 32
- Celsius = (Fahrenheit - 32) x 5/9
- Kelvin = Celsius + 273.15
The converter routes every temperature through Celsius internally. So 100 F becomes (100 - 32) x 5/9 = 37.78 C, and 0 C becomes 0 + 273.15 = 273.15 K. Note the "inverse rate" box is hidden for temperature, since a single ratio does not describe an offset conversion.
| From | To | Multiply by |
|---|---|---|
| Miles | Kilometers | 1.609344 |
| Inches | Centimeters | 2.54 (exact) |
| Pounds | Kilograms | 0.45359237 (exact) |
| Ounces | Grams | 28.349523125 (exact) |
| US gallons | Liters | 3.785411784 (exact) |
| Fluid ounces (US) | Milliliters | 29.5735295625 |
| Miles per hour | km/h | 1.609344 |
| km/h | m/s | 0.2777... (divide by 3.6) |
Worked examples
100 F to C: (100 - 32) x 5/9 = 68 x 5/9 = 37.78 C. This is just above normal body temperature, a warm summer day.
5 miles to km: 5 x 1.609344 = 8.04672 km, displayed as 8.047. A 5-mile run is an 8K, near enough that race organizers round it.
0 C to K: 0 + 273.15 = 273.15 K, the freezing point of water on the absolute scale used in science.
A note on precision
The conversion factors used here are exact definitions, not rounded measurements. The inch is defined as exactly 2.54 centimeters, the pound as exactly 0.45359237 kilograms, and the US gallon as exactly 3.785411784 liters. That means converting 1 inch to centimeters gives precisely 2.54, with no measurement error at all. The only rounding you see is display rounding: results are trimmed to 6 decimal places so a value like 8.04672 km does not print with floating-point noise.
Very large or very small results switch to scientific notation to stay readable. If you need more digits for engineering work, the underlying math is exact, so you can safely extend the decimals yourself. For everyday use, two or three decimals are plenty: no luggage scale cares about the fourth decimal of a kilogram.
Quick mental conversion tricks
Fahrenheit to Celsius: subtract 30, then halve it. 100 F becomes (100 - 30) / 2 = 35 C, close to the true 37.78. Good enough for weather.
Miles to kilometers: multiply by 1.6. Kilometers to miles: multiply by 0.6. The error is under 4 percent either way.
Pounds to kilograms: double it and move the decimal: 150 lb is about 68 kg (true value 68.04). Kilograms to pounds: double and add ten percent.
km/h to mph: multiply by 0.62. So 100 km/h is about 62 mph, which is why European speed limits feel familiar on American roads.
Unit converter FAQs
How do I convert Fahrenheit to Celsius?
Subtract 32, then multiply by 5/9: C = (F - 32) x 5/9. So 100 F is (100 - 32) x 5/9 = 37.78 C. Select Temperature above, enter 100 with From set to Fahrenheit and To set to Celsius, and the converter does it instantly.
How many kilometers are in a mile?
One international mile is exactly 1.609344 kilometers, by definition. So 5 miles is 8.047 kilometers. For a quick mental estimate, multiply miles by 1.6.
Why is temperature conversion different from other units?
Most units convert with a single multiply factor, but Celsius, Fahrenheit, and Kelvin have different zero points, so conversion needs both a scale factor and an offset: F = C x 9/5 + 32, and K = C + 273.15. The converter routes temperature through Celsius internally, then applies the offset.
Which gallon does the volume converter use?
The US liquid gallon of exactly 3.785411784 liters, which is the standard for fuel economy, recipes, and commerce in the United States. The imperial gallon used in the UK is larger, at about 4.546 liters.
How many pounds are in a kilogram?
One kilogram is exactly 2.2046226218 pounds, by the international definition of the pound as 0.45359237 kilograms. For a quick estimate, double the kilograms and add ten percent.
Is my input sent to a server?
No. All conversions run in your browser with JavaScript using fixed conversion factors. Nothing you type leaves your device.