Unit Converter
This free unit converter handles length, weight, and temperature: millimeters to miles, milligrams to metric tons, Celsius to Fahrenheit to Kelvin. Enter an amount in whichever unit you know, and every equivalent appears instantly. Nothing you enter is sent anywhere or stored.
How to Use It
- Choose a category: Length, Weight, or Temperature.
- Enter an amount and choose which unit it’s in.
- Every equivalent unit appears instantly.
How It Works
Length and weight conversions here use exact, internationally-defined conversion factors (1 inch = 0.0254 meters, 1 pound = 453.59237 grams), not an approximation, both categories convert through a shared base unit (meters for length, grams for weight) rather than a separate hardcoded factor between every possible pair of units.
Temperature is handled differently, since Celsius, Fahrenheit, and Kelvin have different zero points, not just different scales: 0°C is 32°F and 273.15 K, and the Celsius and Fahrenheit scales only agree with each other at −40 degrees. Entering a temperature below absolute zero (−273.15°C) returns a clear error rather than a nonsensical result.
A couple of worked examples: 5 miles converts to 8,046.72 meters (5 × 1,609.344), or 8.04672 kilometers. Length always funnels through meters as the shared base, so that multiplication happens regardless of which two units you actually picked. For temperature, 100°F converts through Celsius first: (100 − 32) × 5 ÷ 9 = 37.78°C, then to Kelvin by adding 273.15, giving 310.93 K.
Everything runs client-side in your browser. No numbers you enter are sent anywhere or stored.
If you're converting cooking measurements specifically, weight and volume aren't interchangeable without knowing the ingredient's density. See our guide, A Practical Guide to Kitchen Measurement Conversions, or the ingredient-aware Cooking Measurement Converter.
Frequently Asked Questions
Why are these exact conversions, but the Shoe Size Converter uses a reference chart?
It comes down to what kind of thing is being converted. A length or weight unit is defined by an exact, internationally-agreed conversion factor (1 inch is exactly 2.54 centimeters, by definition), so a formula is the accurate approach. Shoe sizes aren’t defined that way. They’re standardized reference charts that vary slightly by convention, so a lookup table matches published sizing better than a formula would.
Why does temperature need its own logic instead of just a multiplier?
Celsius, Fahrenheit, and Kelvin don't just use different step sizes. They start counting from different zero points. Converting length or weight is a straightforward multiplication because both scales start at zero together; temperature needs the actual formulas (F = C × 9/5 + 32, K = C + 273.15) to account for that offset.
Where's Volume?
It's a natural next addition, but this round focuses on Length, Weight, and Temperature. For volume conversions tied to cooking, especially ingredient-specific conversions like cups of flour to grams, the Cooking Measurement Converter already covers that with ingredient density built in.
How many pounds are in a stone?
Exactly 14. Stone is a unit still commonly used for body weight in the UK and Ireland, defined as exactly 14 pounds, which is why entering 1 stone here converts to exactly 14 lb rather than a rounded approximation.
Why does converting a whole number of degrees give a decimal result?
Because the Fahrenheit-Celsius conversion involves dividing by 9 or 5, whole-number inputs often don't land on another whole number. 100°F, for example, converts to exactly 37.777...°C, shown rounded. This is expected, not a rounding bug. The two scales' degree sizes aren't a clean ratio of each other.
Can I convert between two units in the same system, like inches to miles?
Yes. Every length unit and every weight unit converts through the same shared base (meters for length, grams for weight), so converting between two units in the same measurement system (inches to miles, ounces to stone) works exactly the same way as crossing between metric and US/imperial units. There's no special-casing based on which pair you pick.
Why do I get an error entering a negative weight or length?
Negative amounts of length or weight aren't physically meaningful. You can't have -5 kilograms of something, so this calculator rejects negative input for those two categories with a clear error rather than returning a nonsensical negative conversion. Temperature is the one category where negative values are valid and expected, since both Celsius and Fahrenheit have negative ranges in ordinary use.