Convert between metric and imperial units for length, weight, temperature, area, volume and time. Nothing is uploaded — the maths runs in your browser.

How the conversions work

Each unit has a fixed conversion factor to its category's base unit (the metre, gram, litre, square metre or second). To convert, the value is multiplied into the base unit and then out again to the target unit. Because every factor is defined against the same base, any pair — millimetres to miles, ounces to kilograms — is a single, consistent calculation. Temperature is the exception: Celsius and Fahrenheit do not share a zero point, so those use an affine rule with both a scale and an offset.

Units you can convert

• Length — millimetre, centimetre, metre, kilometre, inch, foot, yard, mile.
• Weight — milligram, gram, kilogram, tonne, ounce, pound, stone.
• Temperature — Celsius, Fahrenheit, Kelvin.
• Area — mm2, cm2, m2, hectare, km2, square inch, square foot, acre.
• Volume — millilitre, litre, cubic metre, teaspoon, tablespoon, fluid ounce, cup, pint, quart, gallon.
• Time — millisecond, second, minute, hour, day, week.

Photo rules are written in mixed units, so the converter pairs well with the photo size chart: turn a country's millimetre figure into inches, or check a US 2x2 inch photo in millimetres. Our passport photo checker works entirely on your device too, and if you need to shrink a file afterwards the image compressor handles the kilobytes.

Frequently asked questions

Is any value I type sent to a server?
No. Every conversion is computed in your browser with built-in JavaScript; the numbers you enter never leave your device.

Which units are supported?
Length (mm, cm, m, km, inch, foot, yard, mile), weight (mg, g, kg, tonne, ounce, pound, stone), temperature (Celsius, Fahrenheit, Kelvin), area (mm2, cm2, m2, hectare, km2, square inch, square foot, acre), volume (ml, litre, m3, teaspoon, tablespoon, fluid ounce, cup, pint, quart, gallon) and time (ms, s, minute, hour, day, week).

Why are temperature conversions handled differently?
Length, weight, area, volume and time have a zero point at 0, so one multiplication by a ratio is enough. Celsius and Fahrenheit do not share a zero point, so temperature uses an affine rule with both a scale and an offset.

How exact are the results?
Conversions use the standard international definitions (for example 1 inch = 25.4 mm exactly and 1 lb = 453.59237 g exactly) and the result is shown to twelve significant figures, so it is accurate for practical purposes.

Can I convert in either direction?
Yes. Pick any unit on each side; the From and To selectors are independent, and the Swap button reverses them in one click.

Does it work for passport photo sizes?
Yes, it is handy for turning the millimetre sizes on our photo size chart into inches or centimetres, or for checking a 2x2 inch US photo in millimetres. Open the photo size chart for the exact per-country figures.