Popular conversions
The measurements people look up most, each with its own formula, table and worked examples.
- Millimeters to inches1 mm = 0.0393701 in
- Inches to centimeters1 in = 2.54 cm
- Feet to meters1 ft = 0.3048 m
- Meters to feet1 m = 3.28084 ft
- Kilometers to miles1 km = 0.621371 mi
- Miles to kilometers1 mi = 1.60934 km
- Kilograms to pounds1 kg = 2.20462 lb
- Pounds to kilograms1 lb = 0.453592 kg
- Grams to ounces1 g = 0.035274 oz
- Degrees Celsius to degrees Fahrenheit°F = °C × 9/5 + 32
- Degrees Fahrenheit to degrees Celsius°C = (°F − 32) × 5/9
- Liters to US gallons1 L = 0.264172 gal
- US gallons to liters1 gal = 3.78541 L
- Miles per hour to kilometers per hour1 mph = 1.60934 km/h
- Square feet to acres1 ft² = 0.0000229568 ac
- US cups to milliliters1 cup = 236.588 mL
Browse by category
From kitchen measures to magnetic permeability.
See all with descriptionsEveryday
20Engineering
14Heat
11Fluids & Chemistry
9Light
6Electricity
14Magnetism
5Radiology
5More tools
7Built on definitions, not lookup tables
One conversion engine powers every page and every calculation in your browser.
- Exact where the definition is exact.An inch is 25.4 mm and a pound is 0.45359237 kg by international agreement, so those conversions carry no rounding error.
1 in = 2.54 cm - Offsets for temperature.Celsius, Fahrenheit, Kelvin and Rankine are converted with their zero points, never by a single multiplier.
°F = °C × 9/5 + 32 - Reciprocal and logarithmic units.Fuel economy, wavelength and decibels use the correct inverse or log formula.
L/100 km = 235.215 ÷ mpg - Tested in both directions.Every unit is round-trip tested against every other unit in its category, plus reference values from NIST SP 811.
38,740 automated checks
Measurement guides
All guides- Metric vs ImperialMetric vs imperial: how the two systems really compareWhere metric and imperial units come from, how they are defined today, and the conversions that matter most in daily life.
- Measurement GuidesSignificant figures and rounding: how precise should a conversion be?Why 1 mile is not “1.609344000 km” on a road sign, how to choose sensible precision, and when exact factors matter.
- Engineering ConversionsTorque, pressure and power: a field guide to engineering conversionsThe engineering conversions that trip people up most — torque vs energy, gauge vs absolute pressure, and the three horsepowers.
- Scientific UnitsThe seven SI base units and how they are defined todaySince 2019 every SI unit is defined by a fixed constant of nature. Here is what that means for the metre, kilogram, second and the rest.
- Cooking MeasurementsCooking conversions: cups, spoons, grams and oven temperaturesUS vs metric cups, tablespoons and teaspoons, why weighing beats measuring by volume, and an oven temperature chart with gas marks.
- Construction MeasurementsConstruction math: square footage, cubic yards and board feetHow to work out floor area, concrete and mulch volumes, lumber board feet and insulation R-values — with the conversions builders use.