A reference of the units used on the site, what each one is, and the formula to convert it. Organised by category, with exact factors where they exist.
Every unit conversion is one multiplication. Find the factor that turns one unit into another, multiply to go forward and divide to go back. One pound is 0.45359237 kilograms, so 10 lb is 10 × 0.45359237 = 4.536 kg, and 4.536 kg is 4.536 ÷ 0.45359237 = 10 lb. The exception is temperature, which also needs an offset because its scales start at different points.
Some factors are exact by definition, such as 1 inch = 25.4 mm and 1 lb = 0.45359237 kg. Others are rounded, such as 1 m = 3.28084 ft. Use the exact factor for precise work and the rounded one for quick estimates.
| Unit | What it is | To convert |
|---|---|---|
| Inch (in) | US and imperial unit, defined as exactly 25.4 mm | mm = in × 25.4 cm = in × 2.54 |
| Foot (ft) | 12 inches, exactly 0.3048 m | m = ft × 0.3048 in = ft × 12 |
| Yard (yd) | 3 feet, exactly 0.9144 m | m = yd × 0.9144 |
| Mile (mi) | 5,280 feet, exactly 1.609344 km | km = mi × 1.609344 mi = km × 0.621371 |
| Metre (m) | SI base unit of length | ft = m × 3.28084 yd = m × 1.09361 |
| Unit | What it is | To convert |
|---|---|---|
| Ounce (oz) | 1/16 of a pound, exactly 28.349523125 g | g = oz × 28.3495 |
| Pound (lb) | 16 ounces, exactly 0.45359237 kg | kg = lb × 0.45359237 lb = kg × 2.20462 |
| Stone (st) | 14 pounds, used for body weight in the UK and Ireland | lb = st × 14 kg = st × 6.35029 |
| US short ton | 2,000 pounds | kg = ton × 907.185 |
| Metric tonne (t) | 1,000 kilograms | lb = t × 2204.62 |
| Troy ounce (ozt) | Precious-metal ounce, 31.1034768 g | g = ozt × 31.1035 ozt = oz × 0.911458 (avoirdupois to troy) |
| Gram (g) and kilogram (kg) | SI units; the kilogram is the base unit | kg = g ÷ 1,000 |
| Unit | What it is | To convert |
|---|---|---|
| Celsius (°C) | Water freezes at 0 and boils at 100 at sea level | °F = °C × 9/5 + 32 K = °C + 273.15 |
| Fahrenheit (°F) | Water freezes at 32 and boils at 212 | °C = (°F − 32) × 5/9 |
| Kelvin (K) | SI base unit; 0 K is absolute zero | °C = K − 273.15 °F = (K − 273.15) × 9/5 + 32 |
| Rankine (°R) | Absolute scale with Fahrenheit-size degrees | °C = (°R − 491.67) × 5/9 |
Quick check: 100°C is 212°F and 0°C is 32°F. A shortcut for Celsius to Fahrenheit is to double and add 30, which is accurate within about 5 degrees for everyday weather.
| Unit | What it is | To convert |
|---|---|---|
| US teaspoon (tsp) | 1/3 of a tablespoon | ml = tsp × 4.92892 |
| US tablespoon (tbsp) | 1/2 US fluid ounce | ml = tbsp × 14.7868 |
| US fluid ounce (fl oz) | 1/128 of a US gallon | ml = fl oz × 29.5735 |
| US cup | 8 US fluid ounces | ml = cup × 236.588 |
| US pint and quart | 2 cups and 4 cups | ml = pint × 473.176 l = quart × 0.946353 |
| US gallon (gal) | 231 cubic inches | l = gal × 3.78541 |
| Imperial gallon | 4.54609 litres, used in the UK | l = imp gal × 4.54609 |
| Litre (l) and millilitre (ml) | 1 l = 1,000 ml = 1,000 cm³ | ml = l × 1,000 |
US and imperial gallons, pints and fluid ounces differ, so always check which one a recipe or label means. See US vs UK weights and measures.
| Unit | What it is | To convert |
|---|---|---|
| Square inch (in²) | Exactly 6.4516 cm² | cm² = in² × 6.4516 |
| Square foot (ft²) | 144 square inches, exactly 0.09290304 m² | m² = ft² × 0.092903 ft² = m² × 10.7639 |
| Square yard (yd²) | 9 square feet, exactly 0.83612736 m² | m² = yd² × 0.836127 |
| Acre (ac) | 43,560 square feet | m² = ac × 4,046.86 ha = ac × 0.404686 |
| Hectare (ha) | 10,000 square metres | ac = ha × 2.47105 |
| Square mile (mi²) | 640 acres | km² = mi² × 2.58999 |
| Unit | What it is | To convert |
|---|---|---|
| Miles per hour (mph) | Common road speed in the US and UK | km/h = mph × 1.609344 m/s = mph × 0.44704 |
| Kilometres per hour (km/h) | Road speed in most countries | mph = km/h × 0.621371 m/s = km/h ÷ 3.6 |
| Metres per second (m/s) | SI unit of speed | km/h = m/s × 3.6 |
| Knot (kn) | One nautical mile per hour | km/h = kn × 1.852 mph = kn × 1.15078 |
| Mach | Speed of sound, about 761.2 mph at sea level at 15°C | mph = Mach × 761.2 |
| Unit | What it is | To convert |
|---|---|---|
| Nautical mile (nmi) | Exactly 1,852 m, about one minute of latitude | km = nmi × 1.852 mi = nmi × 1.15078 |
| Light-year (ly) | Distance light travels in a year | km = ly × 9.4607 × 10¹² |
| Astronomical unit (AU) | Mean Earth-Sun distance, exactly 149,597,870.7 km | km = AU × 149,597,870.7 |
| Metre to mile | SI metre and statute mile | mi = m × 0.000621371 |
| Unit | What it is | To convert |
|---|---|---|
| CSS pixel (px) | 1/96 of an inch on a reference display | in = px ÷ 96 mm = px × 0.264583 |
| Point (pt) | 1/72 of an inch | px = pt × 4/3 pt = px × 0.75 |
| rem | Root font size; 16 px by default | px = rem × 16 rem = px ÷ 16 |
| Pixels at a given PPI | Pixel count of a print size | pixels = inches × PPI |
These values use CSS pixels, where 96 px equal one inch. A printed image's real pixel density comes from its PPI. See px, rem, em, points and DPI.
When no direct factor exists, convert in two steps through a base unit. To change miles per hour to metres per second, go through kilometres per hour or use 0.44704 directly. To change US cups to litres, turn cups into millilitres and divide by 1,000: 3 cups × 236.588 = 709.8 ml = 0.7098 l. Keep a few extra digits until the last step and round once at the end.
For the story of why these systems differ, see imperial vs metric.