The factors, in one table
Most of these are exact by definition. Since 1959 the inch has been exactly 25.4 mm and the pound exactly 0.45359237 kg, and the mile, foot, US gallon and ounce follow from them. The length converter and the other converters here multiply by these exact values and round once, at the end.
| To convert | Factor | Example |
|---|---|---|
| Inches to cm | × 2.54 | 10 in = 25.4 cm |
| cm to inches | ÷ 2.54 | 170 cm = 66.93 in (5 ft 6.9 in) |
| Feet to meters | × 0.3048 | 6 ft = 1.8288 m |
| Miles to km | × 1.609344 | 26.2 mi = 42.16 km |
| Pounds to kg | × 0.45359237 | 150 lb = 68.04 kg |
| kg to pounds | ÷ 0.45359237 (about × 2.20462) | 75 kg = 165.35 lb |
| Ounces to grams | × 28.349523125 | 8 oz = 226.80 g |
| Stone to kg | × 6.35029318 (14 lb) | 11 st 4 lb = 71.67 kg |
| US gallons to liters | × 3.785411784 | 10 US gal = 37.85 L |
| UK gallons to liters | × 4.54609 | 10 UK gal = 45.46 L |
| °C to °F | × 1.8, then + 32 | 25 °C = 77 °F |
| mpg (US) to L/100 km | 235.2145833 ÷ mpg | 30 mpg = 7.84 L/100 km |
| mph to km/h | × 1.609344 | 60 mph = 96.56 km/h |
Length: exact one way, a repeating decimal the other
Inches to centimeters is exact, because 2.54 has two decimal places. Going back is not: 1 ÷ 2.54 is 0.393700787 and the digits never end, so most cm to inches answers are rounded.
Heights are quoted in feet and inches, not decimal feet, and mixing them up is a classic trap. 5.9 ft is 5 ft 10.8 in, because 0.9 × 12 = 10.8, and not 5 ft 9 in. 175 cm is 68.9 in, which is 5 ft 8.9 in.
On the road, miles to km uses the statute mile, 1.609344 km. A nautical mile is 1.852 km, about 15% longer, so check which one a figure means.
Weight: stone, ounces and three kinds of ton
The abbreviation lbs is just the plural of lb, and both mean pounds. The kg to lbs page also converts to pounds and ounces, as a baby's weight is written, and to stone and pounds. A stone is 14 pounds, which is 6.35 kg, and it is used for body weight in the UK and Ireland. 72 kg is 158.73 lb, which is 11 st 4.7 lb.
Some names hide more than one size. The ounce on a food label is 28.35 g, but a troy ounce for precious metals is about 31.1 g. A metric ton, or tonne, is 1,000 kg, a US (short) ton is 2,000 lb or 907.18 kg, and a UK (long) ton is 2,240 lb or 1,016.05 kg. The weight converter lists each one separately.
Temperature: an offset, not just a multiplier
Most conversions are one multiplication. Temperature needs an addition as well, because 0 °C is 32 °F. Multiply by 1.8 first and add 32 second: 25 × 1.8 = 45, and 45 + 32 = 77 °F. Add first and you get (25 + 32) × 1.8 = 102.6, which is wrong. Going back, °C = (°F − 32) ÷ 1.8.
A change in temperature is different. A rise of 10 °C is a rise of 18 °F, because only the size of the degree differs and the 32 does not apply. At −40 the two scales agree. An oven at 180 °C is 356 °F, which US recipes round to 350. The Celsius to Fahrenheit page does the sum, and the temperature converter also covers kelvin.
Volume: the same names, different sizes
The US gallon is exactly 231 cubic inches, 3.785411784 L. The UK (imperial) gallon is exactly 4.54609 L, about 20% bigger. The smaller units follow, and the UK pint holds 20 fluid ounces where the US pint holds 16:
| Unit | US | UK (imperial) |
|---|---|---|
| Fluid ounce | 29.57 mL | 28.41 mL |
| Pint | 16 fl oz, 473.18 mL | 20 fl oz, 568.26 mL |
| Quart | 0.946 L | 1.137 L |
| Gallon | 3.785 L | 4.546 L |
A US cup is 236.59 mL, while many recipes outside the US assume 250 mL, about 6% more. The volume converter keeps US and UK units apart and shows both side by side, and liters to gallons opens on the US gallon with the UK answer one click away.
Fuel economy and speed: reciprocals and a mile
Miles per gallon counts distance per fuel, so bigger is better. Liters per 100 km counts fuel per distance, so smaller is better. Converting between them divides a constant by your number: 235.2145833 ÷ mpg for US gallons and 282.4809363 ÷ mpg for UK gallons. 30 mpg is 7.84 L/100 km on US gallons and 9.42 on UK gallons, and 30 mpg (US) is the same car as 36.03 mpg (UK). Always ask which gallon.
Equal steps in mpg are not equal savings. Over 10,000 miles, going from 10 to 20 mpg saves 500 gallons (1,000 down to 500), while going from 30 to 40 saves only 83.3. The fuel economy converter and the mpg to L/100 km page handle both gallons.
For speed, 1 mph is exactly 1.609344 km/h, so 60 mph is 96.56 km/h and 100 km/h is 62.14 mph, and a knot is exactly 1.852 km/h. The speed converter and mph to km/h pages cover them.
Mental shortcuts, and how wrong they are
Each shortcut below is close enough for a rough idea. The error is a percentage change from the exact answer, as in percentage calculations explained: the shortcut minus the exact value, divided by the exact value.
| Conversion | Shortcut | Example | Exact | Off by |
|---|---|---|---|---|
| cm to inches | × 0.4 | 50 cm = 20 in | 19.69 in | 1.6% high |
| Miles to km (and mph) | × 1.6 | 10 mi = 16 km | 16.09 km | 0.6% low |
| km to miles | × 0.62 | 100 km = 62 mi | 62.14 mi | 0.2% low |
| kg to pounds | Double, add 10% | 50 kg = 110 lb | 110.23 lb | 0.2% low |
| Pounds to kg | Halve, take off 10% | 200 lb = 90 kg | 90.72 kg | 0.8% low |
| °C to °F | Double, add 30 | 30 °C = 90 °F | 86 °F | 4 °F high |
| Liters to US gallons | ÷ 3.8 | 20 L = 5.26 gal | 5.28 gal | 0.4% low |
| mpg (US) to L/100 km | 235 ÷ mpg | 30 mpg = 7.83 | 7.84 | 0.1% low |
The temperature shortcut drifts as it gets warmer, so use double, take off a tenth of that, then add 32 instead. It is the exact formula in disguise, because 1.8 is 2 minus 0.2: for 30 °C that is 60 − 6 + 32 = 86. For kilometers and miles, 5 miles is about 8 km and 8 km is about 5 miles.
When prices are quoted per pound on one side and per kilogram on the other, convert before you compare. The margin vs markup guide shows how a unit mix-up makes a thin margin look generous.