Unit Conversion Cheat Sheet: Length, Weight, Temperature and Time
The conversions people use most, with exact factors, mental shortcuts and common mistakes, plus how to use the converters.
Length
The metric system is built on tens, which makes it easy to scale: 1 kilometre is 1,000 metres, 1 metre is 100 centimetres, and 1 centimetre is 10 millimetres. The imperial units are not so tidy: 12 inches make a foot, 3 feet make a yard and 1,760 yards make a mile.
Exact links between the systems: 1 inch = 2.54 cm exactly, 1 foot = 30.48 cm, 1 yard = 0.9144 m, and 1 mile = 1.609344 km. Shortcuts: multiply miles by 1.6 to get kilometres, and divide kilometres by 1.6 for miles. A foot is about 30 cm, and a metre is a little over 3 feet.
Weight and mass
1 kilogram = 1,000 grams, and 1 gram = 1,000 milligrams. 1 pound = 0.45359237 kg exactly, and 1 ounce is about 28.35 grams. A stone, used in the UK for body weight, is 14 pounds, or about 6.35 kg. Quick check: a kilogram is about 2.2 pounds, so halve the pounds and subtract a tenth to estimate kilograms.
Scientists distinguish mass from weight, but in daily life both are given in kilograms or pounds.
Temperature
Temperature scales do not convert with a single multiplier because they start at different zero points. Fahrenheit = Celsius × 9/5 + 32, and Celsius = (Fahrenheit − 32) × 5/9. Kelvin = Celsius + 273.15. Handy anchors: 0 °C = 32 °F (freezing), 20 °C = 68 °F (a pleasant room), 37 °C ≈ 98.6 °F (body temperature) and 100 °C = 212 °F (boiling at sea level).
A mental shortcut from Celsius to Fahrenheit is to double it and add 30; it is approximate, but close for everyday temperatures. The Unit Converter does the exact maths.
Time
A minute is 60 seconds, an hour is 3,600 seconds and a day is 86,400 seconds. The Time Converter uses those exact values. It does not convert to months or years, because their lengths vary: months have 28 to 31 days, and a year is 365 or 366 days. For calendar maths, such as the time between two dates, use a date calculator or the age calculator instead of a fixed multiplier.
Decimal hours cause confusion: 1.5 hours is 1 hour 30 minutes, not 1 hour 50. To convert decimal hours to minutes, multiply the decimal part by 60: 2.25 hours is 2 hours 15 minutes.
Volume and other units
This site's converter covers length, weight and temperature. For cooking and fuel you may also meet volume: 1 litre = 1,000 millilitres, 1 US gallon is about 3.785 litres, and a UK (imperial) gallon is about 4.546 litres, so check which gallon a source uses. A US cup is about 237 ml, and a tablespoon is about 15 ml, though cup sizes differ between countries.
Avoiding mistakes
Write the unit next to every number. Convert once, at the end of a calculation, and keep extra digits until then. Check whether your answer makes sense: a person weighing 800 kg or a 400 °C room is a sign of a slip. Use Percentage Calculator when you want to express a change between two measurements as a percent.