Unit Conversion Guide: Metric, Imperial and More
By AllConvertor · · 2 min read
Whether you are following a recipe from another country, buying furniture, or reading an engineering datasheet, unit conversion is a daily need. Learning a handful of exact factors and using a reliable unit converter prevents most mistakes.
Why conversion errors are costly
Mixing units has caused real disasters. In 1999 NASA lost the Mars Climate Orbiter because one team used imperial units and another used metric. In 1983 an Air Canada Boeing 767 ran out of fuel mid-flight after fuel was loaded in pounds instead of kilograms, and it glided to an emergency landing. Everyday errors are smaller, but the lesson is the same: always label units.
Key length, mass and volume factors
- 1 inch = 2.54 centimetres (exact)
- 1 foot = 0.3048 metres (exact)
- 1 mile = 1.609344 kilometres (exact)
- 1 pound = 0.45359237 kilograms (exact)
- 1 ounce = 28.3495 grams (approximate)
- 1 US gallon = 3.78541 litres; 1 imperial gallon = 4.54609 litres
Note the US and imperial gallons differ by about 20 percent, which matters for fuel economy comparisons.
Temperature conversions
- Celsius to Fahrenheit: °F = °C × 9 ÷ 5 + 32
- Fahrenheit to Celsius: °C = (°F − 32) × 5 ÷ 9
- Celsius to Kelvin: K = °C + 273.15
Unlike length or weight, temperature scales have different zero points, so you cannot convert by multiplying alone. For example, 20 °C is 20 × 9 ÷ 5 + 32 = 68 °F.
Data units: KB vs KiB
In the SI system, 1 kilobyte (KB) is 1,000 bytes, 1 megabyte is 1,000,000 bytes and so on. In binary, 1 kibibyte (KiB) is 1,024 bytes and 1 mebibyte (MiB) is 1,048,576 bytes. Storage makers use the decimal system, while some operating systems display binary values, which is why a 1 TB drive may show as roughly 931 GiB.
How to convert units accurately
- Write down the starting value with its unit.
- Pick the exact conversion factor, or use the unit converter to apply it for you.
- Multiply or divide in the correct direction, and keep extra decimals until the last step.
- Round only the final result, to the precision your task needs.
- Sanity check the answer: a result in feet should be bigger than the same length in metres.
Tips for everyday use
- Learn rough anchors: 1 kg is about 2.2 lb, 1 km is about 0.62 miles, 1 litre is about 1.06 US quarts
- For cooking, use a scale and grams when possible, since cups and spoons vary by region
- For height, convert feet and inches to centimetres with a height converter instead of mental maths
- When in doubt, include the unit in every number you share