Use the How to Convert Units
To convert units, multiply the original measurement by the correct conversion factor. For reverse conversions, divide by the same factor. Temperature is the main exception because Celsius, Fahrenheit and Kelvin use formulas with offsets.
If 1 unit A equals 2.54 units B, multiply A by 2.54 to get B. To go back from B to A, divide by 2.54.
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Use the main unit converter for length, weight, temperature, volume and speed.
The 4-step method for converting units
Most everyday conversions follow the same simple process. This works for length, weight, volume and speed where the relationship is a straight multiplier.
- Write down the original value. For example, 10 miles.
- Choose the unit you want to convert to. For example, kilometres.
- Find the conversion factor. 1 mile = 1.609344 kilometres.
- Multiply or divide. 10 miles × 1.609344 = 16.09344 kilometres.
converted value = original value × conversion factorFor a quick tool-led answer, use the unit converter. For a deeper explanation of the multiplier, see conversion factor.
How to know whether to multiply or divide
A common mistake is using the right number in the wrong direction. The easiest way to avoid this is to start with a known relationship and ask which side is bigger.
| Known relationship | To convert this way | Use | Example |
|---|---|---|---|
| 1 inch = 2.54cm | inches to cm | multiply by 2.54 | 10in = 25.4cm |
| 1 inch = 2.54cm | cm to inches | divide by 2.54 | 25.4cm = 10in |
| 1 mile = 1.609344km | miles to km | multiply by 1.609344 | 5 miles ≈ 8.05km |
| 1 mile = 1.609344km | km to miles | divide by 1.609344 | 10km ≈ 6.21 miles |
When in doubt, do a quick sense check. If you convert miles to kilometres, the number should usually get bigger. If you convert centimetres to inches, the number should get smaller.
Example: converting length units
Length conversions are common for height, furniture, DIY, travel, screens and property measurements. Common UK conversions include centimetres to inches, metres to feet and miles to kilometres.
inches = centimetres ÷ 2.54
centimetres = inches × 2.54
kilometres = miles × 1.609344
miles = kilometres ÷ 1.609344Use the length converter for millimetres, centimetres, metres, kilometres, inches, feet, yards and miles.
Example: converting weight units
Weight conversions are common for body weight, parcels, food packaging, gym equipment and luggage. In the UK, body weight is often spoken about in stone and pounds, while retail and official weights are usually metric.
pounds = kilograms × 2.2046226218
kilograms = pounds ÷ 2.2046226218
stone = pounds ÷ 14
pounds = stone × 14| Example | Calculation | Result |
|---|---|---|
| 70kg to pounds | 70 × 2.2046226218 | 154.32lb |
| 154lb to stone | 154 ÷ 14 | 11 stone |
| 500g to ounces | 500 ÷ 28.3495 | 17.64oz |
Use the weight converter for grams, kilograms, ounces, pounds and stone.
Example: converting volume and speed units
Volume and speed conversions are useful for travel, fuel, containers, recipes, drinks, liquids and road-speed comparisons.
| Conversion | Formula | Example |
|---|---|---|
| litres to millilitres | ml = litres × 1,000 | 1.5L = 1,500ml |
| litres to UK pints | UK pints = litres × 1.759754 | 2L ≈ 3.52 UK pints |
| mph to km/h | km/h = mph × 1.609344 | 70mph ≈ 112.65km/h |
| km/h to mph | mph = km/h ÷ 1.609344 | 100km/h ≈ 62.14mph |
Use the volume converter or speed converter for these conversions.
Why temperature conversion is different
Temperature is not converted with a simple multiplier only. Celsius, Fahrenheit and Kelvin start from different zero points, so the formulas include both multiplication and addition or subtraction.
Fahrenheit = (Celsius × 9/5) + 32
Celsius = (Fahrenheit - 32) × 5/9
Kelvin = Celsius + 273.15
Celsius = Kelvin - 273.15Use the temperature converter for Celsius, Fahrenheit and Kelvin. For cooking, use the oven temperature converter.
How to round converted units
Rounding should match the job. For quick estimates, one or two decimal places may be enough. For DIY, science, medicine, official forms or product specifications, avoid rounding too aggressively.
| Situation | Rounding approach | Example |
|---|---|---|
| Travel distance | Round to 1 or 2 decimal places | 5 miles ≈ 8.05km |
| Body weight | Use stone and remaining pounds | 154lb = 11 st 0 lb |
| DIY cutting | Keep more precision | Do not round 30.48cm down to 30cm unless the tolerance allows it |
| Temperature | Use sensible whole numbers for everyday use | 20°C = 68°F |
For more guidance, read rounding converted measurements.
Common unit conversion mistakes
- Multiplying when the conversion needs dividing.
- Using US pints or gallons when the question needs UK pints or gallons.
- Treating Celsius to Fahrenheit as a simple multiplier.
- Rounding too early before doing a second calculation.
- Confusing mass or weight with volume, especially in cooking.
- Using decimal stone instead of stone and remaining pounds for UK body-weight examples.
FAQs
What is the easiest way to convert units?
The easiest way is to use a unit converter. If doing it manually, use a conversion factor: multiply to move one way and divide by the same factor to move back.
What is a conversion factor?
A conversion factor is the number used to change one unit into another. For example, 2.54 is the conversion factor from inches to centimetres.
Do all conversions use multiplication?
No. Length, weight, volume and speed usually use multiplication or division, but temperature uses formulas with offsets, such as °F = (°C × 9/5) + 32.
Should I round before or after converting?
Usually round at the end. Rounding too early can create larger errors if you use the result in another calculation.
Are UK and US pints the same?
No. A UK pint is about 568ml, while a US liquid pint is about 473ml. Choose the correct pint type when converting volume.
Sources and notes
This guide uses stable standard conversion factors and UK-friendly examples. For official UK measurement context, refer to GOV.UK weights and measures guidance. For metric unit definitions, refer to the International System of Units.