What is a conversion factor?
A conversion factor is the number you multiply or divide by to change a measurement from one unit into another, such as centimetres to inches, miles to kilometres or kilograms to pounds.
A conversion factor is a fixed number used to convert a measurement from one unit into another. For example, because 1 inch equals 2.54 centimetres, the conversion factor from inches to centimetres is 2.54.
Conversion factors are used in both the metric system and imperial units. They are the reason a converter can turn kilograms into pounds, litres into pints, or miles per hour into kilometres per hour.
Why conversion factors matter
Every unit converter relies on conversion factors. Once the factor is known, the calculation is usually simple: multiply to move one way, and divide to move back the other way.
They are especially useful in the UK because everyday measurements often switch between metric and imperial units. You might see product dimensions in centimetres, body weight in stone and pounds, road speed in miles per hour, and liquids in litres or pints.
Conversion factor formula
The basic rule is:
To reverse the conversion, divide by the same factor:
For example, miles to kilometres uses a factor of 1.609344. So 5 miles is 5 × 1.609344 = 8.04672 kilometres.
Common conversion factors
Here are some common factors used in everyday unit conversions.
| Conversion | Factor | Example |
|---|---|---|
| inches to centimetres | × 2.54 | 10 in = 25.4 cm |
| centimetres to inches | ÷ 2.54 | 30 cm ≈ 11.81 in |
| miles to kilometres | × 1.609344 | 10 miles ≈ 16.09 km |
| kilograms to pounds | × 2.2046226218 | 70 kg ≈ 154.32 lb |
| litres to UK pints | × 1.759754 | 2 L ≈ 3.52 UK pints |
| mph to km/h | × 1.609344 | 70 mph ≈ 112.65 km/h |
Do you multiply or divide?
Whether you multiply or divide depends on the direction of the conversion. If you know the factor from unit A to unit B, multiply by that factor. To convert back from B to A, divide by the same factor.
| Direction | What to do | Example |
|---|---|---|
| inches to centimetres | multiply by 2.54 | 8 in × 2.54 = 20.32 cm |
| centimetres to inches | divide by 2.54 | 20.32 cm ÷ 2.54 = 8 in |
| miles to kilometres | multiply by 1.609344 | 3 miles × 1.609344 = 4.83 km |
| kilometres to miles | divide by 1.609344 | 4.83 km ÷ 1.609344 ≈ 3 miles |
Temperature is different
Most unit conversions use one multiplication factor. Temperature conversions are different because the scales do not share the same zero point.
°C = (°F - 32) × 5/9
K = °C + 273.15
That is why a simple factor works for length, weight, volume and speed, but not for Celsius, Fahrenheit and Kelvin.
Conversion factor examples
Here are a few practical examples using conversion factors:
| Question | Calculation | Answer |
|---|---|---|
| How many cm is 12 inches? | 12 × 2.54 | 30.48 cm |
| How many miles is 10 km? | 10 ÷ 1.609344 | About 6.21 miles |
| How many pounds is 5 kg? | 5 × 2.2046226218 | About 11.02 lb |
| How many km/h is 30 mph? | 30 × 1.609344 | About 48.28 km/h |
Rounding conversion results
Conversion factors can produce long decimal answers. In everyday use, it is usually fine to round the result to one or two decimal places, especially for shopping, travel, cooking and general estimates.
Do not over-round when accuracy matters. For example, technical, medical, engineering or legal measurements may need more precision than an everyday conversion.
Quick answers
| Question | Answer |
|---|---|
| What is a conversion factor? | A conversion factor is the number used to change a value from one unit into another. |
| Do you multiply by a conversion factor? | Usually yes, when converting in the direction the factor is written. To go back, divide by the factor. |
| What is the conversion factor from inches to cm? | 2.54, because 1 inch equals 2.54 centimetres. |
| Do temperature conversions use a conversion factor? | Not in the same simple way. Celsius, Fahrenheit and Kelvin need formulas because their zero points differ. |
FAQs
What does conversion factor mean?
A conversion factor is a number used to convert one unit of measurement into another. For example, 2.54 converts inches into centimetres.
What is an example of a conversion factor?
One example is 1.609344 for miles to kilometres. Multiplying miles by 1.609344 gives the equivalent distance in kilometres.
How do I know whether to multiply or divide?
Multiply when using the factor in its normal direction. Divide by the same factor to reverse the conversion.
Why do temperature conversions need formulas?
Temperature scales have different zero points, so Celsius to Fahrenheit and Fahrenheit to Celsius need formulas rather than one simple multiplication factor.