Unit conversion expresses one quantity in another unit without changing the quantity itself. The safest approach is to identify the kind of quantity first, use a documented conversion factor or formula, keep units visible through the calculation, and round only the displayed answer.
Reference factors you can check
These examples use the international inch, foot and mile, plus the international avoirdupois pound. The factors are based on reference values published in the NIST Guide to the SI, Appendix B.8. Some other units have regional variants, so always check the full unit name.
| Conversion | Factor or formula | Worked example |
|---|---|---|
| Inches to centimetres | in × 2.54 | 12 in = 30.48 cm |
| International feet to metres | ft × 0.3048 | 10 ft = 3.048 m |
| International miles to kilometres | mi × 1.609344 | 3 mi = 4.828032 km |
| Avoirdupois pounds to kilograms | lb × 0.45359237 | 5 lb ≈ 2.26796 kg |
For 3 kilometres to metres, the kilometre factor is 1,000 metres, so 3 × 1,000 = 3,000 m. To convert 10 feet to metres, use 10 × 0.3048 = 3.048 m. The units should cancel to leave the unit you want. Test the values with the Unit Converter.
Temperature needs a formula, not one multiplier
Celsius, Fahrenheit and Kelvin use different zero points. Celsius to Fahrenheit is °F = (°C × 9/5) + 32. Fahrenheit to Celsius is °C = (°F − 32) × 5/9. Kelvin is K = °C + 273.15.
Worked check: 72°F becomes (72 − 32) × 5/9 = 22.222…°C, normally displayed as about 22.2°C. If a converter produces a different result, check the selected source and target units before assuming the arithmetic is wrong. The NIST customary-to-metric reference lists the Fahrenheit conversion formula.
Digital storage: GB and GiB are not the same
Decimal SI prefixes use powers of ten: 1 GB = 1,000 MB = 1,000,000,000 bytes. Binary prefixes use powers of two: 1 GiB = 1,024 MiB = 1,073,741,824 bytes. For example, 1.5 GB is 1,500 MB in the decimal convention, while 1.5 GiB is 1,536 MiB in the binary convention. The BIPM SI-prefix table defines decimal prefixes; the NIST binary-prefix reference explains KiB, MiB and GiB.
Choose the convention required by the task rather than treating KB, MB and GB as synonyms for KiB, MiB and GiB. The Storage Size Converter lets you select a decimal or binary scale for common byte units.
Three mistakes that change the result
- Rounding too early: keep the unrounded intermediate value and round the final display once.
- Reusing a linear factor for area or volume: square the length factor for area and cube it for volume. A length conversion factor alone is not an area or volume factor.
- Mixing units with similar names: a mass ounce is not a fluid ounce; U.S. and imperial volume units can differ; and U.S. survey feet are not the same as international feet.
A reliable repeatable workflow
- Write down the input value and the complete source unit.
- Choose a published factor or formula for that exact unit, including the measurement convention.
- Keep the unit next to every intermediate value so the factors cancel correctly.
- Round for the precision the task needs, then run a reverse or independent check when the result matters.