How Unit Conversion Works
Every conversion is a multiplication by a ratio. To convert a value, express both units in terms of a common base and divide:
Result = Value × (size of the source unit ÷ size of the target unit)
This calculator stores every unit as a multiple of a single base — the metre, kilogram, litre, square metre, metre per second, joule, pascal or byte — so any pair within a category converts exactly, without chaining approximations.
Temperature is the exception. Celsius, Fahrenheit and Rankine have different zero points as well as different scales, so they need an offset as well as a ratio and cannot be handled by simple multiplication.
Temperature Formulas
°F = °C × 9/5 + 32 °C = (°F − 32) × 5/9
K = °C + 273.15 °R = °F + 459.67
| Celsius | Fahrenheit | Kelvin | Reference |
|---|---|---|---|
| −273.15 | −459.67 | 0 | Absolute zero |
| −40 | −40 | 233.15 | Where the two scales cross |
| 0 | 32 | 273.15 | Water freezes |
| 20 | 68 | 293.15 | Room temperature |
| 37 | 98.6 | 310.15 | Human body temperature |
| 100 | 212 | 373.15 | Water boils at sea level |
A useful mental shortcut for Celsius to Fahrenheit: double it and add 30. For 20°C that gives 70 against a true 68 — close enough for weather.
Exact Definitions
Several imperial units were redefined in terms of metric ones by international agreement in 1959, so these conversions are exact rather than approximate:
| Unit | Exact equivalent |
|---|---|
| 1 inch | 25.4 mm |
| 1 foot | 0.3048 m |
| 1 yard | 0.9144 m |
| 1 mile | 1,609.344 m |
| 1 pound | 0.45359237 kg |
| 1 ounce | 28.349523125 g |
| 1 US gallon | 3.785411784 L |
| 1 imperial gallon | 4.54609 L |
US and Imperial Are Not the Same
Britain and the United States share unit names but not always the values, which is a persistent source of error in recipes and fuel figures.
| Unit | US | Imperial (UK) | Difference |
|---|---|---|---|
| Gallon | 3.785 L | 4.546 L | UK is 20% larger |
| Fluid ounce | 29.57 mL | 28.41 mL | US is 4% larger |
| Pint | 473 mL (16 fl oz) | 568 mL (20 fl oz) | UK is 20% larger |
| Ton | 907 kg (short) | 1,016 kg (long) | UK is 12% larger |
A US car doing "30 miles per gallon" achieves 36 mpg on the UK measure for identical fuel use.
Kilobyte or Kibibyte?
Digital storage has two competing conventions. The SI prefixes are powers of 1,000; the binary prefixes are powers of 1,024. Storage manufacturers use the decimal form, while operating systems have historically reported the binary form, which is why a "1 TB" drive shows as roughly 931 GB in Windows.
| Decimal (SI) | Bytes | Binary (IEC) | Bytes | Gap |
|---|---|---|---|---|
| 1 KB | 1,000 | 1 KiB | 1,024 | 2.4% |
| 1 MB | 1,000,000 | 1 MiB | 1,048,576 | 4.9% |
| 1 GB | 109 | 1 GiB | 1,073,741,824 | 7.4% |
| 1 TB | 1012 | 1 TiB | 1,099,511,627,776 | 10.0% |
Both are offered above so you can see which one a given figure is using.
Handy Approximations
| Conversion | Exact | Mental shortcut |
|---|---|---|
| km to miles | × 0.6214 | × 0.6, or use Fibonacci: 5 km ≈ 3 mi, 8 km ≈ 5 mi |
| kg to pounds | × 2.2046 | Double and add 10% |
| cm to inches | ÷ 2.54 | Divide by 2.5 |
| litres to US gallons | ÷ 3.785 | Divide by 4, add a bit |
| metres to feet | × 3.281 | × 3, add 10% |
Why Unit Errors Matter
NASA lost the $327 million Mars Climate Orbiter in 1999 because one team supplied thrust data in pound-seconds while the navigation software expected newton-seconds. The spacecraft approached the planet roughly 170 km lower than intended and broke up. Unit conversion is not a trivial detail in engineering — it is a documented cause of catastrophic failure.
Frequently Asked Questions
Why does my result differ in the last decimal place?
Rounding. This calculator converts through an exact base value and rounds only for display, so intermediate precision is preserved. Chained conversions elsewhere may round at each step.
Which gallon does the calculator use?
Both are listed separately — US and imperial — so you can choose explicitly rather than guess.
Can I convert between categories?
No, because they measure different physical quantities. Litres and kilograms are only relatable through density, which differs by substance: one litre of water is 1 kg, but one litre of oil is about 0.92 kg.
Is a Calorie the same as a calorie?
No. The food Calorie is a kilocalorie — 1,000 small calories, or 4,184 joules. Both are listed in the energy category.