How the conversion works
The converter covers seven categories: length, weight, temperature, area, speed, volume and digital storage. For every category except temperature, each unit stores one number, its size in a base unit (metre, gram, square metre, metre per second, litre or byte). A conversion goes through that base:
result = value × (from factor) ÷ (to factor)
For example 10 km to miles is 10 × 1000 ÷ 1609.344 = 6.21371 mi. Many factors are exact by international definition: 1 inch is exactly 0.0254 m, 1 pound is exactly 0.45359237 kg and 1 mile is exactly 1609.344 m. Results are rounded to 7 significant figures for display.
Temperature is different
Celsius, Fahrenheit, Kelvin and Rankine do not share a zero point, so a multiplier alone would be wrong. The converter goes through Celsius using offsets:
°C = (°F − 32) × 5/9and°F = °C × 9/5 + 32K = °C + 273.15and°R = (°C + 273.15) × 9/5
A temperature below absolute zero (0 K, −273.15 °C, −459.67 °F) shows a message instead of a result.
So 72°F gives (72 − 32) × 5/9 = 22.2°C, and 0°C is 273.15 K.
Where people get tripped up
- US versus imperial gallons. The US gallon is 3.785 L, the imperial gallon is 4.546 L. That 20% difference is why fuel economy figures from the US and UK are not directly comparable. Both are listed separately here, along with imperial pints and fluid ounces.
- Short ton, long ton and tonne. A US ton is 907.18 kg, a UK long ton is 1,016.05 kg and a metric tonne is 1,000 kg. Troy ounces (31.1035 g, for precious metals) are separate from ordinary ounces.
- Fluid ounces versus ounces. The first measures volume, the second mass. They are in different categories on purpose.
- Decimal versus binary storage. 1 GB is 1,000,000,000 bytes (the kilobyte is written kB), while 1 GiB is 1,073,741,824 bytes. That is why a drive sold as 1 TB shows about 931 GiB in an operating system that counts in binary.
Common reference values
| Category | Conversion |
|---|---|
| Length | 1 mile = 1.609344 km |
| Weight | 1 kg = 2.20462 lb |
| Volume | 1 US cup = 236.588 ml |
| Area | 1 acre = 4,046.856 m², about 0.4047 ha |
| Speed | 1 knot = 1.852 km/h |
| Storage | 1 TiB = 1,099,511,627,776 bytes |
Besides the main result, the page lists your value in every unit of the category at once, which is handy when a recipe or spec sheet uses a unit you don't recognize. Dedicated pages exist for the length, weight, temperature, volume and data storage converters. For money, use the currency converter. The background on why two systems exist is in metric vs imperial units.