How to Use This Tool
Pick a category, type a value, and choose units. Every conversion names the exact variant it used, because the variants are where the mistakes live.
The units that share a name
Metric conversions are boring and reliable. The interesting failures are all between systems that use the same word for different quantities:
- Gallon — US 3.785 L, imperial 4.546 L. A 20.1% difference, which is why fuel economy figures between the two countries are not comparable without converting.
- Pint — US 473 ml, imperial 568 ml. Recipes and beer both suffer.
- Fluid ounce — US 29.57 ml, imperial 28.41 ml. Here the US one is larger, reversing the pattern, because the two systems define them from different bases.
- Ton — three of them: metric tonne 1000 kg, US short ton 907 kg, UK long ton 1016 kg.
- Mile — the statute mile is 1609 m, the nautical mile is 1852 m. Aviation and shipping use the second.
Why the fluid ounce reverses
This one catches people who have learned the "imperial is bigger" rule. The imperial gallon was defined as the volume of ten pounds of water, and divided into 160 fluid ounces. The US gallon came from an older wine gallon and divides into 128. So the imperial gallon is larger while the imperial fluid ounce is smaller, and neither is a rounding of the other.
Temperature is not like the others
Every other conversion here is a multiplication, because zero means the same thing in both units. Zero metres is zero feet. Temperature has an offset, so converting requires both a scale and a shift, and crucially:
- Converting a temperature uses the offset: 20°C is 68°F.
- Converting a difference does not: a rise of 20°C is a rise of 36°F.
Applying the temperature formula to a difference is a real and common error — it turns a 20-degree rise into a 68-degree one. Kelvin has no offset relative to Celsius in size, only in origin, which is why scientific work uses it.
Precision
Results are shown to a sensible number of significant figures rather than to fifteen decimal places. A conversion cannot be more precise than the input: converting "about 5 miles" and reporting 8.04672 kilometres implies a precision that does not exist. If you need the full factor it is shown under the result.
