How the unit conversion works
Each unit in a category stores one number: its size relative to the category's base unit. That's the meter for length, the square meter for area, the liter for volume, the kilogram for weight and the second for time. Converting is then one step: multiply by the source factor to reach the base, divide by the target factor to leave it. Routing everything through a base unit, instead of storing every pair, means rounding error never piles up across chained conversions, and adding a new unit takes one number instead of dozens.
Temperature is the exception, and it's worth knowing why. The other categories are ratio scales: zero means none of the quantity, so one multiplier converts correctly. Celsius and Fahrenheit are interval scales. Their zeros are arbitrary points, and neither one means "no heat." So conversion needs a multiplication and an offset. It's also why 20 °C isn't "twice as warm" as 10 °C, and why a temperature difference of 10 °C is 18 °F, not 50 °F.
The unit conversions that trip people up
- The US and UK gallons differ: 3.785 L against 4.546 L, a 20% gap. American recipes, fuel economy figures and UK pub measures don't share a gallon.
- An ounce can be weight or volume. A fluid ounce measures volume and an ounce measures mass. They only line up for water, and only roughly, so this converter keeps them in separate categories on purpose.
- Ton, tonne and short ton are three different weights. The metric tonne is 1,000 kg, the US short ton is 907 kg and the UK long ton is 1,016 kg. Cargo and emissions figures rarely say which one they mean.
- Square units don't convert linearly. A square meter is 10.76 square feet, not 3.28, because the conversion factor gets squared. Cubic units work the same way.
Metric and imperial, briefly
The metric system is decimal by design. Prefixes shift the decimal point, so converting inside it means moving a comma. Imperial units grew out of human-scale references (a foot, a hand, a grain of barley) and their relationships are irregular: 12 inches to a foot, 3 feet to a yard, 1,760 yards to a mile. That irregularity is why you want a tool here instead of doing it in your head.
Since 1959, imperial units have been legally defined in terms of metric ones. An inch is exactly 25.4 mm by international agreement, with no measurement involved. That's why the factors here are exact.
Privacy
Everything runs in your browser as plain arithmetic. Nothing is uploaded and nothing is stored.
Frequently asked questions
Why is temperature handled differently from other units?
Temperature scales have offset zero points. Length is proportional: 0 m is 0 ft, and doubling one doubles the other, so one multiplier does the job. But 0 °C is 32 °F, so converting takes a multiplication and an addition. That's also why the formula that converts a temperature reading won't convert a temperature difference.
Which gallon does the volume converter use?
Both, listed separately. The US liquid gallon is 3.785 liters. The UK imperial gallon is 4.546 liters, about 20% larger. US and UK recipes and fuel figures don't match, and mixing up the two gallons is the most common error in volume conversion.
How precise are the results?
The factors are the exact values set by international definition (an inch is exactly 0.0254 m, a pound exactly 0.45359237 kg), so the only error is floating-point rounding, far below anything you could measure. Results show eight decimal places and switch to scientific notation at the extremes.
Why is a month 30 days?
A calendar month has no fixed length. We use 30 days for a month and 365 for a year so every conversion gives the same answer. When the exact calendar matters, as with contracts, interest or deadlines, count the actual dates.