Definitions, Not Approximations
Most everyday unit conversions are exact by definition rather than measured. An inch has been exactly 25.4 millimetres since 1959, a pound exactly 0.45359237 kilograms, and a mile exactly 1609.344 metres. These are not rounded approximations that happen to be convenient; they are the definitions themselves, agreed internationally to make the two systems commensurable.
This converter uses those defined factors across length, weight, volume, temperature and area.
How to Convert a Unit
Pick the category first, since it determines which units are offered.
- Choose a category: length, weight, volume, temperature or area.
- Enter the value you want to convert.
- Select the unit you are converting from.
- Select the unit you are converting to.
- Read the result, which updates as you type.
- Swap the two units to check the reverse conversion when it matters.
Why Temperature Behaves Differently
Every other category converts by multiplication alone, because the scales share a zero point. Zero metres is zero feet; zero kilograms is zero pounds. Temperature scales do not share one, so Celsius to Fahrenheit needs both a factor and an offset, multiply by 9/5 and add 32, and that offset is why you cannot convert a temperature difference the same way you convert a temperature. A rise of 10 °C is a rise of 18 °F, not 50 °F.
Volume carries a different trap. The US and imperial gallons are not the same size: a US gallon is about 3.785 litres and an imperial gallon about 4.546, a difference of roughly 20 per cent. The same divergence runs through pints and fluid ounces, so a recipe or a fuel-consumption figure means materially different things depending on which side of the Atlantic wrote it.
Defined Conversion Factors
These values are exact by international agreement, not rounded.
| Unit | In base unit | Status |
|---|---|---|
| 1 inch | 0.0254 m | Exact by definition |
| 1 foot | 0.3048 m | Exact by definition |
| 1 mile | 1609.344 m | Exact by definition |
| 1 pound | 0.45359237 kg | Exact by definition |
| 1 ounce | 0.028349523125 kg | Exact by definition |
| 1 US gallon | 3.785411784 L | Exact by definition |
The long decimals are not precision for its own sake. Because the pound is defined exactly, an ounce is one-sixteenth of it exactly, which is where those trailing digits come from, truncating them introduces error that compounds across repeated conversions.
Rounding and Significant Figures
A converter can give you more digits than your input justifies. Converting a measurement of "about 5 feet" to 1.524 metres implies millimetre precision that the original never had. The general rule is to keep the result to the same number of significant figures as the input, and to round only once at the end of a calculation rather than at each step.
Everything here runs in your browser, so conversions work offline once the page has loaded and nothing you enter is transmitted or stored. For anything with legal, medical or engineering consequence, a dosage, a load calculation, a customs declaration, check the figure against the standard or authority that governs it, since the unit definition is only one part of getting such a number right.
One category deliberately absent here is currency. Exchange rates move continuously and a converter that stored them would be wrong within minutes, whereas every factor on this page was fixed by agreement decades ago and will read the same next year. That difference, defined constants against live market data, is why the two kinds of conversion belong in separate tools rather than behind one dropdown.