Unit Converter at a glance
- What it does
- Convert between metric and imperial units for length, weight, temperature, volume, area and speed, with the exact conversion factors shown.
- Where it runs
- Entirely in your browser — no data is uploaded
- Works offline
- Yes, once the page has loaded
- Cost
- Free, with no account and no usage limit
- Category
- Converters
How to use the converter
- Choose a category - length, weight, temperature and so on.
- Enter a value and select the unit it is in.
- Read the converted values and copy whichever one you need.
The conversion factors
These are the definitions, not approximations. Since 1959 the inch has been defined as exactly 25.4 millimetres by international agreement, which makes every imperial length conversion exact.
| From | To | Multiply by |
|---|---|---|
| Inches | Centimetres | 2.54 exactly |
| Feet | Metres | 0.3048 exactly |
| Miles | Kilometres | 1.609344 exactly |
| Pounds | Kilograms | 0.45359237 exactly |
| Ounces | Grams | 28.349523125 |
| US gallons | Litres | 3.785411784 |
| Imperial gallons | Litres | 4.54609 |
| Acres | Hectares | 0.40468564224 |
| Miles per hour | Km/h | 1.609344 |
Temperature is not like the others
Every other conversion here is a simple multiplication. Temperature is not, because Celsius and Fahrenheit have different zero points as well as different degree sizes.
°F = (°C × 9/5) + 32
°C = (°F − 32) × 5/9
K = °C + 273.15This is why you cannot convert a temperature difference with the same formula. A rise of 10 °C is a rise of 18 °F, not 50 °F - for a difference you use only the ratio and drop the offset.
The two scales cross at −40, where −40 °C equals −40 °F. A useful rough conversion for weather: double the Celsius figure and add 30, which is close enough for deciding what to wear.
The units that share a name and differ
A US gallon is 3.785 litres; an imperial gallon is 4.546 litres, about 20% larger. Pints follow: 473 ml in the US, 568 ml in the UK. Fluid ounces differ too, and confusingly in the opposite direction - the US fluid ounce is slightly larger than the imperial one.
Weight has the same trap. A US ton is 2,000 pounds, a UK long ton is 2,240 pounds, and a metric tonne is 1,000 kilograms - roughly 2,205 pounds. Recipes, shipping quotes and fuel economy figures all suffer from this regularly.
Fuel economy is worse still, because miles per gallon and litres per 100 km are inversely related: you cannot convert by a simple factor, and improving from 20 to 25 mpg saves far more fuel than improving from 40 to 45.
When unit errors get expensive
NASA lost the Mars Climate Orbiter in 1999 because one team supplied thrust in pound-seconds and the receiving software expected newton-seconds. The spacecraft entered the atmosphere at the wrong altitude and was destroyed - a 125-million-dollar unit conversion error.
In 1983 an Air Canada 767 ran out of fuel at 41,000 feet after a loading calculation used pounds where the aircraft was calibrated in kilograms. It glided to a landing at a disused airfield with no fatalities, and is still known as the Gimli Glider.
The lesson for anything routine: write the unit next to the number, in the variable name if you are writing code, and convert at the boundary rather than in the middle of a calculation.
Frequently asked questions
The US kept the older English wine gallon of 231 cubic inches. Britain redefined its gallon in 1824 as the volume of ten pounds of water, giving 4.546 litres. Neither has changed since.
No. A metric tonne is 1,000 kg (about 2,205 lb), a US short ton is 2,000 lb, and a UK long ton is 2,240 lb. Always check which is meant in shipping and construction quotes.
Use only the ratio: multiply a Celsius difference by 9/5 to get Fahrenheit, with no offset. The +32 applies to absolute temperatures, not to differences.
The United States, Liberia and Myanmar are the usual answer. In practice the US uses metric extensively in science, medicine and much of industry - it is everyday consumer measurement that remains imperial.
Nothing you enter here leaves your browser
Unit Converter does its work in JavaScript running on your own device. The page loads once, and after that there is no upload step and no server involved — which matters here because exported data frequently contains customer records or transactions.
You can verify this rather than taking our word for it: load the page, disconnect from the internet, and the tool keeps working. Our privacy policy sets out what is and is not collected, and this guide explains why the distinction matters.