About this tool
The Unit Converter handles conversions across six categories: length, weight and mass, temperature, volume, area, and speed. It supports the metric (SI) system, the US customary and British imperial systems, and other common units like knots and acres. All conversions happen instantly in your browser, with no upload and nothing saved.
Unit confusion is more than an inconvenience: a units mismatch destroyed NASA's Mars Climate Orbiter in 1999 when one team used pound-seconds and another used newton-seconds. On a smaller scale, the same trap appears when a recipe in US cups meets a kitchen scale in grams, or a height in feet and inches needs to go on a metric form. This converter keeps the exact international factors so you get the right answer the first time, and the tables below double as a reference you can scan without typing anything.
How unit conversion works
Every category here shares one trick: a base unit. Each unit is stored as how many base units it equals, so converting is two steps, divide out the source unit and multiply in the target. Temperature is the exception because its scales have different zero points, so it uses an offset formula rather than a single factor.
value_in_base = value x factor(from) result = value_in_base / factor(to) # temperature is affine, not a simple ratio: Fahrenheit = Celsius x 9/5 + 32 Kelvin = Celsius + 273.15
- factor(from) and factor(to) are exact ratios to the base unit (metre for length, kilogram for mass, litre for volume).
- Length, weight, volume, area, and speed are pure ratios, so order does not matter and the math is reversible.
- Temperature needs the offset (the +32 and +273.15) because 0 degrees does not mean "no temperature" on the Celsius or Fahrenheit scale.
Worked example: 5 miles to kilometres
- Base factor for the mile: 1 mile = 1,609.344 metres (exact, from the 1959 international yard agreement).
- Convert to base: 5 miles x 1,609.344 = 8,046.72 metres.
- Convert base to target: a kilometre is 1,000 metres, so 8,046.72 / 1,000 = 8.04672 km.
- Sanity check: the memory aid "5 miles is about 8 km" matches, confirming the result.
Common unit conversion factors
| Type | Conversion | Memory aid |
|---|---|---|
| Length | 1 inch = 2.54 cm (exact) | Always exact since 1959 international yard agreement |
| Length | 1 mile = 1.60934 km | 5 miles ~ 8 km |
| Length | 1 foot = 0.3048 m (exact) | |
| Mass | 1 lb = 0.453592 kg | 2.2 lb per kg |
| Mass | 1 stone (UK) = 6.35 kg = 14 lb | British weight, also Ireland |
| Volume | 1 US gal = 3.785 L | US gallon is smaller than UK |
| Volume | 1 UK gal = 4.546 L | Imperial gallon, ~20% larger than US |
| Volume | 1 fl oz US = 29.57 mL | |
| Temperature | F = C x 9/5 + 32 | 0 C = 32 F; 100 C = 212 F |
| Temperature | K = C + 273.15 | Absolute zero = 0 K |
| Energy | 1 kcal = 4.184 kJ | Food labels use kcal even when they say 'calorie' |
| Power | 1 hp (metric) = 735.5 W | Engine horsepower |
| Pressure | 1 atm = 101.325 kPa = 14.696 psi | Atmospheric at sea level |
SI prefixes
| Prefix | Symbol | Power | Example |
|---|---|---|---|
| yotta | Y | 10^24 | Mass of the Sun ~2 YT |
| zetta | Z | 10^21 | Global data created annually |
| exa | E | 10^18 | Atomic bonds in 1 mole |
| peta | P | 10^15 | Petabyte = 1000 TB |
| tera | T | 10^12 | Terabyte |
| giga | G | 10^9 | Billion |
| mega | M | 10^6 | Million |
| kilo | k | 10^3 | Thousand |
| centi | c | 10^-2 | Centimeter, hundredth |
| milli | m | 10^-3 | Millisecond, millimeter |
| micro | µ | 10^-6 | Bacterial size |
| nano | n | 10^-9 | Wavelength of visible light |
| pico | p | 10^-12 |
Imperial vs metric: where each is used
Three countries still officially use imperial: United States, Liberia, Myanmar. The UK uses a mix (metric for trade, imperial for road distance, beer pints). Everywhere else is metric, though horsepower, inches for screens, and feet-and-inches for height are common globally even in metric countries.
Common pitfalls
- US versus UK gallons. A US gallon is 3.785 L and a UK gallon is 4.546 L, about 20 percent larger. A recipe or fuel figure converted with the wrong gallon is off by a fifth.
- Weight versus mass. Kilograms measure mass and stay constant; pounds are often used for weight, which depends on gravity. On Earth the everyday conversion holds, but the words are not interchangeable in physics.
- Temperature is not a ratio. You cannot say "20 degrees C is twice as warm as 10 degrees C". Because of the offset, only Kelvin supports ratio reasoning.
- Fluid ounce versus ounce. A fluid ounce measures volume and an ounce measures mass. They are equal only for water by coincidence of the definition.
- Ton ambiguity. "Ton" can mean a metric tonne (1,000 kg), a US short ton (907 kg), or a UK long ton (1,016 kg). Always confirm which one a source means.
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Frequently asked questions
What units are supported?
Length (km, mi, m, ft, in, cm), weight (kg, lb, oz, g), temperature (°C, °F, K), volume (L, gal, mL, cups), area (m², ft², acres, hectares), speed (km/h, mph, m/s, knots).
Why is 1 US gallon different from 1 UK gallon?
The US gallon (3.785 L) is based on the wine gallon of 1707. The UK / Imperial gallon (4.546 L) was redefined in 1824 as the volume of 10 lb of water at 62°F. They've diverged ever since.
Is Celsius or Fahrenheit better for weather?
Celsius scales 0-100 over freezing-to-boiling of water, useful for science. Fahrenheit scales roughly 0-100 over 'very cold' to 'very hot' for human experience, useful for daily weather. Most of the world uses Celsius; the US uses Fahrenheit.
How many significant figures does this conversion preserve?
Most factors used are exact (defined by international agreement) or accurate to 6+ significant figures. Display rounding may show fewer digits. Underlying math uses full IEEE 754 precision.
Are pound (mass) and pound-force the same?
On Earth at sea level, approximately yes. Strictly: 1 lb mass weighs 1 lbf force in standard gravity. In space or precise engineering, the distinction matters. Most everyday usage conflates them.
What's the difference between metric ton, short ton, and long ton?
Metric ton (tonne) = 1,000 kg (universal). Short ton (US) = 2,000 lb = 907 kg. Long ton (UK historical) = 2,240 lb = 1,016 kg. Shipping uses metric tons; US road weight limits use short tons.