Answer: The All-in-One Unit Converter produces your output instantly from the input you provide — everything runs in your browser, free, with no signup required.
Convert between any units — length, weight, temperature, volume, and more
Unit conversion looks trivial until you meet it in the wild: a European recipe in grams, a torque spec in newton-meters on a car built for foot-pounds, a shipping weight in kilograms on a runway measured in feet. This converter handles the everyday families — length, weight, volume, temperature, area, speed — with exact conversion factors, so the output is the same number you would get from a reference table, not a rounded approximation of it.
The foundation of modern conversion is exactness. Since 1959, the international yard and pound agreement has defined 1 yard as exactly 0.9144 meter and 1 pound as exactly 0.45359237 kilogram, which makes every imperial–metric conversion in length and mass a matter of multiplying a fixed rational number. Volume units trace to the liter (1 L = 0.001 m³) with the US gallon defined as exactly 231 cubic inches. Temperature is the odd one out: it converts by formula, not factor, because the Celsius and Fahrenheit scales have different zero points.
What this means practically is that a converter can and should be exact. Below are the exact or standard factors the tool uses, the same ones published by national metrology institutes such as NIST in their special publication on metric conversion.
Using it is simple: pick a category, choose the units on each side, and type a value — the result updates instantly, and you can flip the direction with one click. Because the factors are exact, converting back and forth loses no precision, which matters for recipes, medication doses, engineering specs, and customs paperwork where a rounded conversion can compound into a real error.
The table lists the anchor conversions from which all others in each family derive — miles chain through feet (1 mi = 5,280 ft exactly), ounces through pounds, cups through the legal US definition of the fluid ounce (exactly 29.5735295625 mL).
One caution: US and imperial (UK) units diverge where volume is concerned. A US gallon is 3.785 L; an imperial gallon is 4.546 L. A US fluid ounce is about 29.57 mL; an imperial one about 28.41 mL. Recipes and fuel economy figures are the classic victims — always confirm which system the source used before converting.
Significant figures deserve a mention too. A factor may be exact, but your input rarely is: converting 150 pounds to kilograms gives 68.0388555 kg on paper, yet a bathroom scale reading of 150 lb justifies no more than three significant figures of output. Report what the measurement supports, not everything the arithmetic produces — the tool gives you the exact number and leaves that judgment to you.
A note on notation: the converter accepts decimals and simple fractions where units are traditionally written fractionally, such as 5.5 or 5 1/2 inches in woodworking.
Temperature cannot be converted by a single factor because the scales do not share a zero. The exact relations are: °F = °C × 9/5 + 32, and K = °C + 273.15. The 273.15 is exact by definition of the kelvin. Water freezes at 0 °C / 32 °F / 273.15 K and boils at 100 °C / 212 °F at standard pressure; −40 is the single point where Celsius and Fahrenheit numerically agree — a handy fact for checking any temperature converter.
| Quantity | Conversion | Factor (exact unless noted) |
|---|---|---|
| Length | 1 inch | 2.54 cm exactly |
| Length | 1 foot | 0.3048 m exactly |
| Length | 1 mile | 1.609344 km exactly |
| Mass | 1 pound | 0.45359237 kg exactly |
| Volume | 1 US gallon | 3.785411784 L exactly |
| Temperature | °F to °C | (°F − 32) × 5/9 |
History's memorable unit mistakes are instructive. The 1999 Mars Climate Orbiter was lost because one team used pound-force seconds and another newton-seconds — a factor of about 4.45 between software systems. Medicine dosing errors between milligrams and grains, and aviation fuel miscalculations between kilograms and pounds (the 1983 Gimli Glider incident), show the same lesson: the arithmetic is easy, but the discipline of naming the unit before converting is what prevents the error. Type the number, confirm both units, then trust the answer.
How many centimeters in an inch?
Exactly 2.54 — the inch has been defined as 2.54 cm by international agreement since 1959. Twelve inches is exactly 30.48 cm, and a foot is exactly 0.3048 meter.
How do I convert kilograms to pounds?
Multiply by 2.20462 (or divide pounds by 0.45359237, which is the exact definition of the pound in kilograms). 10 kg ≈ 22.05 lb.
What is the formula for Celsius to Fahrenheit?
°F = °C × 9/5 + 32. So 20 °C = 68 °F. Reverse it with °C = (°F − 32) × 5/9. At −40 degrees the two scales read the same number.
Is a US gallon the same as a UK gallon?
No. A US gallon is about 3.785 liters; an imperial (UK) gallon is about 4.546 liters. US and imperial fluid ounces also differ (29.57 mL vs 28.41 mL). Always confirm which system the source uses.
Why do converters give slightly different answers?
Good ones do not — the core factors are exact by international definition. Differences come from rounding displayed digits, or from using the wrong system (US vs imperial volume, statute vs nautical miles).