CodeMaster AcademyWhy unit conversion isn't just multiplication
Most unit conversions — length, weight, volume — really are a single multiplication by a fixed ratio: 1 mile is exactly 1.60934 kilometers, so converting either direction is one multiply or divide. But not every unit family works that way. Temperature is the classic exception: converting Celsius to Fahrenheit requires both a multiplication and an addition (°F = °C × 9/5 + 32), because the two scales don't share a common zero point — 0°C is 32°F, not 0°F. This tool handles both cases correctly, applying the right formula for whichever unit family you're converting, rather than treating every conversion as a simple ratio.
The unit families covered here
- Length: millimeters, centimeters, meters, kilometers, inches, feet, yards, miles.
- Weight/mass: milligrams, grams, kilograms, ounces, pounds, metric and imperial tons.
- Volume: milliliters, liters, US and imperial fluid ounces, cups, pints, gallons.
- Temperature: Celsius, Fahrenheit, and Kelvin — each with its own conversion formula rather than a shared ratio.
Where rounding causes confusion
Most real-world conversion factors are irrational or long decimals — a mile isn't a clean multiple of kilometers, and a US gallon isn't a clean multiple of liters. Different tools round to different decimal places, which is why you'll occasionally see a conversion vary by a tiny fraction between two calculators even though both are "correct." For engineering or scientific work where precision matters, always check how many decimal places a tool is actually using rather than assuming rounded figures are exact.
Metric vs. imperial: a quick note
Most of the world uses the metric system (meters, kilograms, liters, Celsius) because its units scale by clean powers of ten, making mental conversion between, say, millimeters and meters straightforward. The US and a handful of other countries primarily use imperial/US customary units (feet, pounds, gallons, Fahrenheit), which trace back to older, less systematic historical standards. This tool exists specifically because the two systems don't convert cleanly in your head, and getting a conversion wrong in cooking, travel, or shipping paperwork is an easy, avoidable mistake.
Using this converter
Choose the unit category (length, weight, volume, or temperature), select your "from" and "to" units, and enter a value — the converted result updates instantly. All conversions run locally in your browser.
Common questions
Why do my results differ slightly from another converter?
Small differences almost always come down to rounding precision — the underlying conversion factors are standardized, but how many decimal places a tool displays can vary.
Is a US gallon the same as an imperial (UK) gallon?
No — a US gallon (3.785 liters) is smaller than an imperial gallon (4.546 liters). This is a common source of recipe and fuel-economy conversion errors between US and UK sources.
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Page last reviewed: July 2026