What this does
Converts a number between any two units in the same category. Pick a tab — Length, Weight, Temperature, Volume, Area, Speed, Data, or Time — choose a unit on each side, and type. The page opens on Length with 100 centimeters entered, shown as 39.370079 inches. Both boxes are live: edit either one and the other recomputes on every keystroke. The ⇄ button swaps the two units while keeping the number on the left, so 100 cm → in becomes 100 in → cm. Everything runs in your browser and keeps working offline.
Which units can I convert?
- Length: millimeters, centimeters, meters, kilometers, inches, feet, yards, miles, nautical miles.
- Weight: milligrams, grams, kilograms, metric tons, ounces, pounds, stone.
- Temperature: Celsius, Fahrenheit, Kelvin.
- Volume: milliliters, liters, cubic meters, and the US teaspoon, tablespoon, fluid ounce, cup, pint, quart, and gallon.
- Area: square millimeters to square kilometers, hectares, square inches, square feet, acres, square miles.
- Speed: meters per second, km/h, mph, feet per second, knots.
- Data: bytes, decimal KB/MB/GB/TB (powers of 1000), binary KiB/MiB/GiB/TiB (powers of 1024).
- Time: milliseconds, seconds, minutes, hours, days, weeks, 30-day months, 365-day years.
How do the conversions work?
Every non-temperature unit is stored as a multiplier to its category's base unit (meters, grams, liters, square meters, m/s, bytes, seconds), so a conversion is value × from-factor ÷ to-factor. The factors are the defined ones, not approximations: 1 inch = 0.0254 m, 1 pound = 453.59237 g, 1 US gallon = 3.785411784 L, 1 knot = 1.852 km/h. Temperature can't use a multiplier because the scales have different zero points, so it routes through Celsius: °F = °C × 9/5 + 32 and K = °C + 273.15.
How precise are the results?
Results are rounded to six decimal places with trailing zeros dropped; anything at or above 10¹⁵ or below 10⁻⁶ switches to exponential notation. Worked example: 100 cm ÷ 2.54 = 39.37007874… inches, shown as 39.370079. Switch the right-hand unit to feet and the same 100 cm reads 3.28084 — the exact value is 3.280839895…, and the sixth decimal rounds to a zero that is then trimmed.
What's the difference between MB and MiB?
A megabyte is 1,000,000 bytes; a mebibyte is 1,048,576 bytes (1024²). Drives are sold in decimal units and operating systems often report binary ones, which is why a "1 TB" disk shows up as about 931 GiB — enter 1 terabyte to gibibytes on the Data tab and you get 931.322575. Each unit is labelled with its base so the two families can't be confused.
Why are cups and gallons marked "US"?
Because imperial and US customary volumes disagree. A US cup is 236.588 mL and a US gallon 3.785 L; the UK imperial pint is 568 mL and the imperial gallon 4.546 L. The Volume tab uses US definitions throughout, which is what American recipes and fuel prices mean. Reading a British recipe, treat "pint" as 568 mL rather than converting it here.
Is there a page for just one pair?
Yes — the most-searched pairs have preset pages with the dropdowns already chosen and a pair-specific explainer: cm to inches, inches to cm, km to miles, miles to km, meters to feet, feet to meters, kg to lbs, lbs to kg, Celsius to Fahrenheit, and Fahrenheit to Celsius. They hide the category tabs but still offer every unit in their category.