Unit Converter

What is a unit converter?

A unit converter turns a number in one unit of measurement into the equivalent number in another, so you can turn 10 miles into kilometers, 98.6 degrees Fahrenheit into Celsius, or 500 grams into ounces without doing the arithmetic yourself. This one covers eight categories — Length, Mass, Volume, Temperature, Area, Speed, Data, and Time — spanning 67 units in total, from millimeters and metric tons to teaspoons, knots and gibibytes.

Pick a category, type a value, and the result appears instantly in large type next to a badge marked EXACT or ROUNDED, with a line underneath naming the conversion factor it used. Below the main result sits a table converting the same value into every other unit in that category at once, so you can see it in inches, feet, and meters side by side without reopening the tool. A copy button lets you grab the number you need without retyping it.

How to convert a unit

1. Choose a category. Use the eight buttons at the top — Length, Mass, Volume, Temperature, Area, Speed, Data, or Time — to load the units that matter for what you are converting.
2. Enter a value and pick your units. Type the number, choose the From unit and the To unit from the dropdowns, and hit the swap button if you picked them backward.
3. Read the result and the table. The converted value appears instantly with an EXACT or ROUNDED badge and the factor used, and the table underneath shows the same value in every other unit in that category; copy any figure with the copy button.

The traps: gallons, fluid ounces, and kilobytes

Two mistakes account for more bad conversions than everything else combined, and both come from unit names that sound interchangeable but are not.

The first is gallons. A US gallon is exactly 3.785411784 liters; an imperial gallon is exactly 4.54609 liters — about 20% larger, and a different unit rather than a rounding difference. The fluid ounces built on top of them are just as inconsistent: a US pint holds 16 US fluid ounces, but an imperial pint holds 20 imperial fluid ounces. A recipe or a fuel-economy figure carried across the Atlantic without noticing which gallon it started as is a genuine, common error, which is why this tool lists US and imperial gallons — and US and imperial fluid ounces — as separate units rather than one ambiguous gallon.

The second is storage. A kilobyte is 1000 bytes; a kibibyte is 1024 bytes. Worked out exactly, 1 terabyte is 931.322574615478515625 gibibytes, which is precisely why a drive sold as 1 TB shows up as roughly 931 GB in Windows. Nothing is missing and nobody is lying — a terabyte and a tebibyte are simply different units, and this converter keeps the two families separate rather than blending them.

Exact factors, not approximations

Almost every customary unit is not measured against the metric system — it is defined by it, by international agreement. The 1959 international yard and pound agreement fixed the yard at exactly 0.9144 meter and the avoirdupois pound at exactly 0.45359237 kilogram, and nearly everything else follows from those two numbers: an inch is exactly 25.4 millimeters, a mile is exactly 1609.344 meters, a US gallon is exactly 3.785411784 liters, an imperial gallon is exactly 4.54609 liters, and an international nautical mile is exactly 1852 meters.

This tool stores every one of those factors as an exact fraction and does the arithmetic in exact fractions too, rounding only once, when the answer is displayed — which is how it can honestly label each result EXACT or ROUNDED rather than rounding early and guessing. The badge is telling you something real: of the 632 non-temperature conversions between two different units, 421 — 66.6 percent — produce a terminating decimal. Imperial-to-metric conversions tend to terminate and metric-to-imperial ones tend not to, because the defining factors are built from powers of 2 and 5 in one direction only: 1 foot is exactly 0.3048 meter, but 1 meter is 3.28083989501... feet, forever.

The factors were checked against NIST Special Publication 811 Appendix B.9, which publishes standard conversion factors rounded to seven significant digits. Ten were checked directly — pound to kilogram, ounce to gram, short ton and long ton to kilogram, inch and mile to meter, US and imperial gallon to liter, and both fluid ounces to milliliter — and every one matched. Where NIST's published table shows 1 pound as 0.4535924 kilogram, this tool shows 0.45359237 kilogram exactly, because NIST's table is rounded for practical use while the underlying legal definition is not. Separately, all 641 ordered pairs of units were round-tripped at five different values with no loss, and conversions were checked to be transitive across 6,598 triples — converting A to B to C always lands on exactly the same answer as converting A to C directly.

Temperature does not play by these rules. Celsius, Fahrenheit, and kelvin have different zero points, so no single multiplying factor connects them — the conversion is affine, not proportional. That has a practical consequence: a temperature reading and a temperature difference convert differently. 10 degrees Celsius as a reading is 50 degrees Fahrenheit, but a rise of 10 degrees Celsius is a rise of only 18 degrees Fahrenheit. Minus 40 is the one point where the two scales agree, and absolute zero is exactly -459.67 degrees Fahrenheit.

Two more honest notes. The US survey foot is not offered here: NIST and the National Geodetic Survey retired it on December 31, 2022, and it differs from the ordinary international foot by about two parts per million — small enough, and official enough to have been formally withdrawn, that offering it would only invite mistakes. And JavaScript's own Intl API lists 45 units, but only for formatting numbers with labels — it does not convert between them, and its fluid-ounce unit never says whether it means the US ounce or the imperial one. The factor table behind this tool is its own.

Why is it free?

Every conversion in this tool runs on the arithmetic in your own browser. Nothing you type is uploaded anywhere, there is no account to create, and the result carries no watermark. That is true whether you are converting one value or scrolling through the full table of 67 units — the page costs nothing to run and nothing to use.