Overview
What this tool does
A unit converter turns a value in one unit into the equivalent in another. This converter covers eight of the most-needed categories: Length (millimeter to light-year), Mass (milligram to long ton), Volume (milliliter to UK gallon, both US and imperial), Temperature (Celsius, Fahrenheit, Kelvin, Rankine), Area (square millimeter to square mile, plus acres and hectares), Data (binary KiB/MiB/GiB AND SI kB/MB/GB so you can spot the difference disk manufacturers use), Energy (Joule to kWh to BTU to electron-volt), and Time (millisecond to year). Pick a category, type a value, pick from and to. Conversions are exact for everything except temperature, which uses the standard linear formulas. A second pane shows the same source value converted into the other common units in that category, so you can spot-check at a glance. 100% client-side, no signup.
How to
Use it in 3 steps
- Pick a category from the top row: Length, Mass, Volume, Temperature, Area, Data, Energy, or Time.
- Enter your value in the Value field.
- Pick the source unit in 'From' and the destination unit in 'To'. The Result updates as you type.
- Use the swap arrow between the columns to flip direction without re-entering anything.
- Below the main converter, a 'common conversions' panel shows the same source value in several other units of the same category for quick comparison.
FAQ
Frequently asked questions
▶Why are there both binary (KiB/MiB) and decimal (kB/MB) units in the Data category?
Because the same shorthand 'GB' means different things depending on context. RAM, file sizes inside operating systems, and binary-prefix usage = 1024-based (KiB, MiB, GiB). Disk manufacturers, network speeds, and SI usage = 1000-based (kB, MB, GB). A 1 TB hard drive holds about 931 GiB once formatted, not 1024. We include both so you can be precise.
▶Is the temperature conversion exact?
Yes. Celsius ↔ Kelvin is a simple offset, Celsius ↔ Fahrenheit uses the standard 9/5 ratio, Kelvin ↔ Rankine is a simple ratio. There's no rounding loss except in the final display, which shows up to ten significant digits.
▶Why does a 'year' equal 365.25 days here?
Because we use the Julian year (365.25 days), which is the standard astronomical and SI convention. It accounts for leap years on average. Real calendar years are 365 or 366; this matters only for very precise time-of-flight or astronomical calculations.
▶Where do volume gallons / pints come from?
US and UK pint/gallon/quart are different sizes (US gallon = 3.785 L, UK gallon = 4.546 L). We label them explicitly so you don't accidentally use the wrong one.
▶Is my data sent anywhere?
No. Conversions are pure JavaScript that run in your browser. Nothing leaves your device.
▶Can I add custom units?
Not yet. We may add a 'custom factor' input in a future revision. If you need a niche unit (slugs, furlongs per fortnight, etc.) and can give us the conversion factor to SI base, we can add it.