How to use the unit converter
- Choose a quantity, such as Length or Data size.
- Type a value and select the unit you are converting from.
- Select the unit to convert to; the result updates immediately.
- Use the swap button to reverse the direction, or scan the table of all units below.
How it works
Each unit is stored as an exact multiple of a base unit (metre, kilogram, square metre, cubic metre, metre per second, byte, second). A conversion is result = value × factor(from) ÷ factor(to). Factors come from their legal definitions: the 1959 international yard and pound (1 in = 0.0254 m, 1 lb = 0.45359237 kg), the US gallon of 231 cubic inches, the imperial gallon of 4.54609 litres, and the values tabulated in NIST Special Publication 811.
Temperature is not a simple multiple because the scales have different zero points, so it uses the exact formulas °F = °C × 9/5 + 32 and K = °C + 273.15. For data, SI prefixes are powers of 1000 (1 MB = 1,000,000 bytes) and IEC binary prefixes are powers of 1024 (1 MiB = 1,048,576 bytes).
Worked example
A drive sold as “1 TB” holds 1,000,000,000,000 bytes. Divided by 2³⁰ (1,073,741,824 bytes per GiB) that is about 931.32 GiB — which is why an operating system that counts in binary units shows less than the label says.
Limitations
- Results are shown to 10 significant digits; floating-point arithmetic can differ from exact values in the last digits.
- US survey units (survey foot, survey mile) are not included; they were retired by NIST at the end of 2022.
- Months and years use mean Gregorian lengths (30.436875 and 365.2425 days), not calendar months.
- Cooking units are US customary; Australian and metric cups or spoons are slightly different sizes.
Frequently asked questions
What is the difference between MB and MiB?
MB (megabyte) is 1,000,000 bytes in the SI system used by storage makers and network speeds. MiB (mebibyte) is 1,048,576 bytes, the binary unit many operating systems use even when they label it “MB”.
Are US and UK gallons the same?
No. A US gallon is 3.785411784 litres; an imperial (UK) gallon is 4.54609 litres, about 20% larger. Fluid ounces differ too.
How accurate are the conversions?
The factors are exact by definition, and calculations use double-precision arithmetic, which is accurate to about 15 significant digits. The display rounds to 10.
Why can't I enter −300 °C?
You can, but the tool warns you because it is below absolute zero (−273.15 °C), the lowest temperature physically possible.
Is a knot the same as a mile per hour?
No. A knot is one nautical mile (1852 m) per hour, about 1.151 mph or 1.852 km/h.