Unit Conversion Tools
Pre-set converters for the most common unit pairs — each with the formula, a quick-reference table and a working converter. Or use the full Unit Converter for any pair.
Length
Weight / Mass
Temperature
Volume
Speed
Data Storage
Are these conversion factors exact?
It surprises people that an inch is exactly 2.54 centimetres rather than approximately. Since the 1959 international yard and pound agreement, imperial units have been defined in terms of metric ones: the yard is exactly 0.9144 m, which fixes the foot at 0.3048 m and the inch at 25.4 mm, and the avoirdupois pound is exactly 0.45359237 kg. These are definitions rather than measurements, so they carry no uncertainty and will not be revised. The metric base units are themselves now defined by physical constants — since 2019 the kilogram via the Planck constant, the metre via the speed of light. The practical upshot is that any imprecision in a converted result comes from rounding in the display, never from the factor. Where a conversion genuinely is approximate, such as an average month length, the relevant page says so.
The two places where the answer genuinely depends
Volume and data storage both have competing definitions, and choosing the wrong one produces errors far larger than any rounding. For volume, US and imperial systems share names and not sizes. A US gallon is 3.785 L while an imperial gallon is 4.546 L — a 20 percent difference — and pints, quarts and fluid ounces differ similarly. These pages use US customary measures, so a British recipe quoting pints needs care. For data, the conflict is between binary and decimal. One gigabyte is 1024 megabytes under the binary convention that operating systems report, and 1000 megabytes under the decimal convention that drive manufacturers use in marketing. These pages use binary, which matches what your computer tells you. The discrepancy compounds at each step, which is exactly why a drive sold as 1 TB shows as 931 GB.
Converting differences, and keeping precision honest
Temperature has a subtlety that catches people out. Converting a temperature and converting a temperature difference are different operations. A rise of 10 °C is a rise of 18 °F, using only the 1.8 scale factor with no offset — whereas 10 °C as an absolute temperature is 50 °F. Applying the offset to a difference is a common and significant error. On precision, a converted value should not claim more accuracy than its input had. If a room is described as 4 metres wide, that is one significant figure of implied precision, and reporting 13.1234 feet suggests a measurement nobody made. Carry full precision through intermediate steps and round only at the end, to roughly the significant figures of the least precise input. That last point matters most in multi-step conversions, where rounding at each stage accumulates error across a long chain.
Frequently asked questions
Are these converters accurate?
- Yes. Length, mass and temperature use the exact factors fixed by international agreement, so results are precise to the digits displayed rather than approximated.
Do I need an account?
- No. There is no sign-up, no limit and no tracking. Every conversion runs in your browser and nothing you type is transmitted.
Which gallon and pint do these use?
- US customary. A US gallon is 3.785 L against 4.546 L for the imperial gallon — roughly 20% larger — so check which system your source uses.
Is 1 GB 1000 MB or 1024 MB?
- These pages use 1024, the binary convention operating systems report. Drive manufacturers use 1000, which is why a 1 TB drive displays as 931 GB.
How do I convert a temperature difference?
- Use only the scale factor and drop the offset. A rise of 10 °C is a rise of 18 °F, whereas 10 °C as a temperature is 50 °F.
Why is an inch exactly 2.54 cm?
- The 1959 international yard and pound agreement defined it that way. Imperial units are defined in terms of metric ones, so the factor is exact.
How many decimal places should I keep?
- Roughly as many significant figures as your least precise input. Reporting six decimals from a rough measurement implies accuracy that was never there.