Unit converter, with exact defined factors
Convert length, weight, temperature, volume, speed and more.
1 mm = 0.1 cm
All unit conversion pages
Every pair has its own page with the formula, worked examples and a quick-reference table.
This converts across eight categories — length, weight, temperature, area, volume, speed, time and data — using the exact factors defined by international agreement. Most conversions are a single multiplication by a defined constant; temperature is the exception, because Celsius and Fahrenheit have different zero points as well as different degree sizes, so it needs an offset as well as a scale. The core constants are exact by definition rather than measured: one inch is precisely 25.4 mm, one pound is 0.45359237 kg, and °F = °C × 9/5 + 32. Two conventions are worth checking before trusting a result elsewhere. Volume here is US customary, so a gallon is 3.785 L rather than the UK's 4.546 L, and a cup is 236.6 ml. Data storage uses the binary convention where 1 GB is 1024 MB, matching what operating systems report but not what drive manufacturers print on the box.
About the Unit Converter
Convert between metric and imperial units across length, weight, temperature, area, volume, speed, time and data storage. Pick a category, type a value, and see the converted result instantly. All math runs in your browser using verified conversion factors.
How to use the Unit Converter
- 01Choose a category (Length, Weight, Temperature, and so on).
- 02Pick the unit you're converting from and the unit you want to convert to.
- 03Type the value you want to convert — the result updates as you type.
- 04Tap the swap button to flip the direction of the conversion.
Why use our Unit Converter
- 01
8 unit categories
Length, weight, temperature, area, volume, speed, time and data storage.
- 02
Verified factors
Uses the same constants as the international standards (1 in = 25.4 mm, 1 lb = 0.453592 kg).
- 03
Live results
No 'convert' button — the answer updates the moment you type.
- 04
Browser-only
All conversions happen locally. Nothing you type is uploaded or logged.
Are unit conversion factors exact?
It surprises people that an inch is exactly 2.54 centimetres, not approximately. Since the 1959 international yard and pound agreement, the 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, not measurements, so they carry no uncertainty and will not be revised. The metric base units themselves are now defined by physical constants — since 2019 the kilogram is defined via the Planck constant rather than by a platinum-iridium cylinder in Paris, and the metre by the speed of light. The practical upshot is that conversions between length and mass units are exact to as many digits as you care to write, and any imprecision in a result comes from rounding in the display rather than from the factor.
The two places where ambiguity is real
Volume and data storage both have genuinely competing definitions, and picking the wrong one produces errors far larger than any rounding. For volume, the 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 US and imperial pints, quarts and fluid ounces differ similarly. This converter uses US customary measures throughout, which is the right default for a US-focused audience but means a British recipe quoting pints needs care. For data, the conflict is between binary and decimal: 1 GB is 1024 MB under the binary convention that operating systems report, and 1000 MB under the decimal convention that drive manufacturers use for marketing. This converter uses binary, which matches what your computer tells you. That discrepancy compounds at each step and is exactly why a drive sold as 1 TB shows as 931 GB.
Precision, significant figures and not over-reporting
A converted value should not claim more precision than its input had. If someone tells you a room is 4 metres wide, they have given you one significant figure of implied precision, and reporting 13.1234 feet suggests a measurement accuracy nobody made. The convention is to carry full precision through intermediate steps and round only at the end, to roughly the same number of significant figures as the least precise input. This matters most in multi-step conversions, where rounding at each stage accumulates error — converting inches to centimetres to metres with rounding at each step drifts noticeably over a long chain. Temperature has an additional subtlety worth remembering: converting a temperature difference is not the same as converting a temperature. A rise of 10 °C is a rise of 18 °F, using only the 1.8 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.
Exact defined conversion factors
| Relationship | Exact value | Defined by |
|---|---|---|
| 1 inch | 25.4 mm | 1959 international agreement |
| 1 foot | 0.3048 m | 1959 international agreement |
| 1 yard | 0.9144 m | 1959 international agreement |
| 1 mile | 1609.344 m | Derived from the yard |
| 1 pound | 0.45359237 kg | 1959 international agreement |
| 1 US gallon | 3.785411784 L | US customary definition |
| 1 imperial gallon | 4.54609 L | UK Weights and Measures Act |
These are definitions rather than measurements, so they carry no uncertainty.
Frequently asked questions
Which gallon, cup and pint does this use?
US customary throughout — US gallon 3.78541 L, US cup 236.588 mL, US pint 473.176 mL. Imperial measures are roughly 20% larger for the gallon.
Is 1 GB 1024 MB or 1000 MB?
This converter uses the binary convention of 1024, which matches what operating systems report. Drive manufacturers use 1000, which is why a 1 TB drive shows as 931 GB.
How accurate are the temperature conversions?
Exact. They use the defined formulas °F = °C × 9/5 + 32 and K = °C + 273.15, so results are precise to the digits displayed.
Why is an inch exactly 2.54 cm?
Because the 1959 international yard and pound agreement defined it that way. Imperial units are now defined in terms of metric ones, so the factors are exact.
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.
How many significant figures should I keep?
Roughly as many as the least precise input. Reporting 13.1234 feet from a room described as '4 metres' implies an accuracy nobody measured.
Can I link to a specific conversion?
Yes. There are dedicated pages for the most common pairs, such as /units/km-to-miles, /units/kg-to-lbs and /units/celsius-to-fahrenheit.
Why do multi-step conversions drift?
Rounding at each intermediate step accumulates error. Carry full precision through and round only the final answer.
Is anything I type uploaded?
No. All conversions happen locally in your browser with no network request.
Last updated
Factors per the 1959 international yard and pound agreement, the BIPM SI Brochure (9th edition) and NIST Special Publication 811. Binary prefixes per IEC 80000-13.