Unit converters online that are exact, and the ones that are not
Length, mass, area, volume, energy, pressure, speed, power, angle, data and more. Every factor here is traceable to a defined standard rather than a number someone remembered.
16 tools here, 150+ across the site
1 mile=1.609344 km
exact by definition, not rounded
All tools
Angle ConverterConvert degrees, radians, and gradiansUnit Converters
Area ConverterConvert square meters, feet, acres, hectares, and moreUnit Converters
Data Storage ConverterConvert bytes, KB, MB, GB, TB, PB accuratelyUnit Converters
Data Transfer Speed ConverterConvert Mbps, Gbps, MB/s, and moreUnit Converters
Energy ConverterConvert joules, kWh, calories, BTU, and moreUnit Converters
Frequency ConverterConvert Hz, kHz, MHz, GHzUnit Converters
Fuel Efficiency ConverterConvert between MPG, L/100km, and km/LUnit Converters
Length ConverterConvert between meters, miles, feet, inches, km, and moreUnit Converters
Pixel to REM / EM ConverterConvert pixel values to CSS rem or em unitsUnit Converters
Power ConverterConvert watts, kilowatts, horsepower, and moreUnit Converters
Pressure ConverterConvert Pascal, bar, PSI, atm, torrUnit Converters
Speed ConverterConvert km/h, mph, m/s, knots, and moreUnit Converters
Temperature ConverterConvert between Celsius, Fahrenheit, and KelvinUnit Converters
Torque ConverterConvert Nm, ft-lb, kgf-m for engineering useUnit Converters
Volume ConverterConvert liters, gallons, cups, ml, cubic feetUnit Converters
Weight ConverterConvert between kg, pounds, ounces, grams, stoneUnit Converters
Foundations
Seven definitions the whole system rests on
Since May 2019 every SI base unit is defined by a fixed constant of nature rather than a physical artefact. The last metal object was retired that year.
sSecond, from the caesium 133 hyperfine transition
mMetre, from the speed of light in vacuum
kgKilogram, from the Planck constant
AAmpere, from the elementary charge
KKelvin, from the Boltzmann constant
molMole, from the Avogadro constant
cdCandela, from luminous efficacy
Until 2019 the kilogram was a platinum iridium cylinder in a vault near Paris, and it was slowly losing mass relative to its own copies. Redefining it around the Planck constant means anyone with the right equipment can now realise a kilogram from first principles. The full definitions are published by the BIPM.
Precision
Rounding is where conversions quietly go wrong
Three habits cause most of the disagreement between two people converting the same number.
| Habit | What goes wrong | Better |
|---|---|---|
| Chaining conversions | Inches to feet to yards to metres compounds error at every hop | Convert once, straight to the target |
| Rounding early | Truncating a factor to 2.2 instead of 2.20462 shifts a 100 kg result by 46 grams | Round only the final displayed value |
| Assuming symmetry | Converting there and back does not always return the original because of floating point | Keep the source value, do not round trip |
Significant figures carry meaning. A room measured as 4 metres is not 13.1233595800525 feet. That output claims precision to a tenth of a micrometre from a measurement good to maybe a centimetre. Match your output precision to your input precision, otherwise the number lies about how well you know it.
Families
Which system you are actually working in
Most conversion confusion comes from two units sharing a name across different systems.
GallonThe US gallon is 3.785 litres, the imperial gallon is 4.546 litres. A 20 percent difference hiding behind one word, which is why fuel economy comparisons between the US and the UK need care.
TonMetric tonne is 1,000 kg. US short ton is 907 kg. UK long ton is 1,016 kg. Three different masses, one spelling that nearly matches.
Fluid ounceUS fluid ounce is 29.57 ml, imperial is 28.41 ml. The imperial pint has 20 of them, the US pint has 16.
CalorieThe small calorie is 4.184 joules. The kilocalorie, written Calorie on food packaging, is a thousand of those. Nutrition labels almost always mean the second.
Kilobyte1,000 bytes under SI, 1,024 under the binary prefix that should be called a kibibyte. Drive makers use the first, operating systems often use the second, which is why a 1 TB drive shows as 931 GB.
HorsepowerMechanical horsepower is 745.7 watts, metric horsepower is 735.5. Car specifications quote whichever flatters the market they are sold in.
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Other things to convert
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From the blog
Reading around units
Where measurement systems came from and why two of them still argue.
FAQ
Questions about unit converters online
On exactness, rounding and the units that share a name.
Are your conversion factors exact or rounded?
Most are exact by definition. Since 1959 the international yard is defined as exactly 0.9144 metres, which makes the inch exactly 25.4 mm and the mile exactly 1.609344 km. Temperature and a few engineering units involve genuine measurement, but the length, mass and volume factors carry no error at all.
Why does my answer differ slightly from another converter?
Usually rounding, sometimes unit choice. We convert once through the SI base unit rather than chaining several steps, which avoids accumulating floating point error. If the difference is large rather than tiny, check whether one tool is using a US gallon and the other an imperial one.
Which gallon does the volume converter use?
Both, listed separately. The US gallon is 3.785 litres and the imperial gallon is 4.546 litres, a 20 percent gap hiding behind one word. The same applies to fluid ounces and pints, which is why US and UK recipes are not interchangeable by volume.
Is a kilobyte 1,000 or 1,024 bytes?
Both definitions are in use, which is the whole problem. SI says 1,000 and the binary prefix kibibyte says 1,024. Drive manufacturers use the first, many operating systems display the second, and that is why a 1 TB drive shows up as roughly 931 GB. The converter handles both explicitly.
How many decimal places should I keep?
Match your input. A room measured as 4 metres is not meaningfully 13.1233595800525 feet, because that output claims precision to a fraction of a micrometre from a measurement good to maybe a centimetre. Significant figures carry information about how well you know the number.
Do you support Mach and the speed of light?
Yes, in the speed converter. Worth knowing that Mach is not a fixed speed, it is a ratio against the local speed of sound, which changes with temperature and altitude. Mach 1 at sea level is about 343 m/s and drops to roughly 295 m/s at cruising altitude.
Why are there two different horsepower figures?
Mechanical horsepower is 745.7 watts and metric horsepower is 735.5 watts, about a 1.4 percent difference. Car specifications quote whichever is standard in the market they are sold in, which is why the same engine can be advertised with two different numbers.
Is anything I convert uploaded?
No. It is arithmetic against a static factor table running in your browser. Nothing you type is transmitted, which is admittedly a little ironic for the bandwidth converter but accurate nonetheless.
Accuracy
Explore all tools
Why the factors here are exact
A converter is only as good as the constants inside it. Here is where these come from.
Defined, not measuredSince 1959 the international yard is defined as exactly 0.9144 metres. That makes the inch exactly 25.4 mm and the mile exactly 1.609344 km. These are definitions, so they carry no measurement error.
SI as the pivotEvery conversion routes through the SI base unit rather than chaining factor to factor. Chaining accumulates floating point error; pivoting once does not.
Nothing is uploadedAll arithmetic runs in your browser. No values are transmitted, logged or stored.