Proportional conversion through the base unit
y = x × f(A) ÷ f(B)
f is each unit’s factor to the base unit; this covers the vast majority of units.
Universal Unit Converter
Universal unit converter covering 28 quantities (length, area, volume, mass, temperature, speed, pressure, energy, power, torque, flow, fuel economy, digital storage, electricity, amount of substance) with exact factors and clear explanations of affine and inverse conversions.
Choose a quantity, then enter a value in any field: every other value updates instantly.
Enter a value in any row to convert instantly.
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How does this tool work?
This universal converter brings everyday quantities (length, area, volume, weight, temperature, speed, time), physics and mechanics (pressure, energy, power, force, torque, flow, density), chemistry (moles, molar concentration), electricity and digital units together on one page. Pick a quantity, type a value in any row, and every other unit on screen updates. Use it to read a foreign spec sheet, a European recipe, an energy bill, a tire placard or the real capacity of a drive.
Each quantity has a base unit, usually the SI unit: the meter for length, the cubic meter for volume, the kilogram for mass, the pascal for pressure, the joule for energy, the watt for power. Every unit is stored as a factor to that base: 1 foot = 0.3048 m, 1 pound = 0.45359237 kg, 1 psi = 6,894.757 Pa. Converting from A to B means multiplying by A’s factor and dividing by B’s, so no table of pair-by-pair coefficients is needed.
Most of these factors are exact by definition: the inch has been 2.54 cm and the pound 0.45359237 kg since the 1959 international yard and pound agreement, the US gallon is 3.785411784 L, the imperial gallon 4.54609 L, the standard atmosphere 101,325 Pa and the metric horsepower 735.49875 W. A few are rounded to about ten significant digits (BTU, mmHg, mechanical horsepower), far beyond any practical need.
Two families do not follow simple multiplication. Temperatures are affine because their zero points differ: °F = °C × 9/5 + 32 and K = °C + 273.15. A temperature difference of 10 °C, however, is 18 °F, without the 32 offset. Fuel economy is inverse: L/100 km measures volume per distance while km/L and mpg measure distance per volume, so mpg (US) = 235.215 ÷ (L/100 km). Halving consumption doubles the mpg figure.
Several units share a name but not a value. The imperial gallon (4.546 L) is 20% larger than the US gallon (3.785 L), and UK mpg figures are 20% higher for the same car. The metric ton or tonne (1,000 kg) is not the US short ton (2,000 lb = 907.18 kg). A food Calorie is a kilocalorie, worth 4.184 kJ. Mechanical horsepower (745.7 W) is about 1.4% more than metric horsepower, PS or CV (735.5 W). And 1 MB is 1,000,000 bytes while 1 MiB is 1,048,576 bytes (IEC binary prefixes).
How to use this tool
Pick the quantity
Categories are grouped into common conversions, physics and mechanics, electricity and digital; each quantity opens with its most-used units already listed.
Type a value in your starting row
The row you edit becomes the reference. Commas and decimal points are both accepted, and the other rows are recalculated at full precision with no intermediate rounding.
Adjust the units shown
Change a row’s unit with its selector, add a row to compare, say, US and imperial gallons side by side, or remove the ones you don’t need.
The three kinds of conversion
A universal converter combines proportional, affine and inverse conversions.
Affine conversion (temperature)
°F = °C × 9/5 + 32
The constant term comes from the different zero points of the two scales.
Inverse conversion (fuel economy)
mpg US = 235.215 ÷ (L/100 km)
235.215 = 100 × 3.785411784 ÷ 1.609344; with the imperial gallon the constant becomes 282.481.
- x, y
- starting value and converted value
- f(A), f(B)
- factors of units A and B to the base unit (for example 0.3048 for the foot, in meters)
- C
- fuel consumption in liters per 100 kilometers
Example: 12 psi in bar = 12 × 6,894.757 ÷ 100,000 = 0.827 bar; 30 °C = 30 × 9/5 + 32 = 86 °F; 5 L/100 km = 235.215 ÷ 5 = 47.0 mpg US.
Use cases
Foreign spec sheets
Compare torque in N·m with lb·ft, pressure in bar with psi, power in kW with hp, or flow in L/min with gallons per minute across manufacturers.
Cooking and travel
Switch between grams and ounces, liters and quarts or gallons, °C and °F for an oven or a weather forecast abroad.
School and lab work
Check a conversion from mmol/L to mol/m³, kgf to newtons or degrees to radians in an assignment or protocol.
IT and networking
Understand the gap between advertised and displayed drive capacity, or turn a Mbit/s connection speed into MB/s download speed.
Examples
Filling up in Europe
40 ÷ 3.785411784 = 10.567 gal. The same 40 liters are only 8.80 imperial gallons.
Input
40 LResult
10.57 US gallonsCar fuel economy
100 ÷ 6.5 = 15.385 km/L, then ÷ 0.4251437 = 36.19 mpg US. The same car looks more frugal in British figures.
Input
6.5 L/100 kmResult
36.19 mpg US (43.46 mpg UK)Engine power
150 × 735.49875 W = 110,325 W; divided by 745.6999 W, that is 147.95 mechanical horsepower.
Input
150 PSResult
110.32 kW, or 147.95 hpSSD capacity
500 × 10⁹ ÷ 2³⁰ = 465.66 GiB, which Windows labels “GB”.
Input
500 GBResult
465.66 GiBTire pressure
35 × 6,894.757 Pa = 241,317 Pa = 2.41 bar, the value on a European gauge.
Input
35 psiResult
2.41 barOven temperature
(350 − 32) × 5/9 = 176.67 °C; recipes usually round it to 180 °C.
Input
350 °FResult
176.67 °CTips and best practices
- For a single pair you use often, the dedicated converters (km to miles, kg to lbs, °C to °F, MB to MiB) add a reference table and focused explanations.
- Always check where a gallon, pint or mpg comes from: an American text means US units, a British one usually means imperial units.
- To convert a temperature difference, multiply by 9/5 (or 5/9) only: the 32 offset applies to absolute temperatures, not to changes.
- Divide a speed in Mbit/s by 8 to get MB/s: a 100 Mbit/s connection downloads at most 12.5 MB/s.
- Add a row to compare two look-alike units, such as US mpg and UK mpg, instead of redoing the conversion.
Sources and references
- NIST, Guide for the Use of the International System of Units (SP 811), Appendix B.8: Factors for Units Listed Alphabetically (opens in a new tab)Page in English
- BIPM, The International System of Units (SI Brochure) (opens in a new tab), Bureau international des poids et mesuresPage in English
- NIST, Prefixes for binary multiples (opens in a new tab)Page in English
Frequently asked questions
How does a unit converter work?
It turns the value into a base unit (usually SI), then converts it into each target unit. Example: 3 ft × 0.3048 = 0.9144 m, then 0.9144 m ÷ 0.0254 = 36 inches.
What is the difference between a US and an imperial gallon?
A US gallon is 3.785411784 L, an imperial gallon 4.54609 L, about 20% more. That is why UK mpg figures are 1.2 times US mpg figures for the same car.
How do I convert L/100 km to mpg?
Divide 235.215 by the consumption for US mpg, or 282.481 for UK mpg. Example: 8 L/100 km = 29.4 mpg US = 35.3 mpg UK.
Why does my 1 TB drive show 931 GB?
Manufacturers count in powers of 10 (1 TB = 10¹² bytes), operating systems often in powers of 2: 10¹² ÷ 2³⁰ = 931.32 GiB, displayed as “GB” by Windows.
How many kJ are in a calorie?
A kilocalorie (the food Calorie) is exactly 4.184 kJ, so a 600 kcal meal is 2,510 kJ. A small calorie is 4.184 J.
Is hp the same as PS?
Almost: metric horsepower (PS, CV, ch) is 735.49875 W, mechanical horsepower 745.7 W. 100 kW is 135.96 PS but 134.10 hp.
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