Convert units exactly, with the source of every factor
The converter works between 268 units of 24 quantities, from length, mass and volume to pressure, energy and power, up to fuel consumption, data and typography. It calculates with exact fractions instead of floating point numbers, understands input such as “3.5 t in lb” or “5 ft 3 in in cm” and shows the legal definition of every unit.
What the converter does
- Exact arithmetic
- Every factor is stored as a fraction, the way it is defined: 1 in = 0.0254 m, 1 lb = 0.45359237 kg, 1 gal (US) = 231 in³. Only the display is rounded. 0.1 + 0.2 gives 0.3, and a conversion there and back returns exactly the value you started with.
- Natural input
- Decimal point or comma, Swiss digit grouping (1'234.5), fractions (1/4 in, 1 1/2 in), compound values (5 ft 3 in, 2 h 30 min), sums and products such as 2 m × 3 m, and unit names in English, German, French, Italian and Spanish.
- Two readings
- When an input can mean two things, such as 1,234 (thousands or a decimal comma), gal (US or UK) or °C as a temperature or a temperature difference, the converter shows the reading it chose and offers the other one with a click.
- All units at once
- Below the result you see every unit of the quantity. A click makes a unit the target, favourites can be pinned. A logarithmic scale shows where the value sits among the units.
- Exact view
- The result with all its digits, as a repeating decimal, as a fraction or as a multiple of π, with the definition of each unit involved and its source.
- Old Swiss measures
- Fuss (30 cm), Zoll, Linie, Elle (60 cm), Klafter (1.8 m), Juchart (36 a), Mass (1.5 l), Pfund (500 g) and Zentner as set by the concordat of 1838, in force until the metric system of 1877.
Definitions and formulas
1 in = 0.0254 m · 1 ft = 12 inInternational inch and foot since 19591 lb = 0.45359237 kgInternational pound; 1 oz = 1/16 lb, 1 st = 14 lb1 gal (US) = 231 in³ = 3.785411784 lUS gallon after NIST Handbook 441 gal (UK) = 4.54609 lImperial gallon after the Weights and Measures Act 1985°F = 1.8 × °C + 32Temperature; as a difference, 1 K = 1.8 °Fmpg (US) = 235.214583 / (l/100 km)Consumption and economy are reciprocals1 BTU = 1055.05585262 JInternational Table BTU, from 1 cal = 4.1868 J1 PS = 75 kgf · m/s = 735.49875 WMetric horsepower; 1 hp = 550 ft · lbf/s
Example: 3.5 t in pounds
Input “3.5 t in lb”, decimal point, 10 digits:
| Result | 7716.179176 lb |
|---|---|
| In pounds and ounces | 7716 lb 2.8668 oz |
| Factor | 1 t ≈ 2204.622622 lb |
| The other way | 1 lb = 0.00045359237 t |
| Exact | 50,000,000,000/6,479,891 lb |
The exact value is a fraction because 1 lb is defined as 7000 grains of 64.79891 mg; its decimal expansion never ends. For 72 °F the converter shows 22.22222222 °C, exactly 200/9 °C.
Frequently asked questions about the unit converter
How exact is the converter?
It calculates with exact fractions. Only the display is rounded, to 10 significant digits by default, adjustable from 6 to 15. A value that ends within 12 digits is shown in full. A ≈ in front of the number marks every rounding.
Where do the factors come from?
From the definitions: the BIPM SI Brochure, NIST Special Publication 811 and Handbook 44, the UK Weights and Measures Act 1985, IEC 80000-13 for bits and bytes, and the Historical Dictionary of Switzerland for the old Swiss measures. Every unit names its source.
What is the difference between kB and KiB?
1 kB is 1000 bytes (SI prefix), 1 KiB is 1024 bytes (IEC binary prefix). For gigabytes the difference is already 7.4 %: 1 GB is 0.931 GiB.
Is a gallon the same in the US and the UK?
No. The US gallon holds 3.785 l, the imperial gallon 4.546 l. For “gal” the converter takes the US gallon and offers the British one as a second reading; “gal (UK)” selects it directly. The same applies to pint, quart, fluid ounce and mpg.
Can I share a conversion?
Yes. The input is part of the page address, and “Link” copies it. Pinned units and the recent list stay in the browser on this device.