Unit Converter

about this tool

Exact by construction

Every factor in the table is a definition, not a rounded figure. The international inch is exactly 25.4 mm (since 1959), the avoirdupois pound exactly 0.45359237 kg, the US gallon exactly 231 cubic inches, the imperial gallon exactly 4.54609 litres, the acre exactly 4840 square yards, the standard atmosphere exactly 101 325 Pa, the mechanical horsepower exactly 550 foot-pounds per second, the electronvolt exactly 1.602176634 × 10⁻¹⁹ J since the 2019 SI. Each unit is stored as its size in the category's base unit, so converting is a single multiplication and division: nothing chains through a rounded intermediate, and a test round-trips every unit in every category.

Temperature is the exception, because its scales differ by an offset as well as a scale. It converts through kelvin with the textbook formulas, which are shown beside the result: 100 °C is 212 °F, −40 is the same in both, and 0 K is −273.15 °C.

What the numbers mean

Results are shown to four, six or ten significant figures, with trailing zeros trimmed, so 0.1 + 0.2 reads 0.3 rather than 0.30000000000000004, and exponent form appears only beyond 10¹⁵ or below 10⁻⁶. The factor to multiply by is printed under the result to ten figures, which is what you need when you are writing the conversion into code. Below that, the value is shown in every unit of the category at once.

The value field accepts commas as thousands separators, exponent notation, simple fractions (3/4), and feet-and-inches in either form (5'11", 5 ft 11 in), which is read as a length in inches whatever unit is selected, with a note saying so.

Choices worth knowing

The calorie is thermochemical (4.184 J); the BTU is the International Table one (1055.05585262 J). Mach is the speed of sound in standard sea-level air at 15 °C, a reference figure rather than a constant. The month is a twelfth of a Julian year and the year 365.25 days, which is what a duration conversion usually wants; a calendar year is the date tool's business. Torr and mmHg differ by a few parts in ten million and the tool uses the mmHg definition, noting the difference. The light- year uses the Julian year, as the IAU does.

Limits

No currency, no data sizes (a separate tool), no shoe sizes or other conventional scales, and no compound unit arithmetic (km/h to m/s is there as speed, but you cannot type 3 m × 4 m). Non-SI units are the US and imperial ones people actually meet; historical and regional units are not included.

Digital quantities are a separate problem, because KB and KiB differ — the data size & bandwidth converter handles those. For arithmetic on the results, the math expression evaluator.

questions

How accurate are the conversions?

Exact, as far as the definitions go. Since 1959 an inch is exactly 25.4 mm and a pound exactly 0.45359237 kg; a US gallon is exactly 231 cubic inches and an acre exactly 4840 square yards. Each unit is stored as its size in the base unit, so a conversion is one multiplication and one division, with no rounded steps in between. The precision control only changes how many figures are shown.

Why is a US gallon different from an imperial gallon?

They were defined differently and never reconciled: the US gallon is 231 cubic inches (3.785 litres), the imperial gallon 4.54609 litres, about a fifth larger. Pints, quarts and fluid ounces differ the same way, so the volume list names which system each one belongs to.

Which calorie is this?

The thermochemical calorie, exactly 4.184 J, and the kilocalorie is a thousand of them, which is the Calorie on food labels. The International Steam Table calorie is 4.1868 J, a difference of less than a tenth of a percent that matters only in thermodynamics.

Why does typing 5'11" convert from inches even though I chose another unit?

Because feet-and-inches is a number with its unit built in. The tool reads it as 71 inches and converts from there, and says so under the field. Type a plain number to use the unit you selected.

What is Mach here?

The speed of sound in standard air at sea level and 15 °C, 340.29 m/s. Real Mach numbers depend on the actual air temperature, so this is a reference figure rather than a fixed unit.