Conversion Calculator

Use this conversion calculator to convert between commonly used units of length, temperature, area, volume and weight. Pick the unit you have on the left, the unit you want on the right, and type a value into either box; typing on either side converts towards the other.

The factors behind it are exact definitions wherever one exists. The inch has been exactly 25.4 millimetres since 1959, the avoirdupois pound exactly 0.45359237 kilograms, and the light year exactly 9,460,730,472,580,800 metres. Nothing here is a rounded approximation standing in for a definition.

Different systems of units

A system of units is a collection of units of measurement together with rules relating them to each other, and a unit of measurement is a defined magnitude used as a standard for the same kind of quantity: length, weight, volume and so on.

Historically many such systems existed. Most were defined locally and could rest on something as arbitrary as the length of a king's thumb. That works within a village and falls apart the moment two regions trade or two scientists compare results, which is why more universal and consistent systems developed over time. The ones still in use are the metric system, the imperial system and the United States customary units.

The International System of Units, SI, is the standard metric system in use today. It is built on seven base units, and since a redefinition that took effect in 2019 every one of them is fixed to a constant of nature rather than to a physical object. The kilogram was the last to go: until 2019 it was defined by a platinum-iridium cylinder held near Paris, an artefact that could and did drift.

Quantity	Base unit	Symbol	Defined from
Length	metre	m	the speed of light
Mass	kilogram	kg	the Planck constant
Time	second	s	the caesium-133 hyperfine transition
Temperature	kelvin	K	the Boltzmann constant
Electric current	ampere	A	the elementary charge
Luminous intensity	candela	cd	the luminous efficacy of 540 THz radiation
Amount of substance	mole	mol	the Avogadro constant

SI is used almost universally in science, including in the United States. Some countries nonetheless keep their own everyday system, largely because changing one carries substantial financial and cultural cost against a benefit that mostly accrues elsewhere. US customary units are deeply entrenched in daily American life and SI already covers the applications where standardisation genuinely matters, so everyday USC is unlikely to disappear. Which is why converters like this one exist and will continue to.

US customary and imperial are not the same thing

This catches people constantly, and it is the single most common source of a wrong answer in a volume conversion. Both systems use words like gallon, quart, pint and fluid ounce, and the quantities behind them differ.

Unit	US customary	Imperial
Gallon	3.785411784 L	4.54609 L	imperial is 20% larger
Quart	0.946352946 L	1.1365225 L	imperial is 20% larger
Pint	473.176473 mL	568.26125 mL	imperial is 20% larger
Fluid ounce	29.5735295625 mL	28.4130625 mL	imperial is 4% smaller

The imperial gallon is about 20% larger than the US one, yet the imperial fluid ounce is about 4% smaller. Both are true at once because a US gallon holds 128 fluid ounces and an imperial gallon holds 160. A recipe calling for a pint of milk means 473 mL in Boston and 568 mL in Birmingham, which is enough to matter.

The calculator lists the two families separately for this reason. Pick the one your source actually used rather than the one that shares your spelling.

History of the pound

In the eighth and ninth centuries CE, Arab civilisation flourished across the Middle East and Spain. Coins served as a measure of weight, since a minted coin could not easily be cut or shaved down without the loss showing, which made it a dependable standard. The silver dirhem was the basic unit, weighing roughly what 45 fully grown grains of barley weighed. Ten dirhems made a Wukryeh, translated into Latin as an uncia, which is where the word ounce comes from.

As trade spread from the Mediterranean into Europe, including the northern German city states, a pound of 16 ounces of silver, or 7,200 grains, became a common measure across many regions.

England adopted it too, but a shortage of silver led King Offa to cut the pound to 5,400 grains so that smaller coins could be struck. When William the Conqueror took the throne he kept the 5,400-grain pound for minting and reverted to the 7,200-grain pound for everything else.

The avoirdupois system arrived during the reign of Queen Elizabeth in the sixteenth century. It was built around the weight of coal, and its name comes from the French avoir de pois, goods of weight. An avoirdupois pound is 7,000 grains, made up of 256 drams of 27.344 grains or 16 ounces of 437.5 grains. Since 1959 it has been defined in most English-speaking countries as exactly 0.45359237 kilograms, which is the figure this calculator uses.

Measurement systems developed independently elsewhere. In ancient India a weight called the Satamana equalled the weight of 100 gunja berries. In China, Shi Huang Di, the first emperor, established a system of weights and measures in the third century BCE, with weight based on the shi, roughly 132 pounds. The Chi and the Zhang were lengths of about 25 centimetres and 3 metres, both close to 9.8 inches and 9.8 feet. The Chinese also checked accuracy acoustically, using a bowl of a specific size that rang at a particular pitch when struck; if the note was off, so was the measure.

A short history of the metric system

In 1668 John Wilkins proposed a decimal system linking length, area, volume and mass to each other, using as its base length a pendulum with a beat of one second. In 1670 Gabriel Mouton proposed a decimal system based instead on the circumference of the Earth, an idea supported by prominent scientists of the day including Jean Picard and Christiaan Huygens. Neither took hold for roughly another century.

By the middle of the eighteenth century it was clear to nations that traded and exchanged scientific work that standardisation was needed. In 1790 Charles Maurice de Talleyrand-Périgord approached the British, represented by John Riggs-Miller, and the Americans, represented by Thomas Jefferson, with a proposal to define a common standard of length based on a pendulum. That same year Jefferson presented his Plan for Establishing Uniformity in the Coinage, Weights, and Measures of the United States, advocating a decimal system with units related by powers of ten. A committee of leading French scientists reached a similar conclusion.

None of the three efforts landed together. Congress considered Jefferson's report and did not adopt it. Riggs-Miller lost his parliamentary seat in the 1790 election. So the system was implemented only in France, formally defined in French law in 1795 and officially adopted in 1799, though still not universally observed across the country.

It spread slowly. Areas annexed by France during Napoleon's reign were the first to take it up. By 1875 two thirds of the European population and nearly half the world's population had adopted the metric system. By 1920, roughly 22% of the world's population used the imperial or US customary system, 25% used mainly metric, and 53% used neither.

The International System of Units was published in 1960 and is now the most widely used system of measurement in the world. Every developed country has adopted it except the United States, and even there it is standard in science and used heavily by the military.

Where conversions go wrong

The first trap is weight against mass. A kilogram is a mass and a pound is a weight, and the conversion here treats them as equivalent because that is what everyone means in practice. On the Moon it would not hold.

The second is the US and imperial volume families described above. If a source does not say which it used, the size of the container usually gives it away.

The third is treating temperature like a scale factor. Celsius, Fahrenheit and Kelvin have different zero points as well as different degree sizes, so a temperature converts through an offset and cannot be handled by multiplication. Doubling 20 °C does not give twice the temperature in any physical sense. Kelvin is the exception, being an absolute scale.

The fourth is carrying more digits than the input deserves. The calculator returns twelve significant figures because the definitions support it, but a measurement of "about 6 feet" does not become more precise by converting to 1.8288 metres.