Length, Weight, Volume and Speed Converter

How the conversion is done

Each category has one base unit: meters for length, kilograms for weight, liters for volume, bytes for storage. Every unit is stored as a single number, its size in that base. Converting is then one multiply and one divide, and the rounding happens once, at the last moment, on the way to the screen.

That matters more than it sounds. Converting kilometers to feet by going through miles and rounding at each hop compounds the error. Going through a base unit means there is only ever one rounding step, no matter which pair you pick.

Imperial units are defined in metric, exactly

An inch is 2.54 centimeters. Not roughly — exactly, by the international yard and pound agreement of 1959, which redefined the inch, foot, yard, mile, ounce and pound in terms of the meter and the kilogram. A pound is exactly 0.45359237 kg. A mile is exactly 1609.344 m. So metric-to-imperial conversions are not measurements with error bars, they are definitions.

The one survivor of the old system was the US survey foot, very slightly longer and used in land surveying. It was officially retired at the end of 2022. If you are reading coordinates off a pre-2023 US survey, that is the difference you may be chasing.

US and imperial volumes are different units with the same names

A US gallon is 3.785 liters. An imperial gallon is 4.546 liters, about 20% bigger. Both are on the list above, labelled, because a recipe or a fuel figure that does not say which one it means is genuinely ambiguous.

Ounces are stranger. An imperial fluid ounce is 28.41 ml, smaller than the US one at 29.57 ml. But an imperial pint holds 20 of them against the US pint's 16, so the imperial pint still comes out larger — 568 ml against 473 ml. This is why a pint of beer changes size when you cross the Atlantic.

The cup is the worst unit in the kitchen

This tool uses the US customary cup, 236.588 ml, which is 8 US fluid ounces. But US nutrition labels use a legal cup of exactly 240 ml, an Australian metric cup is 250 ml, and older British recipes use a cup of 284 ml. There is no single correct answer, only the one the recipe author meant.

Worse, a cup measures volume and flour is sold by weight. Depending on whether it was scooped or spooned, a cup of flour ranges from roughly 120 to 150 grams. If a baking recipe matters to you, weigh the ingredient — no converter can fix a measurement that was imprecise at the source.

Two kinds of ounce, and mass versus weight

The ounce here is the avoirdupois ounce, 28.3495 grams, which is what food, post and body weight use. Precious metals use the troy ounce, 31.1035 grams — about 10% heavier. If you are converting a gold price, this tool will give you the wrong number.

Strictly, kilograms measure mass and pounds measure force. Everyone treats them as interchangeable because everyone is standing on Earth, and so does this tool. On the Moon the conversion would be wrong, which is not a limitation most people will hit.

Why your 1 TB drive shows as 931 GB

Drive manufacturers use decimal prefixes: 1 TB means 1,000,000,000,000 bytes. Windows divides by 1024 three times and still writes the result as "GB", giving 931. Both numbers describe the same drive. The unambiguous binary units are the kibibyte, mebibyte and gibibyte, all listed separately above, and macOS has reported decimal sizes since Snow Leopard.

What this tool will not do

• Temperature. Celsius and Fahrenheit have different zero points as well as different degree sizes, so the conversion needs an offset, not just a factor. That breaks the model this tool is built on, which is why it lives in the separate temperature converter.
• Fuel economy. Miles per gallon and liters per 100 km are inverses of each other, not multiples. Doubling the mpg halves the liters figure. A factor table cannot express that.
• Currency. Rates change by the second and would need a server. Nothing on this site talks to one.
• Unlimited precision. Arithmetic uses JavaScript numbers, which carry about 15 to 17 significant decimal digits. Twelve are shown. Good enough for cooking, shipping and homework; not a substitute for a certified calculation in engineering or trade.

One parsing quirk worth knowing: a comma between groups of three digits is read as a thousands separator, so 1,500 means one thousand five hundred. A comma with any other digit pattern, like 1,5, is read as a decimal point.

Frequently asked questions

How many centimeters are in an inch?

Exactly 2.54. This has been a definition rather than a measurement since 1959, when the inch was formally fixed in terms of the meter. Going the other way, one centimeter is 0.3937007874 inches, which does not come out round.

Is a US gallon the same as a UK gallon?

No. A US gallon is 3.785 liters and an imperial gallon is 4.546 liters, roughly 20% bigger. Both are listed separately in the volume category. Fuel economy figures quoted in mpg are almost always wrong by that 20% if you assume the wrong one.

Why does my 1 TB hard drive show up as 931 GB?

The manufacturer counted 1,000,000,000,000 bytes. Windows divides by 1024 three times and labels the result GB, which gives 931. Nothing is missing. Convert terabytes to tebibytes above and you get the same 0.909 figure from the other direction.

Can it convert temperatures?

No, and that is deliberate. Temperature scales have different zero points, so converting needs an addition as well as a multiplication, which does not fit a factor table. Use the temperature converter instead.

How accurate are the results?

The factors are exact where an exact definition exists, which covers all the metric and imperial units here. The arithmetic uses standard floating point, so the last of the twelve digits shown can be off by one. Do not use it as the final word on a legal or engineering measurement.

Last updated September 19, 2026