Measurement systems: SI, US customary and imperial
Three systems, one set of exact factors between them, and a handful of units that share a name but not a size. Where the numbers on this page come from, and where a unit conversion stops being the right question.
The three systems in one paragraph each
SI, the International System of Units, is the metric system as maintained by the BIPM in Paris. Seven base units (second, metre, kilogram, ampere, kelvin, mole, candela) and a fixed set of prefixes from yocto to quetta. Since 2019 every base unit is defined by a constant of nature, so a kilogram is no longer a lump of platinum in a vault. Every conversion on UnitGlide passes through an SI base unit.
US customary units are the inch, foot, mile, pound, ounce, gallon and their relatives as used in the United States. They descend from English units but froze at an earlier point than the British ones, which is why the US kept the old wine gallon and the old ale-and-corn dry measures.
Imperial units are the British reform of 1824: the same names, one gallon for everything (defined then as the volume of 10 pounds of water), and pints and fluid ounces sized from that gallon. The UK, Ireland, Canada, Australia and New Zealand used it; most have since gone metric, with the UK keeping miles on roads and pints in pubs.
The exact factors that tie the systems together
The link between metric and the English-derived units is not a measurement, it is a definition. In 1959 the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa agreed on an international yard of 0.9144 metre and an international pound of 0.453 592 37 kilogram, both exact. Everything below follows from those two numbers and from the US definition of the gallon as 231 cubic inches. The values are read from the converter’s own unit registry, so this table cannot disagree with a result you get elsewhere on the site.
| Unit | Symbol | Exact value | Defined by |
|---|---|---|---|
| Inch | in | 0.0254 m | 1959 international yard and pound agreement |
| Foot | ft | 0.3048 m | 1959 (12 inches) |
| Mile | mi | 1609.344 m | 1959 (5,280 feet) |
| Pound | lb | 0.45359237 kg | 1959 international yard and pound agreement |
| Ounce | oz | 0.028349523125 kg | 1959 (1/16 pound) |
| Gallon | gal | 3.785411784 l | US gallon, 231 cubic inches |
| Fluid ounce | fl oz | 0.0295735295625 l | US fluid ounce, 1/128 gallon |
| Cup | cup | 0.2365882365 l | US customary cup, 8 fluid ounces |
| Tablespoon | tbsp | 0.01478676478125 l | US tablespoon, 1/2 fluid ounce |
| Teaspoon | tsp | 0.00492892159375 l | US teaspoon, 1/3 tablespoon |
Same name, different size
Length and mass are safe: a foot is a foot and a pound is a pound on both sides of the Atlantic. Volume is not. The 1824 imperial gallon and the 231-cubic-inch US gallon differ by about a fifth, and every unit cut from them (quart, pint, gill, fluid ounce) differs with them. Because the imperial pint has 20 fluid ounces and the US pint 16, the fluid ounces nearly meet again, with the US ounce slightly larger.
| Unit | US | Imperial or metric | Note |
|---|---|---|---|
| Gallon | 3.785 411 784 L | 4.546 09 L | The imperial gallon is about 20% larger. |
| Fluid ounce | 29.573 5 mL | 28.413 1 mL | Close, but not equal: the US ounce is the larger one here. |
| Pint | 473.176 mL | 568.261 mL | A US pint is 16 US fl oz, an imperial pint 20 imperial fl oz. |
| Quart | 946.353 mL | 1.136 52 L | Both are a quarter of their own gallon. |
| Cup | 236.588 mL | 250 mL (metric cup) | US nutrition labels use a legal cup of 240 mL; Australia and NZ use 250 mL. |
| Ton | 907.184 74 kg (short) | 1,016.046 908 8 kg (long) | The metric tonne, 1,000 kg, is a third unit with the same word. |
| Hundredweight | 45.359 237 kg | 50.802 345 44 kg | 100 pounds in the US, 112 pounds in the UK. |
Where a unit stops being enough
A unit converter answers one kind of question: the same quantity in a different unit. Three everyday questions look like that and are not, and each needs data that no factor table contains.
Cups to grams is a density question. A cup is a volume; grams are a mass; the bridge between them is the density of what is in the cup. On a 240 mL cup, water is 240 g, all-purpose flour 125 g and granulated sugar 200 g, and the spread is why a recipe converted with a single factor fails. The per-ingredient densities on cupsngrams.com are calibrated against USDA FoodData Central portion weights, which is the reference we would point a baker to.
Clothing sizes are not units at all. A US 9 shoe and an EU 42 are labels on scales with different zero points and different step sizes: EU sizes follow the Paris point of two thirds of a centimetre, US and UK sizes the barleycorn of a third of an inch, with a different starting point for men and women. The physical anchor is foot length in centimetres, which is how the US to EU shoe size table on mysizecharts.com is built, and it is the only way to convert between them honestly.
Running pace is an inverse speed. Minutes per kilometre and minutes per mile convert through the same 1.609 344 factor as any length, but in the other direction: a 5:00 min/km pace is 8:03 min/mile, not 3:06. The pace conversion table on paceslab.com lays the two columns side by side with the speed in km/h and mph for each row, which is easier than doing the division on a track.
Frequently asked questions
Is the imperial system the same as US customary?
No. Both grew out of English units, and length and mass are identical since the 1959 agreement fixed the yard and the pound for both. Volume is where they split: the imperial gallon, pint and fluid ounce, defined in the UK in 1824, are different sizes from the US ones, which kept the older wine gallon of 231 cubic inches.
Why is an inch exactly 25.4 mm?
Because six English-speaking countries agreed on it in 1959. Before that the US inch and the British inch differed by about two parts in a million. The agreement defined the international yard as 0.9144 metre exactly, which makes the inch 25.4 mm exactly and the foot 0.3048 m exactly.
Which cup does a recipe mean?
American recipes mean the US customary cup, 236.6 mL, though US nutrition labelling rounds it to a legal cup of 240 mL. Australian, New Zealand and many metric recipes mean a 250 mL cup. The difference is about 5%, which matters for flour and sugar and barely at all for a cup of stock.
What are the seven SI base units?
The second (time), metre (length), kilogram (mass), ampere (electric current), kelvin (temperature), mole (amount of substance) and candela (luminous intensity). Since 2019 all seven are defined from fixed constants of nature rather than physical artefacts.
Sources
- BIPM, The International System of Units (SI Brochure), 9th edition
- NIST Special Publication 811, Guide for the Use of the International System of Units
- NIST, US customary units and their SI equivalents (Handbook 44, Appendix C)
- UK Weights and Measures Act 1985, Schedule 1 (definitions of imperial units)
Reviewed end to end on 2026-09-04. How the converter applies these factors is on the methodology page; the citable tables live under reference data.