Imperial units are the system of measurement Britain standardized in the Weights and Measures Act of 1824: inches, feet, yards, miles, ounces, pounds, stones, pints, gallons and their relatives. The act replaced a mess of local and trade-specific measures with one set of national standards, and that set spread across the British Empire.
Today the system survives in everyday use in the UK, in modified form in the US, and in patches across Canada and a handful of other countries. The numbers below are the exact definitions, not rounded approximations.
Contents
- TLDR
- Imperial units chart
- Imperial vs US customary units
- Why the pint and gallon differ
- Where imperial units are still used
- Worked conversion examples
- Memory tricks
- FAQ
TLDR
- Imperial units were standardized in Britain in 1824 and are defined today by exact metric values.
- 1 inch = 25.4 mm, 1 yard = 0.9144 m, 1 mile = 1.609344 km, 1 pound = 0.45359237 kg, 1 imperial gallon = 4.54609 L.
- US customary units share the same inch, foot, yard, mile and pound, but the US pint, gallon, hundredweight and ton are different sizes.
- The UK still uses miles on road signs, pints for beer and milk, and stone for body weight.
Imperial units chart
Since the 1959 international yard and pound agreement, every imperial unit is defined by an exact metric value. The UK government’s weights and measures guidance lists which of these units are still legal to use in trade. The tables below use the same exact definitions, so you can convert without rounding errors creeping in. For every other unit of measurement, with its symbol and SI equivalent, see this complete reference of 98 units.
Length
| Unit | Abbreviation | Imperial equivalent | Exact metric value |
|---|---|---|---|
| Inch | in | 1/12 foot | 25.4 mm |
| Foot | ft | 12 inches | 0.3048 m |
| Yard | yd | 3 feet | 0.9144 m |
| Chain | ch | 22 yards | 20.1168 m |
| Furlong | fur | 10 chains | 201.168 m |
| Mile | mi | 8 furlongs (1,760 yards) | 1.609344 km |
The chain and furlong look like museum pieces, but they survive in surveying, horse racing and property deeds. A furlong is the length of a traditional ploughed furrow, and a horse race of eight furlongs is exactly one mile.
Area
| Unit | Abbreviation | Imperial equivalent | Metric value |
|---|---|---|---|
| Square inch | sq in | 1 in x 1 in | 6.4516 cm² |
| Square foot | sq ft | 144 sq in | 0.09290304 m² |
| Square yard | sq yd | 9 sq ft | 0.83612736 m² |
| Acre | ac | 4,840 sq yd (43,560 sq ft) | 4,046.856 m² (about 0.4047 ha) |
| Square mile | sq mi | 640 acres | 2.589988 km² |
Volume (liquid capacity)
| Unit | Abbreviation | Imperial equivalent | Exact metric value |
|---|---|---|---|
| Fluid ounce | fl oz | 1/20 pint | 28.4130625 mL |
| Gill | gi | 5 fl oz | 142.0653125 mL |
| Pint | pt | 20 fl oz | 568.26125 mL |
| Quart | qt | 2 pints | 1.1365225 L |
| Gallon | gal | 4 quarts (8 pints) | 4.54609 L |
Weight (mass)
| Unit | Abbreviation | Imperial equivalent | Exact metric value |
|---|---|---|---|
| Ounce | oz | 1/16 pound | 28.349523125 g |
| Pound | lb | 16 ounces | 0.45359237 kg |
| Stone | st | 14 pounds | 6.35029318 kg |
| Hundredweight | cwt | 112 pounds (8 stone) | 50.80234544 kg |
| Ton | t | 2,240 pounds (20 cwt) | 1,016.0469088 kg |
The imperial ton is 16 kg heavier than the metric tonne (1,000 kg). That gap is small enough to ignore in conversation and large enough to cost you money on a freight invoice.
Imperial vs US customary units
Most people say “imperial” for both systems, which is how the confusion starts. The two are close relatives that split apart in the early 1800s. Here’s where they match and where they don’t.
| Unit | Imperial (UK) | US customary |
|---|---|---|
| Inch, foot, yard, mile | Same | Same |
| Pound, ounce | Same | Same |
| Fluid ounce | 28.41 mL | 29.57 mL |
| Pint | 20 fl oz (568 mL) | 16 fl oz (473 mL) |
| Quart | 40 fl oz (1.137 L) | 32 fl oz (946 mL) |
| Gallon | 4.546 L | 3.785 L |
| Hundredweight | 112 lb (50.8 kg) | 100 lb (45.4 kg) |
| Ton | 2,240 lb (1,016 kg) | 2,000 lb (907 kg) |
| Stone | 14 lb, in everyday use | Not used |
Length and mass line up because both countries agreed in 1959 to define the yard and the pound by the same metric values. Everything involving liquid volume or the “short” versions of the heavy weights diverges.
The US calls its 2,000-pound ton the short ton and the 2,240-pound version the long ton. When a historical text says “ton” without context, check which country wrote it. The Titanic’s displacement of roughly 52,000 tons, for example, is in long tons.
One odd detail: the imperial fluid ounce is smaller than the US one, yet the imperial pint is bigger. That’s because the imperial pint holds 20 of its ounces and the US pint only 16.
Why the pint and gallon differ
The US didn’t adopt the 1824 reform. It kept the older British wine gallon, defined as 231 cubic inches, which dates to a 1707 statute under Queen Anne. At the time Britain also had a separate ale gallon (282 cubic inches) and a corn gallon, so a “gallon” depended on what you were buying.
The 1824 act scrapped all of them and created one imperial gallon, defined as the volume of ten pounds of water at a set temperature. That works out to 4.54609 litres. The US stayed with the wine gallon, now defined as exactly 3.785411784 litres.
The ratio between them is 1.20095. As a rule of thumb, an imperial gallon is a fifth bigger than a US gallon, and the same is true of the pint and quart. This is also why fuel economy figures don’t match: a car rated at 40 mpg in the UK gets about 33 mpg on the US scale, even though the car and the road are identical.
Where imperial units are still used
UK roads. Distances and speed limits are in miles and miles per hour. Converting road signs to kilometres has been discussed for decades and keeps getting dropped because of the cost of replacing every sign.
Pubs and milk. Draught beer and cider are sold by the pint, and the law specifies the imperial pint. Milk in returnable bottles is sold in pints too, which is one of the few cases where the UK government gave imperial units a formal exemption from metrication.
Body weight and height. British people typically give their weight in stone and pounds (12 stone 4 pounds) and their height in feet and inches. A UK doctor will record kilograms, but the patient will answer in stone.
The US. American customary units cover most daily life: miles, pounds, gallons, Fahrenheit, feet and inches. Science, medicine and the military have used metric for decades.
Canada. Canada officially switched to metric in the 1970s. Road signs, fuel and weather are metric, but people still give their height in feet and inches, their weight in pounds, and buy lumber in feet. Cooking and baking are often a mix of both.
Elsewhere. Countries that were part of the British Empire often kept imperial units in specific trades long after switching to metric: acres for land in India and Pakistan, for example, or feet for aircraft altitude, which is standard in international aviation almost everywhere.
Worked conversion examples
Start with the exact factors, then multiply.
Height: 5 ft 10 in to centimetres. Convert to inches first: (5 x 12) + 10 = 70 in. Then 70 x 2.54 = 177.8 cm.
Body weight: 12 stone 4 lb to kilograms. 12 x 14 = 168 lb, plus 4 = 172 lb. Then 172 x 0.45359237 = 78.0 kg.
Speed: 70 mph to km/h. 70 x 1.609344 = 112.65 km/h.
Volume: 3 imperial pints to millilitres. 3 x 568.26 = 1,704.8 mL, or about 1.7 L.
Imperial gallons to US gallons. Multiply by 1.20095. A 10-imperial-gallon tank holds about 12 US gallons. To go the other way, divide by 1.20095 (or multiply by 0.8327).
Metric to imperial: 5 km to miles. Divide by 1.609344, which gives 3.11 miles.
Memory tricks
You don’t need to memorize the tables. A few anchors do most of the work:
- 1 inch = 2.54 cm. This is exact, and it’s the single most useful number in the system.
- 1 mile is about 1.6 km. A 5 km run is a little over 3 miles. A 10 km run is 6.2 miles.
- 1 kg is about 2.2 lb. Multiply a kilogram figure by 2.2 for a quick pound estimate.
- 1 litre is about 1.76 imperial pints, or roughly 2.1 US pints.
- A pint of water weighs 20 ounces in the imperial system. The old rhyme “a pint’s a pound the world around” is a US-only truth: a US pint of water weighs about 1.04 lb. The British version says “a pint of pure water weighs a pound and a quarter,” which is 20 ounces, and it’s accurate.
- Temperature shortcut: to convert Fahrenheit to Celsius, subtract 32 and multiply by 5/9. Fahrenheit isn’t an imperial unit in the strict sense, but it travels with the system in the US.
For a deeper reference on how metric and customary units relate, the US National Institute of Standards and Technology publishes Special Publication 811, the official guide to the International System of Units with conversion factors for non-SI units.
FAQ
What is the difference between imperial and metric units? Metric units are decimal: every step is a factor of 10, 100 or 1,000. Imperial units use mixed bases (12 inches in a foot, 3 feet in a yard, 1,760 yards in a mile, 16 ounces in a pound), which makes mental arithmetic harder but gives small units that suit human-scale tasks.
Is the US system the same as imperial? No. The inch, foot, yard, mile, ounce and pound match. The pint, quart, gallon, hundredweight and ton do not. US customary units descend from the pre-1824 English system, while imperial units are the post-1824 reform.
Which countries use the imperial system? The UK uses imperial units in several everyday contexts alongside metric. The US uses US customary units, which are related but not identical. Canada uses a metric and imperial mix. Almost every other country uses metric as its official system.
Why does the UK still use miles? Changing road signs would cost a large sum, and neither major political party has found the benefit worth it. The UK adopted metric for most trade from 1965, but kept miles, pints of draught beer and a few other units by law.
How many ounces are in an imperial pint? Twenty fluid ounces. A US pint has sixteen.
How many pounds are in a stone? Fourteen. A stone is 6.35029318 kg.
Is a long ton the same as a metric tonne? No. A long ton is 2,240 lb (1,016.05 kg). A metric tonne is 1,000 kg, and a US short ton is 2,000 lb (907.18 kg).
