The Imperial system of units

The Imperial system is the set of weights and measures that the United Kingdom made official in 1824. It replaced a tangle of local and trade-specific standards with one legal set. Its units are the familiar ones: the inch, the foot, the yard and the mile for length; the ounce, the pound and the stone for mass; the pint and the gallon for volume. Most of the world has since moved to the metric system, and the United Kingdom itself now defines every Imperial unit in metric terms. The units have not disappeared, though. They still measure road distances in Britain, the height of a person, the weight of a newborn, and a glass of beer.

The system is often confused with US customary measure. The two share most of their names and many of their sizes, but they are not the same system and they were never fully aligned. The gallon is the clearest example: an Imperial gallon is about a fifth larger than a US gallon. This page explains where the Imperial system came from, how it is defined now, and where it still differs from the American units that look identical on paper.

Where the Imperial system came from

Before 1824, British measurement was inconsistent. A bushel of grain in one county was not a bushel in the next. Trades kept their own units, and the Crown maintained several competing standards at once, including the Winchester measures that dated to the fifteenth century. Disputes over quantity were common, and the law had no single answer to them.

The Weights and Measures Act of 1824 settled the question. Parliament defined one yard, one pound and one gallon, and made them the legal basis of every other unit. The gallon was newly defined as the volume of ten pounds of distilled water at a specified temperature and pressure, which is why the Imperial gallon does not match the older wine gallon the American colonies had already adopted. That single decision is the origin of most Imperial-versus-US differences that still cause confusion today.

The Act came into force in 1826 and applied across the British Empire. This is why Imperial units remain in partial use in Canada, Ireland, India, Australia and New Zealand, even though all of those countries have since adopted the metric system as their legal standard. A second Act in 1878 tightened the definitions and created the physical prototype bars and weights that served as the standard until the twentieth century.

How Imperial units are defined today

Imperial units are no longer defined by physical artefacts. They are defined by exact conversion to metric units. The turning point was the international yard and pound agreement of 1959, signed by the United Kingdom, the United States, Canada, Australia, New Zealand and South Africa. It fixed the yard at exactly 0.9144 metres and the pound at exactly 0.45359237 kilograms.

Every other Imperial unit of length and mass follows from those two numbers. The inch became exactly 25.4 millimetres. The foot became exactly 0.3048 metres. The mile became exactly 1609.344 metres. Because the definitions are exact rather than measured, conversion between Imperial and metric units involves no approximation at all. A conversion from inches to millimetres is a change of notation, not an estimate.

Volume was handled separately. The United Kingdom redefined the Imperial gallon in 1985 as exactly 4.54609 litres, which is close to but not identical with the 1824 water-based definition. The pint, quart and fluid ounce are all fixed fractions of that gallon. This is the reason an Imperial pint is 568 millilitres while a US liquid pint is 473 millilitres.

Imperial units of length

Imperial length is built on the yard. The smaller units divide it and the larger units multiply it, but the ratios are not decimal. Twelve inches make a foot, three feet make a yard, and 1760 yards make a mile. The awkward 1760 is inherited from the furlong, an agricultural unit equal to the length of a ploughed furrow, of which there are eight in a mile.

Two of these deserve a note. The hand survives almost entirely to measure horses, and is written in a peculiar way: 15.2 hands means fifteen hands and two inches, not fifteen and one fifth. The nautical mile is not really an Imperial unit at all. It is defined as 1852 metres by international agreement, chosen to approximate one minute of latitude, and it is used at sea and in the air regardless of which system a country otherwise follows.

Imperial units of mass

Imperial mass uses the avoirdupois pound, the commercial pound of sixteen ounces. Two older systems, troy and apothecaries' weight, used a different pound for precious metals and medicines; troy weight survives today only for gold, silver and gemstones.

The stone is the unit most tied to British habit. It equals fourteen pounds, and it is still the normal way to state a person's body weight in the United Kingdom and Ireland. It is almost unknown in the United States, where the same weight is given in pounds alone. The hundredweight compounds the confusion: the Imperial long hundredweight is 112 pounds, while the American short hundredweight is 100 pounds. The same applies to the ton. An Imperial long ton is 2240 pounds, a US short ton is 2000 pounds, and a metric tonne is 1000 kilograms, or about 2205 pounds.

Where Imperial and US customary units diverge

Length and mass are safe. Since 1959 an inch is an inch and a pound is a pound on both sides of the Atlantic. Volume is where the systems part company, and the difference is large enough to matter in cooking, fuel economy and engineering.

The Imperial gallon is 4.54609 litres. The US gallon is 3.785411784 litres, because the United States kept the older English wine gallon that Britain abandoned in 1824. That is a difference of roughly 20 per cent. It cascades down: an Imperial pint is 568 millilitres against 473 for the US liquid pint, and an Imperial fluid ounce is 28.41 millilitres against 29.57 for the US fluid ounce. The fluid ounce is the odd one out, because the Imperial unit is the smaller of the two while every other Imperial volume unit is larger.

The practical consequence is that a recipe, a fuel figure or a tank capacity is ambiguous unless the system is stated. British miles per gallon and American miles per gallon are not comparable numbers. When a source is unclear, the safest approach is to convert to litres and compare there.

Where the Imperial system is still used

The United Kingdom is formally metric, but the law carves out exceptions. Road signs give distances in miles and speeds in miles per hour. Draught beer and cider are sold in pints, and so is milk in returnable bottles. Precious metals are traded in troy ounces. Beyond those legal cases, habit keeps Imperial units alive: people give their height in feet and inches and their weight in stones and pounds.

Ireland switched its road signs to kilometres in 2005 but retains the pint. Canada uses metric officially while construction, aviation and everyday speech still lean on feet and pounds. The United States remains the largest economy using non-metric units in daily life, but the units it uses are US customary rather than Imperial. Recognising which of the two a figure belongs to is usually more important than the conversion itself.

Imperial units of area

Imperial area units are squares of the length units, with one important exception. The square inch, square foot, square yard and square mile all behave as expected: each is the area of a square whose side is the corresponding length. Because the length ratios are not decimal, the area ratios are their squares, so a square yard is nine square feet and a square mile is 3,097,600 square yards.

The acre is the exception. It is not the square of any length unit. It is defined as one chain by one furlong, which is 4840 square yards or about 4047 square metres. The shape is agricultural in origin: a chain wide and a furlong long was the area a team of oxen could plough in a day. The acre remains the standard unit for land in the United Kingdom, Ireland and the United States, and it is one of the few Imperial units still used in official land registration. Its metric counterpart, the hectare, is 10,000 square metres, which makes one hectare roughly 2.47 acres.

Speed, pressure and temperature

Imperial speed units are simply a length over a time. Miles per hour governs British road signs and remains the unit in which most British drivers think. Feet per second appears in ballistics and engineering. The knot, one nautical mile per hour, is the standard at sea and in aviation everywhere, metric countries included.

Pressure in Imperial units is measured in pounds per square inch, usually written psi. It is still the common unit for tyre pressure in Britain and North America, and for hydraulic and pneumatic equipment worldwide. One psi is about 6895 pascals. Larger pressures are given in thousands of psi, written ksi.

Temperature is the one Imperial quantity with no metric relationship of the simple multiply-by-a-factor kind. The Fahrenheit scale places the freezing point of water at 32 degrees and the boiling point at 212, a span of 180 degrees where Celsius uses 100. Converting therefore requires both a multiplication and an offset, which is why Fahrenheit to Celsius is the only conversion on this site that adds a number as well as scaling one.

Converting Imperial units

Because every Imperial unit now has an exact metric definition, conversion is arithmetic rather than estimation. The three worth memorising are that an inch is 2.54 centimetres, a pound is about 0.454 kilograms, and a mile is about 1.609 kilometres. For anything else, or for the volume units where Imperial and US values differ, use the converters linked in the tables above.

You can browse every Imperial unit on this site from the unit dictionary, or compare them against the metric equivalents in the SI system.