Understanding Imperial Units

Despite the fact that imperial units of measurement are still in widespread use in the UK, the USA and many other countries, most young people in Britain today are not taught how to use them. To remedy that, the BWMA has put together the following guide to teach the most basic and commonly used units.


The basic unit of length is the inch. There are 12 inches in a foot. This means that a foot can easily be divided by 2, 3, 4 or 6 to give a round number: one third of a foot is 4 inches, one quarter of a foot is 3 inches, half a foot is 6 inches, and so on. Feet and inches are sometimes expressed with a prime and double prime (inverted commas), so 3 foot 6 inches can be expressed as 3′ 6″. This is often seen on road signs.

There are 3 feet in a yard, or 36 inches. So:

1 yard = 3 feet = 36 inches

1/2 yard = 1 foot 6 inches = 18 inches

1/3 yard = 1 foot = 12 inches

1/4 yard = 9 inches (3/4 of a foot)

1/6 yard = 6 inches (1/2 of a foot)

The inch is typically divided into fractions of one sixteenth for measuring very small lengths. This means it can also be expressed in halves, quarters and eighths of an inch.

The other main units of length after the yard are the chain, furlong and mile. There are 22 yards in a chain, 10 chains in a furlong and 8 furlongs in a mile. This means that:

1 chain = 22 yards = 66 feet

1 furlong = 10 chains = 220 yards = 660 feet

1 mile = 8 furlongs = 80 chains = 1760 yards = 5280 feet

This means that there is a unit for every size of distance people may need to measure in different situations. Feet and inches are used for human heights, yards are used for road sign distances, chains are used for railway measurements, furlongs for measuring fields and horse races, and miles for the distances between towns and cities. There is rarely ever a need to convert directly from miles to yards; the chain and furlong fit in between to give an appropriate unit for each kind of distance.


An acre is an area equivalent to one chain by one furlong, or 22 by 220 yards. This means an acre can easily be divided up into 8 square chains, each of which can be divided into 484 square yards (giving 4840 square yards in an acre). Ten acres make a square furlong, and 64 of these (8×8 in a grid), or 640 acres, make a square mile.


The main imperial weights you will encounter in day-to-day life are the ounce (oz), pound (lb) and stone (st). 16 ounces are in a pound, allowing division by 2, 4 and 8. The stone provides a unit ideally sized for human weights in between the pound and the next unit up, the hundredweight (cwt). There are 14 pounds in a stone and 8 stones in a hundredweight, meaning there are 112 pounds in a hundredweight. The last unit above that is the ton, which has 20 hundredweight or 2240 pounds.

Fluid Measures

A fluid ounce (fl oz) is the amount of water that weighs one ounce at room temperature (62 degrees Fahrenheit). This means that in day-to-day usage, the weight of water in oz is equal to its volume in fl oz. There are 20 fluid ounces in a pint (pt) and 8 pints in a gallon. There are other fluid measures but these are the ones typically encountered today.


The Fahrenheit degree (°F) is smaller than the Celsius degree (5/9 the size), meaning that round numbers of Fahrenheit degrees can express smaller changes in temperature. The freezing point of water is 32 degrees Fahrenheit, human body temperature is 98.6 degrees Fahrenheit and the boiling point of water is 212 degrees Fahrenheit. Absolute zero (the coldest temperature possible in the universe) is −459.67 °F. The Rankine scale uses the same Fahrenheit degree but sets the 0 at absolute zero, the same way the Kelvin scale does with the Celsius degree.

Nautical Measures

In the aviation industry, altitudes are expressed in thousands of feet, while distances are expressed in nautical miles. A nautical mile is one 60th of one degree of the Earth’s equator, or 1.151 miles. Aircraft speeds are measured in knots, a knot being one nautical mile per hour. Knots and nautical miles are also used in the maritime and shipping industries.