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CONVERT IT! Imperial Units Metric System Traditional Units

Traditional Units

The basic traditional unit of weight, the pound, originated as a Roman unit and was used throughout the Roman Empire. The Roman pound was divided into 12 ounces, but many European merchants preferred to use a larger pound of 16 ounces, since a 16-ounce pound is conveniently divided into halves, quarters, or eighths. Medieval English merchants often used a larger pound called the "mercantile" pound (libra mercatoria) containing 15 troy ounces. The weight system used as the basis for English coinage during the medieval era was known as the tower pound containing 12 tower ounces, named after the Tower of London where the Royal Mint was located. During the Middle Ages there were many different pound standards in use, some of 12 ounces and some of 16.

The Grain

The basic unit of weight in the imperial system, originally defined as the weight of a barleycorn. There are 5400 grains to the tower pound, 5760 grains to the Troy pound,  7000 to the avoirdupois pound and 7200 grains to the mercantile pound.

The Carat

The carat (weight) was the weight unit for diamonds and other precious stones was defined by law to be 3.2 troy grains, which was about 207 milligrams. Jewellers everywhere now use a metric carat defined in 1907 to be exactly 200 milligrams.

The carat is also used as a standard to define the purity of gold, a traditional measure of proportion equal to 1/24th. Thus "14-carat gold" is legally required to be at least 14/24, or 58.3%, gold.

The Troy Pound

The oldest English weight system has been used since the time of the Saxon kings. It is based on the 12 ounce troy pound, which provided the basis on which coins were minted and gold and silver were weighed.  The troy pound weighs 5760 grains, and the ounces weigh 480 grains. Twenty pennies weighed an ounce, and therefore a pennyweight was 24 grains. The troy pound was abolished in 1878 but the smaller troy weights also known apothecaries weights quickly became highly specialized and used only for precious metals and for pharmaceuticals, while the avoirdupois pound was used for everything else. The apothecaries system was outlawed for dispensing medicines, in favour of the metric system in 1969.

Troy WeightMetric Equivalent
Grain (gr)64.8 milligrams
Scruple (s) = 20gr1.2960 grams
Pennyweight (dwt) = 24gr1.5552 grams
Dram (dr) = 3s = 60gr3.8879 grams
Ounce (oz) = 8dr = 480gr = 20dwt      31.1 grams
Pound (lb) = 12oz - 5760gr373.242 grams

Imperial Units

Avoirdupois Pound (lb)

Around 1300 the mercantile pound was replaced in English commerce by the 16-ounce avoirdupois pound. The pound unit that was in common use in Britain up to the end of 1999.


Since at least 1400 a standard weight unit in Britain has been the hundredweight, which is equal to 112 avoirdupois pounds. There were very good reasons for the odd size of this "hundred", the  hundredweight can be divided conveniently into 4 quarters of 28 pounds, 8 stone of 14 pounds, or 16 cloves of 7 pounds each. The ton, originally a unit of wine measure, was defined to equal 20 hundredweight or 2240 pounds.

Avoirdupois WeightMetric Equivalent
Dram (dr)1.7718 grams
Ounce (oz) = 16dr28.3495 grams
Pound (lb) = 16oz = 7000gr453.592 grams
Stone (st) = 14lb6350.29 grams = 6.35 kg
Quarter (Q) = 2st = 28lb12.7 kg
Hundredweight (cwt) = 4Q = 8st = 112lb    50.8 kg
Ton (T) = 20cwt = 2240lb1016.047 kg

The clove was very rarely used and has not been included in the above table.

Note that asking for a quarter of sweets in a shop was a reference to pound or 4 ounces and not the quarter in the table above.

The Metric System

The base unit of the SI system for weight is the kilogram (kg) equal to about about 2.20lb avoirdupois. The prefix kilo means x 1,000 : 1kg = 1,000 grams. For smaller weights used for medicines etc. the gram can be prefixed by milli - 1/1,000th (m) or micro - 1/1,000,000th (). Larger weights can be measured in Tonnes equivalent to 1000 kilograms.

1 tonne (t)1000 kilograms (kg) 2203 pounds (lb)
1 kilogram (kg)1000 grams 2.203 pounds (lb)
1 gram (g)1000 milligrams (mg) 0.035 ounces (oz)
1 milligram1000 micrograms (g) 0.015 grains (gr)


The  controls below will convert the various units to and from metric. Enter a value in one of the boxes click outside the box and the remaining boxes in the row will show the results.

pounds kilograms
ounces grams


Page creator - David Haworth.

Russ Rowlett