Units of weight

Units of weight

How can we convert between the units of weight used in the imperial unit system of the United Kingdom? And furthermore how does the conversion work for units in the US customary an metric system? And finally how can we convert between the proceeding unit systems?

This article will review the mass units used in the UK imperial, US customary and metric system. The imperial and US customary system consists of three subsystems. Namely the avoirdupois, apothecary and troy weight systems. In addition, the metric weight units scale up and down with a factor of 10. Finally, each table contains the conversion factor to the SI unit of weight kilogram (kg).

Units of weight avoirdupois system

The people in the United states and United Kingdom uses the avoirdupois weight units for day-to-day usage. Avoirdupois derives from the French word for goods of weight. Furthermore the abbreviation of av indicates the use of the avoirdupois system.

The weight and measurement act (WMA) of 1963 agreed upon the definition of 1 pound av as 0,453 592 37 kg. Unit conversion becomes a possibility as a result of this agreement.

UK imperial avoirdupois units of weight

People in the United Kingdom makes use of the ounce, pound, stone, hundred weight and UK ton units of weight. Although units from the metric system are also used. See table 1 for internal and SI units of weight conversions.

Firstly a pound (lb av) is built out of 16 ounces (oz av).

Furthermore a stone (st av) consists of 14 pound.

In addition a hundred weight (cwt) is the equivalent of 8 stone.

Finally a UK ton consists of 20 hundred weights.

By continuation of the proceeding factors other conversion can be derived.

UK imperial avoirdupois units of weight

Table 1 UK imperial avoirdupois units of weight

US customary avoirdupois units of weight

The people in the United States uses more avoirdupois units of weight than people in the United Kingdom. Since the grain and dram expands their vocabulary. They also use two definitions of the hundredweight (cwt) and ton. Namely the short variant (US) and long edition (UK).

A grain (gr av) is the smallest weight unit in the US customary weight measurement and equivalents 64,8 milligram (mg) of the metric system.

A dram (dram av) furthermore consists of 875/32 = 27,34 grains.

In addition a US short hundred weight equivalents 100 pounds (instead of 14*8 = 112 for the UK long edition).

Therefore a short US ton consists of 20 US short hundredweights or 20*(100/112) = 17,86 UK long hundredweights.

Furthermore a metric tonne equivalents 1000 kg. Therefore you should always use unit abbreviations. For example: “I’d like to order 1 US ton of pancake batter”. Not specifying the type of ton could saddle you up with an 12% surplus of UK ton.

Table 2 contains the internal and SI units of weight conversion factors.

US customary avoirdupois units of weight

Table 2 US customary avoirdupois units of weight

Apothecaries units of weight

Apothecaries in the United Kingdom and United states used special weight units in the past for their small measurements. They have however become obsolete after prohibition by the governments.

Troy units of weight

The Troy units of weight accommodates the units used for measurement of precious metals and valuable gemstones. The traders in the United Kingdom converted to the metric system. However, the traders in the United States (and therefore international traders) still use this units of weight system.

The name Troy derives from the town of Troyes in Northern France. This town was well-known for its commercial fairs during the medieval times.

Table 3 contains the Troy units of weight, namely: grain, pennyweight, ounce and pound.

One troy pound (lb troy) equivalents 0,3732 SI kilogram. It’s therefore significantly different from the avoirdupois pound (lb av), which equivalents 0,4536 kilogram.

A troy grain (gr troy) equals the avoirdupois grain (gr av).

Furthermore a pennyweight (dwt) consists of 24 grains.

Also one troy ounce (oz troy) equivalents 20 pennyweight. However, the troy ounce specifies a different amount of mass than the avoirdupois ounce (oz av).

And finally the troy pound (lb troy) consists of 12 troy ounce.

US Troy units of weight

Table 3 US Troy units of weight


The carat unit is used in the branch of jewelery and indicates how pure gold is. Namely 24 carats equivalents 100 wt% pure gold. Furthermore 12 carats equals 50 wt% and so on.

The metric carat equivalents 3 grains in the imperial and 200 mg in the metric unit system.

Metric units of weight

The metric units of weight scale up and down in a similar method as in previous articles about lentharea and volume

The tonne, kilogram, gram and milligram are the most used metric units of weight. However other units are also used in specific field. Table 4 displays the metric weight units with conversion to SI kilogram and imperial counterparts.

Metric units of weight

table 4 Metric units of weight

@ Engineerings fundamentals

How did you like the article?

Follow chemE blog on Facebook ChemE RSS feed ChemE email list