What is the avoirdupois weight system?
Avoirdupois is the everyday weight system used in the United States and historically across the English-speaking world. It is built on the pound, which is divided into 16 ounces, with each ounce containing 16 drams. Larger units include the stone (14 lb), the quarter (28 lb), the hundredweight (cwt) and the ton. This calculator uses the US "short" definitions: 1 short hundredweight = 100 lb and 1 short ton = 2,000 lb.
How to use this calculator
Enter the numeric value you want to convert, pick the unit you are converting from, then pick the unit you are converting to. The result updates with the converted amount. You can move freely between any pair of avoirdupois units — for example pounds to ounces, stone to pounds, or tons to hundredweight.
The formula explained
Every unit is expressed in terms of the smallest unit, the dram. The conversion multiplies your input by the ratio of the source unit's factor to the target unit's factor: $$\text{value}_{out} = \text{value}_{in} \times \frac{\text{factor}_{in}}{\text{factor}_{out}}$$ Because \(1\ \text{oz} = 16\ \text{dr}\) and \(1\ \text{lb} = 256\ \text{dr}\), converting 1 lb to oz gives \(1 \times (256 / 16) = 16\ \text{oz}\).
Worked example
Convert 5 pounds to ounces. The factor for pounds is 256 drams and for ounces is 16 drams, so: $$5 \times \frac{256}{16} = 5 \times 16 = 80\ \text{ounces}$$ To go the other way, 80 oz to lb = \(80 \times (16 / 256) = 80 / 16 = 5\ \text{lb}\).
FAQ
Is this the same as the imperial hundredweight? No. This tool uses US short units (cwt = 100 lb, ton = 2,000 lb). The UK long hundredweight is 112 lb and the long ton is 2,240 lb.
How many drams are in a pound? There are 256 drams in one avoirdupois pound (16 ounces × 16 drams).
What is a stone? A stone equals 14 pounds and is still widely used for body weight in the UK and Ireland.
