How this instrument works
Avoirdupois — from Old French aveir de peis, goods sold by weight — is the system English wool merchants settled on, and it counts everything in grains. One avoirdupois pound is 7000 grains; one avoirdupois ounce is 437.5 grains. Divide 7000 by 437.5 and 16 comes out with nothing left over, which is why lb to oz has never needed rounding.
Sixteen looks arbitrary until you picture balance-beam trade. Pounds halve into 8 oz, halve again into 4, then 2, then 1, so merchants carrying one doubling set of weights could settle any order down to the single ounce. Rival pounds lost because they could not: Tower pounds held 12 ounces, libra mercatoria held 15, and neither survived England's commerce in wool, spice and metal goods. Curiously, 'ounce' descends from Latin uncia, meaning one twelfth — that name stayed put while the fraction beneath it changed.
Both units are now pinned to metric. Under the 1959 international yard and pound agreement, an avoirdupois pound became 0.45359237 kg exactly, which makes one ounce 28.349523125 g exactly. That treaty ended decades of drift between British and American standards, so pounds weighed in Sheffield hold 16 ounces of identical mass to pounds weighed in Seattle. Fluid ounces and gallons were left untouched and still differ across that ocean; mass units do not.
- Type your weight into the Pounds (lb) field — 5 lb loads by default, so results appear straight away.
- Read the Ounces (oz) line beneath it; that figure recomputes on every keystroke.
- For mixed weights such as 3 lb 4 oz, turn the ounce part into decimals first: 4 ÷ 16 = 0.25, so enter 3.25.
- Working backwards from ounces, divide by 16 — 40 oz is 2.5 lb, 6 oz is 0.375 lb.
- Fractions are fine. Input steps in thousandths of one pound, or 0.016 oz, finer than any kitchen scale resolves.
Worked example — one 5 lb sack of bread flour
Leave 5 in Pounds (lb) and Ounces (oz) reads 80.0. American flour sacks print both figures on their front for exactly this reason: baking formulas run on ounces, while shelf labels and shipping tables run on pounds. Dough needing 20 oz per batch gets four batches out of that sack, with nothing left over.
Check it through grains and you land in one place: 5 × 7000 = 35,000 grains, divided by 437.5 grains per ounce, is 80. Check it through metric and you land there again — 5 lb is 2.26796185 kg exactly; dividing by 28.349523125 g gives 80.0, with no remainder to round away.
Questions
Is 16 ounces per pound exact, or rounded?
Exact, by definition rather than by measurement. Avoirdupois units are both defined in grains — 7000 per pound, 437.5 per ounce — and that ratio is exactly 16. Anchoring to metric preserves it: since 1959 one pound has been 0.45359237 kg exactly, putting one ounce at 28.349523125 g exactly. Multiplying by 16 therefore introduces no error at all, so any stray digits you see come from your own input, never from this factor.
Can I use this for troy ounces of gold or silver?
No — troy is its own system, with 12 ounces to its pound. Troy ounces run 480 grains (31.1034768 g), heavier than avoirdupois ounces at 437.5 grains (28.349523125 g), while troy pounds of 5760 grains come in lighter than avoirdupois pounds of 7000. That inversion is why one pound of gold weighs less than one pound of sugar even though gold ounces weigh more. Bullion quotes and apothecaries' weights use troy; groceries, parcels and body weight use avoirdupois.
Does one fluid ounce weigh one ounce?
Only roughly, and only for water. Fluid ounces measure volume; ounces measure mass. US pints hold 16 fl oz of water weighing about 16.7 oz on the scale, which is where 'a pint's a pound the world around' comes from — close enough to rhyme, off by some 4%. Imperial pints hold 20 imperial fluid ounces, so British pints of water run nearer 20 oz by mass. Honey, oil and flour throw that approximation out entirely, which is why baking by weight beats baking by cup.
Do British and American pounds hold the same number of ounces?
Yes — 16 in both, and identical ounces at that. Britain, the United States, Canada, Australia, New Zealand and South Africa adopted one international avoirdupois pound of 0.45359237 kg in 1959, so mass conversions carry across without adjustment. Volume is where those systems still part company: US fluid ounces run 29.5735 mL against an imperial 28.4131 mL. British practice also keeps the stone, 14 lb or 224 oz, which American usage dropped long ago.
How do I enter weights written as pounds and ounces, like 7 lb 8 oz?
Turn its ounce part into decimal pounds and add: 8 ÷ 16 = 0.5, so 7 lb 8 oz becomes 7.5, returning 120 oz. Halves, quarters and eighths cover most cases — 8 oz is .5, 4 oz is .25, 2 oz is .125, 1 oz is .0625. Birth weights, boxing classes and deli orders all arrive in this mixed form, so those five fractions are worth learning by heart.