Convert ounces, pounds, grains, drams, and tons in the avoirdupois system. Free with steps.
The avoirdupois system — ounces, pounds, drams, grains, and tons — weighs nearly everything in American commerce, from deli meat to freight trucks. Its ratios are gloriously irregular: 16 ounces to the pound, 16 drams to the ounce, 437.5 grains to the ounce. Our free Avoirdupois Weight Converter translates between all six units with exact metric-based factors, showing the gram intermediate so every answer is verifiable.
“Avoirdupois” comes from Old French aveir de peis — “goods of weight” — the system medieval merchants used for everything except precious metals and medicine (those got troy and apothecaries’ systems). It became the international pound’s home: since 1959, the avoirdupois pound is defined as exactly 0.45359237 kg, making every conversion on this page exact by definition.
Use it for recipes scaling ounces to pounds, ammunition measured in grains, freight quoted in tons, and anywhere the troy-vs-avoirdupois ounce distinction matters (they differ by 10% — a costly confusion in metals).
The system’s backbone is the grain — historically a grain of wheat, now defined as exactly 64.79891 mg. Everything builds upward: 27.34375 grains = 1 dram, 16 drams = 1 ounce (437.5 grains), 16 ounces = 1 pound (7,000 grains), 2,000 pounds = 1 short ton. The grain’s exact metric definition makes the whole tower exact.
Two impostors cause the famous confusions. The troy ounce (31.103 g, for precious metals) is ~10% heavier than the avoirdupois ounce (28.350 g) — gold’s “ounce” is not your kitchen’s ounce. The long ton (2,240 lb, British) exceeds the short ton (2,000 lb, American) by 12% — freight quotes must name which ton they mean.
Why 16 ounces? Because 16 = 2⁴ halves cleanly four times: half a pound is 8 oz, a quarter is 4, an eighth is 2 — the binary subdivisions butchers and merchants needed before decimal scales. Base-16 divisibility, like base-12 inches, is a pre-decimal ergonomics fossil that still works.
Key terms:
Since 1959 the pound is defined metrically (0.45359237 kg exactly), so avoirdupois-to-metric conversions are exact — no empirical fudge factors anywhere in the chain.
A recipe calls for 2.5 lb of flour. How many ounces?
The shortcut (lb × 16) agrees — 2.5 × 16 = 40 — but the gram route verifies it exactly.
A box of 9mm ammunition is labeled 115 grains per bullet — express one bullet in pounds.
Bullet weights live in grains because the numbers stay human-sized: 115, 124, 147 — versus 0.016 lb.
A shipper quotes “$1,800 for 9 tons” of machinery from Ohio to Texas. The buyer’s rig is rated in pounds: 9 short tons = 18,000 lb — within the 20,000 lb trailer rating. But the quote came from a Canadian forwarder thinking metric tonnes (1,000 kg = 2,204.6 lb): 9 tonnes = 19,841 lb — still legal, but only 159 lb of margin. And had it been 9 long tons, 20,160 lb would have overloaded the trailer. Three “tons,” three answers; the converter plus a clarifying question prevents a roadside fine.
The grain scale appears in pharmaceuticals’ shadow: supplements dosed at “500 mg” are 7.7 grains — irrelevant until a compounding recipe from 1920 says “gr. v” (5 grains = 324 mg). Historical formulas in grains convert through the exact 64.79891 mg factor, no folklore required.
A second scenario: a butcher breaks down a 180 lb half-cow. In ounces that is 2,880 oz of retail cuts; the 16-oz steaks number 180 before trim. Butchers think in pounds, customers in ounces, and the register in dollars per pound — the converter is the bilingual cashier between them.
A third scenario: coffee roasting. A roaster buys green beans at $3.40/lb in 132 lb bags (60 kg) — the international coffee trade’s standard bag. Roasting loses ~16% moisture weight, so 132 lb green becomes ~111 lb roasted, or 1,776 oz; at 16 oz retail bags that is 111 bags per sack. The lb→oz conversion prices every bag on the shelf, and the moisture-loss math explains why roasted coffee costs more per pound than green.
Postal scales complete the picture: USPS postage steps in ounces — a 3.2 oz letter pays the 4-oz rate — so shippers weigh in ounces and think in thresholds. A product trimmed from 16.4 oz to 15.9 oz drops a full postage tier; the avoirdupois ounce is literally money.
The pound’s ancestors are Roman: the libra (~327 g), whose abbreviation “lb” we still write. Medieval England hosted a chaos of pounds — Tower, Troy, London, merchant’s — until the 14th century’s avoirdupois pound (7,000 grains) won for general goods. The 1959 international agreement finally froze it at 0.45359237 kg, ending centuries of drift between national pounds.
The grain outlasted them all as the defining unit: when the pound was redefined metrically, the grain became exactly 64.79891 mg, preserving every historical ratio (7,000 grains = 1 lb) while anchoring the system to the kilogram. It is the oldest continuously defined mass unit in English law.
The dram’s afterlife is charming: “a dram of whisky” (a small pour), the apothecaries’ fluid dram, and the postal scale’s dram all descend from the same 1/16 ounce. Language preserved what commerce discarded.
Metrication keeps threatening the system — the US officially “prefers” metric since 1975 — but the pound’s network effects (recipes, gym plates, freight, baby weights) make it immortal in practice. The converter is diplomacy between the two regimes.
One last curiosity: the hundredweight (cwt) — 100 lb in the US, 112 lb in Britain — still prices livestock, grain, and steel. “$18 per hundredweight” means $360/ton, and the US/UK hundredweight split has caused exactly the trade disputes you would expect.
The system’s legal metrology is stricter than its folk image: commercial scales in the US must meet NIST Handbook 44 tolerances, and packaged goods follow the Fair Packaging and Labeling Act — net weight in both ounces/pounds and grams. That dual labeling is why American packages read “NET WT 16 OZ (1 LB) 454 g”: the avoirdupois pound and its exact metric definition, printed side by side on every box in the country.
Avoirdupois via grams: grain=0.06479891, dram=1.7718451953125, oz=28.349523125, lb=453.59237 (exact), short ton=907184.74, long ton=1016046.9088 g. Chain: 7000 gr=1 lb; 16 oz=1 lb; 2000 lb=1 short ton; 2240 lb=1 long ton. Distinct from troy ounce (31.103 g). Source: Sigma Calculator, Avoirdupois Weight Converter, https://sigmacalculator.net/math-calculators/avoirdupois-weight-converter/
Cite this page as: Avoirdupois Weight Converter, Sigma Calculator, https://sigmacalculator.net/math-calculators/avoirdupois-weight-converter/
Last updated: October 12, 2026