Pounds to Ounces (lbs to oz)

One avoirdupois pound (lb) is exactly 0.45359237 kilograms — a value fixed by the 1959 International Yard and Pound Agreement and unchanged since. The pound divides into 16 ounces of 437.5 grains apiece, with the grain itself defined as exactly 64.79891 milligrams; the apparently arbitrary factor exists because there are exactly 7,000 grains in a pound, and seven was a convenient divisor for the gunpowder, apothecary and assay measurements that drove early standardisation. In engineering and physics texts, "pound" without qualification can mean either pound-mass (lbm), a unit of mass, or pound-force (lbf), the gravitational force on one pound-mass at standard gravity (9.80665 m/s²). The two are numerically equal at sea level but represent different physical quantities; the gravitational conversion constant gc = 32.174 lbm·ft/(lbf·s²) is the bridge between them. The pound is not part of the International System of Units (SI) but is recognised by the United States National Institute of Standards and Technology for customary use under Federal Register notice 24 FR 5445.

The pound's lineage runs unbroken from the Roman libra, a weight of roughly 328.9 grams in everyday imperial use that bequeathed the modern English word ("pound" from the Latin pondo, meaning "by weight") and the curious abbreviation "lb" (from libra itself). When Roman administration receded from western Europe, regional pounds multiplied: the Tower pound used at the English Royal Mint settled near 350 g, the merchant's pound favoured in continental commerce sat closer to 437 g, the troy pound for gold and silver was fixed at exactly 373.24 g, and the avoirdupois pound — the pound of grocers and general goods — landed near 454 g. By the late Middle Ages the avoirdupois pound had won out for English trade, but the country lacked a single legally-binding artefact for it until the Weights and Measures Act 1855, passed two decades after the 1834 fire at the Palace of Westminster destroyed the original imperial standards. Even after that, US and UK definitions of the pound drifted apart by parts per million — invisible at a kitchen scale, but enough to misalign aviation tables and ballistics charts on opposite sides of the Atlantic. The International Yard and Pound Agreement of 1 July 1959, signed by the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa, fixed the international avoirdupois pound at exactly 0.45359237 kilograms; the United Kingdom transposed the same value into domestic statute via the Weights and Measures Act 1963, which took effect on 31 January 1964. Since the kilogram itself was redefined in May 2019 against the Planck constant, the pound is today, by transitivity, anchored to a fundamental constant of nature rather than to any physical artefact.

Pounds remain the dominant everyday unit of weight across the United States. The Fair Packaging and Labeling Act and FDA labelling rules require net weight on consumer goods in both pounds-and-ounces and grams, and that dual layout has standardised the "lb / g" pair on virtually every American grocery shelf. The United Kingdom officially metricated for trade in 2000 but retains pounds in informal usage: butcher-counter signs, bathroom scales and doctor's-office shorthand still default to pounds, with body weight in NHS settings almost always quoted in stones and pounds (one stone is 14 lbs). Aviation worldwide records empty weight, fuel load and maximum take-off mass in pounds even in fully-metric jurisdictions, because Boeing-era certification documents and pilot training material were written in customary units and the entire airworthiness ecosystem inherited the convention. Combat sports — boxing, wrestling and mixed martial arts — denominate weight classes in pounds globally; professional boxing's heavyweight floor is 200 lbs. North American freight, dimensional lumber and fastener specifications still default to pounds, and US firearm cartridges measure projectile weight in grains (one seven-thousandth of a pound).

The English word "ounce" refers to four distinct units in 2026, three of mass and one of volume. The avoirdupois ounce (oz) is 1/16 of the avoirdupois pound, equal to exactly 28.349523125 g via the 1959 International Yard and Pound Agreement. This is the ounce of US food packaging, postal rates and everyday goods. The troy ounce (oz t, ozt) is 1/12 of the troy pound, equal to exactly 31.1034768 g — about 9.7% heavier than the avoirdupois ounce — and is the global trading unit for gold, silver, platinum and palladium, with spot prices on every major precious-metals exchange (LBMA, COMEX, Shanghai Gold Exchange, Tokyo Commodity Exchange) quoted in US dollars per troy ounce. The apothecaries' ounce, numerically identical to the troy ounce at 31.1034768 g, was the unit of British pharmacy weight under the 1618 Pharmacopoeia Londinensis system but was abolished from UK pharmacy by the Weights and Measures Act 1976; it survives only in occasional historical references and in some US compounding-pharmacy texts. The fluid ounce is a unit of volume rather than mass, and even within volume splits into the US fluid ounce at 29.5735 mL and the UK imperial fluid ounce at 28.4131 mL — a 4% gap that catches recipe transcription between the two systems. Neither fluid ounce should be confused with the mass ounces despite the shared word.

The ounce takes its name from the Roman uncia, the 1/12 subdivision of the libra (the Roman pound) used as both a mass and a linear measure: one uncia of mass was about 27.3 g, one uncia of length one twelfth of the Roman pes (foot), and the same word served both. The mass and length senses survived the empire as separate units, with the linear uncia becoming the inch in English and the mass uncia becoming the ounce — a divergence whose etymological echo survives in the shared root of the two modern words. Two parallel mass ounces emerged in late-medieval English commerce. The avoirdupois ounce, 1/16 of the avoirdupois pound, became the unit of general goods through the same merchant standardisation that fixed the avoirdupois pound. The troy ounce, named for the great medieval trading fair at Troyes in Champagne, was the unit of precious metals: a lighter pound and a 12-ounce subdivision were inherited from the French fair through Anglo-French commerce, and Henry VIII's Coinage Act of 1527 fixed troy weight as the legal standard for English coinage and bullion, the role it has held in English-speaking precious-metals trade ever since. A third ounce — the apothecaries' ounce, numerically identical to the troy ounce at 31.1034768 g but operating within an entirely separate pharmacy-weight system that subdivided into drachms, scruples and grains — was formalised in the Pharmacopoeia Londinensis published by the Royal College of Physicians in 1618 and remained the legal British pharmacy unit for the next three and a half centuries. The Weights and Measures Act 1976 abolished apothecaries' weight from UK pharmacy, leaving the troy ounce as the surviving 31.1-gram unit. The avoirdupois ounce, meanwhile, was given its modern precise value through the 1959 International Yard and Pound Agreement's fixing of the avoirdupois pound at 0.45359237 kg, making 1 oz exactly 28.349523125 g.

Precious-metals trading is the troy ounce's domain. Spot prices on the London Bullion Market Association (LBMA), the COMEX division of CME Group in New York, the Shanghai Gold Exchange and the Tokyo Commodity Exchange are all quoted in US dollars per troy ounce, and gold and silver bars sold to investors are stamped with their troy-ounce weight rather than any metric figure. The London Platinum and Palladium Market and the LBMA Silver Price likewise denominate in troy ounces. The avoirdupois ounce dominates US everyday-goods commerce. The Federal Trade Commission's Fair Packaging and Labeling Act and FDA labelling rules require net weight on consumer goods in pounds-and-ounces or ounces alone, with metric grams alongside, and US food-packaging weights — the 8-oz brick of cream cheese, the 16-oz peanut butter jar, the 5-oz can of tuna — are everyday avoirdupois figures. USPS first-class letter rates in 2026 step up at 1, 2 and 3.5 ounce thresholds, with the 1-ounce minimum the rate break embedded in nearly all US letter mailings. Boxing gloves are denominated in avoirdupois ounces by every major sanctioning body. The WBC, WBA, IBF and WBO standardise glove weight by class: 8-oz gloves for professional bouts up to 147 lb (welterweight), 10-oz gloves above that, with 12-, 14- and 16-oz gloves used in training and amateur competition. The 8/10-oz divide at welterweight is one of the few places in modern professional sport where the imperial unit is the binding contractual specification rather than a converted figure. US spirits and cocktail measurement uses the fluid ounce: a standard US "shot" is 1.5 fl oz (44.4 mL), and US bar-mix recipes denominate every ingredient in fl oz. Soft-drink cans in the US sell as 12 fl oz (354.9 mL), against the European 330 mL standard — a 25 mL gap per can between US Coca-Cola and the same brand's European equivalent.