How this instrument works
A gram is one thousandth of a kilogram, and since 20 May 2019 that kilogram has been defined by fixing Planck's constant at exactly 6.62607015 × 10⁻³⁴ joule-seconds. Before that morning it was one platinum-iridium cylinder held at Sèvres outside Paris, and every gram on Earth traced its size to that single object. Redefinition changed nothing about how heavy one gram is; it changed only what grams are anchored to.
An ounce, unless someone says otherwise, means an avoirdupois ounce — one sixteenth of an international pound. That name comes from Old French aveir de peis, goods sold by weight, and it reached England with medieval wool merchants. Its modern size was settled on 1 July 1959, when six national standards bodies agreed to define a pound as exactly 0.45359237 kilograms; one sixteenth of that is 28.349523125 g, certain in every digit. The grain was left alone at 64.79891 mg, and 7000 grains to the pound still multiplies back out to 453.59237 g.
Invert 28.349523125 and you get 0.03527396194958…, whose digits never close, because reciprocals are not definitions; twelve significant figures of it drive this sheet. Grams rule nutrition panels, pharmacy counters and laboratory benches, while ounces persist in American kitchens, in boxing gloves sized 8 to 16, in denim sold as 14 oz per square yard, and in postal rates that step up one ounce at a time. Packaging is where both systems meet, since US law wants net contents declared in customary and metric side by side.
- Enter your mass in the Grams (g) field. It opens at 100 g, a quantity European labels lean on as their reference serving.
- Read Ounces (oz) directly below; it recalculates on every keystroke and carries six decimal places.
- Working backwards from ounces? Multiply your figure by 28.349523125 to land in grams — that reciprocal is the exact one.
- Pricing gold, silver or platinum? Bullion trades in troy ounces of 31.1034768 g, so divide by that figure instead.
Worked example — 100 g of butter, in American units
A Belgian recipe asks for 100 g of butter. Put 100 into Grams (g) and Ounces (oz) answers 3.527396, or 3.52739619496 written out in full. An American stick of butter weighs 4 oz, so you want a shade past seven eighths of one: cut at 3.5 oz on the printed wrapper, then add a shaving.
Those trailing digits are not noise; they descend from a legal definition. Rounding to 3.53 oz costs about 0.07 g on a 100 g portion, which no kitchen scale will ever show you. Trusting folk arithmetic that calls an ounce 30 g, though, turns 100 g into 3.33 oz — more than five percent light, enough to matter for salt in dough or a weighed espresso dose.
Questions
Is one ounce exactly 28.349523125 grams?
Yes, exactly — a legal definition rather than a measured quantity. The 1959 international yard and pound agreement fixed an avoirdupois pound at 0.45359237 kg, and one ounce is a sixteenth of it, so every digit of 28.349523125 g is certain. Only the direction shown on this sheet, 0.0352739619496 ounces per gram, involves any rounding, because dividing 1 by 28.349523125 produces an unending decimal.
Are American and British ounces the same size?
For mass, yes — both countries signed in 1959, so an avoirdupois ounce weighs 28.349523125 g on either side of the Atlantic. Divergence starts above and below that unit: a British ton is 2240 lb against America's 2000 lb, a stone of 14 lb is British-only, and fluid ounces differ outright. Before 1959 both pounds disagreed around the seventh decimal place, which mattered to standards laboratories and almost nobody else.
Does this converter work for gold and silver?
No. Precious metals trade in troy ounces, which weigh 31.1034768 g — roughly ten percent more than an avoirdupois ounce. A troy ounce holds 480 grains where an avoirdupois ounce holds 437.5, a leftover from two medieval systems that shared a grain and little else. Divide grams by 31.1034768 for bullion, and remember a troy pound counts only 12 ounces, not 16.
Is a fluid ounce the same as an ounce?
No — a fluid ounce measures volume, and both units only look related because an imperial fluid ounce was built to hold one ounce of water. An imperial fluid ounce is 28.4130625 mL; an American one is about 29.5735 mL. Away from water they part company fast: 8 fl oz of honey weighs roughly 12 oz, while 8 fl oz of flour weighs about 4.5 oz. Recipes written by mass sidestep that trap entirely.
How wrong is the rule of thumb that an ounce is 28 grams?
It runs about 1.2 percent low. Trimming 28.349523125 down to 28 shortens every ounce by 0.35 g, so a 16 oz pound arrives at 448 g instead of 453.6 — a 5.6 g gap any digital scale will display. For one sandwich portion nobody cares; across a 20 kg batch of dough, or for anything priced by mass, that shorthand quietly costs more than a percent.
Why does the result carry six decimal places?
Because arithmetic is free and truncating early hides useful information. Nothing you weigh at home resolves to a microgram, but agreement out to six decimals is a quick way to confirm that two published tables started from identical definitions. Round the displayed figure to whatever your scale can honestly read: one or two decimals for kitchen work, three for a jeweller's balance.