How this instrument works
An ounce weighs one sixteenth of its parent pound, so its size is settled entirely by which pound you mean. Since 1 July 1959 only one answer has been worth using: the international avoirdupois pound of exactly 0.45359237 kilograms, agreed by standards laboratories across six English-speaking countries to retire national pounds that had drifted apart around their seventh decimal place. Halving that mass four times means dividing by powers of two, so nothing repeats — 0.45359237 ÷ 16 lands on 0.028349523125 kg, or 28.349523125 g, and stops there.
Grams sit on firmer ground still. One gram is one thousandth of the kilogram, and since May 2019 that kilogram has been defined by fixing Planck's constant at 6.62607015 × 10⁻³⁴ joule seconds, replacing the platinum-iridium cylinder kept in its vault outside Paris. Redefinition left this conversion untouched, because ounces were never lumps of metal: each one simply states so many kilograms, however that kilogram is realised in the laboratory.
You meet ounces on American nutrition panels, postal rate cards, boxing gloves rated 10 or 12 oz, denim sold by weight per square yard, and hospital notes recording newborns at 7 lb 6 oz. Grams handle nearly everything else, including all packaged-food labelling across Britain and Europe. One trap: jewellers quoting gold in ounces mean troy ounces of 31.1034768 g, roughly one tenth heavier than kitchen ounces.
- Type your figure into the Ounces (oz) field; 8 oz — half of one pound — arrives preloaded.
- Read the Grams (g) line below it; results recompute on every keystroke, with no button to press.
- Fractions are fine: 1.5 oz, 0.25 oz and 12.75 oz all work, since that field steps in thousandths.
- Working from pounds and ounces? Multiply pounds by 16 and add before typing — 3 lb 4 oz is 52 oz.
- Going backwards, divide any gram figure by 28.349523125: 500 g returns 17.637 oz, just over one pound.
Worked example — an 8 oz block, weighed in grams
Your kitchen scale reads grams only, and the recipe in front of you calls for an 8 oz block of cream cheese. Put 8 into the Ounces (oz) field and Grams (g) returns 226.796185 — half of one pound exactly, since 8 is half of 16.
Nothing was rounded before display: 8 × 28.349523125 gives precisely 226.796185 g, and that decimal terminates on its own. Set your scale to 227 g and you sit 0.2 g heavy, which no cheesecake will ever detect. Supermarket packets tend to print 226 g, rounded down by manufacturers who wanted tidier numbers on the foil.
Questions
Is 28.349523125 exact, or has it been rounded?
Exact. It follows from definition rather than measurement: the international avoirdupois pound is precisely 0.45359237 kg, and one ounce is one sixteenth of that. Because sixteen is two to the fourth, dividing yields decimals that terminate after nine places, so 28.349523125 g per ounce carries no uncertainty whatever. Any rounding you notice happens at the very last step, when results are printed to fixed width.
Is the troy ounce the same thing?
No, and that gap matters whenever money is involved. One troy ounce weighs exactly 31.1034768 g — about 9.7% heavier — and gold, silver, platinum and palladium are all priced in it. Troy weight also subdivides differently: 12 troy ounces make one troy pound, against 16 avoirdupois ounces to the avoirdupois pound. This sheet handles avoirdupois, which is what food, post and freight mean by ounce.
Do fluid ounces weigh an ounce?
Only for water, and really only in Britain. The imperial gallon was originally defined as ten pounds of water, so an imperial fluid ounce — one 160th of that gallon — holds very nearly one ounce of it. Its US namesake holds 29.5735295625 mL, or roughly 1.043 oz of water. Either way, fluid ounces measure volume, so 8 fl oz of honey and 8 fl oz of oil weigh nothing alike. Recipes mixing both senses ruin more baking than bad ovens do.
Why do American nutrition labels print 28 g for one ounce?
Because labelling rules allow servings to be declared in whole grams, and 28.349523125 rounds down to 28. Such labels claim no equivalence; they state serving size to convenient precision. Scale it up and the shortfall shows: an 8 oz portion declared as 224 g falls nearly 3 g short of its true 226.796185 g.
Is the British ounce different from the American one?
Not since 1959, when America, Britain, Canada, Australia, New Zealand and South Africa adopted one shared pound of 0.45359237 kg. Before that, their two ounces differed by little more than one part in ten million — irrelevant at the bread counter, awkward inside standards laboratories. Other imperial measures never converged so neatly, though: pints and fluid ounces still differ across the Atlantic today.
How can I turn grams back into ounces by hand?
Divide by 28.349523125, or multiply by its reciprocal 0.0352739619…, which never terminates. Rough anchors help: 100 g sits just over 3.5 oz, 250 g runs about 8.8 oz, and one kilogram comes to 35.27 oz, slightly more than two pounds. For anything certified or dispensed, use every digit of that factor rather than the old shortcut 28.35.