What an ounce weighs, and which ounce you mean
The avoirdupois ounce — the one on food packaging, postal scales and every recipe outside the metric world — is exactly 28.349523125 grams. That precision is not an affectation. The international pound was fixed at exactly 0.45359237 kg in 1959, and the ounce is exactly one sixteenth of it, so the conversion carries no measurement uncertainty. Whether you round it to 28.35 g for a recipe or carry all eleven digits for a laboratory reagent is your choice, not the unit's.
Two other ounces exist and neither is interchangeable with this one. The troy ounce, at 31.1034768 g, is used exclusively for gold, silver, platinum and palladium; it is 9.71% heavier than the avoirdupois ounce, so a price quoted per ounce means measurably different amounts of metal depending on which system the quote follows. The fluid ounce is not a mass at all — it measures volume, 29.5735 mL in the US system, and only coincides with the weight ounce for water, and even then only roughly.
The system's odd internal numbers come from its history. There are 16 ounces to the pound, 7,000 grains to the pound, and therefore 437.5 grains to the ounce. The grain, at exactly 64.79891 mg, is the one unit shared by the avoirdupois, troy and apothecary systems, which is why it survives in ammunition, pharmacy and precious-metal assay long after the rest of those systems disappeared.
The arithmetic, and the pounds-and-ounces split
Ounces to grams is one multiplication by 28.349523125. Grams to ounces is the division. For quick kitchen work, 28.35 g per ounce is accurate to one part in 20,000, which is far finer than any domestic scale resolves.
The subdivisions all follow from the pound without new constants. Sixteen ounces make a pound, so pounds to grams is 453.59237. A thousand grams make a kilogram, so a kilogram is 1,000 ÷ 28.349523125 = 35.27396 ounces, or 2.20462 pounds. Grains come from the 7,000-per-pound rule: 453.59237 ÷ 7,000 = 0.06479891 g exactly.
To produce a scale-style reading, split the decimal pound figure. Take the whole-number part as the pounds, multiply the decimal remainder by 16, and that is the ounces. One kilogram is 2.204623 lb, so it is 2 whole pounds plus 0.204623 × 16 = 3.27 ounces — a postal scale would show 2 lb 3.3 oz. Watch the boundary: a remainder that rounds up to 16.00 oz must be carried into the next whole pound, which this calculator does rather than displaying an impossible reading.
Going the other way, from a scale reading to grams, convert both parts and add: 3 lb 5 oz is (3 × 453.59237) + (5 × 28.349523125) = 1,360.777 + 141.748 = 1,502.52 g. The alternative route — 3 lb 5 oz is 53 ounces, and 53 × 28.349523125 = 1,502.52 g — gives the same answer and is usually faster.
Worked example: an 8 oz block of butter in a metric recipe
A US recipe calls for 8 ounces of butter and 4 ounces of flour, and your kitchen scale reads only grams.
- The butter. 8 × 28.349523125 = 226.796 g. Most metric recipes would write this as 225 g, a 0.8% difference that no butter recipe will notice.
- The flour. 4 × 28.349523125 = 113.398 g, usually written as 115 g or 110 g depending on the author's rounding habit.
- The total. 12 oz of ingredients is 12 × 28.349523125 = 340.194 g. Check: 226.796 + 113.398 = 340.194. Converting each ingredient separately and converting the total must agree, because multiplication distributes over addition.
- As a scale reading. 340.194 g is 340.194 ÷ 453.59237 = 0.75 lb, so 0 whole pounds plus 0.75 × 16 = 12.00 oz, which is where you started.
- The rounding decision. If you round each ingredient to 225 g and 115 g you get 340 g, which is within 0.06% of the exact total. Rounding is safe here precisely because the recipe's ratio, not its absolute mass, is what governs the result — see the baker's percentage calculator for the formal version of that idea.
How much precision the number deserves
Match the digits to the instrument and to the consequence. A domestic kitchen scale resolves to 1 g or 5 g, so quoting 226.796 g is theatre; write 227 g, or 225 g if you are following a recipe's own rounding. A postal scale resolves to 0.1 oz, and postage bands are set at whole ounces, so what matters there is which side of a boundary you fall on rather than the decimal itself.
Two contexts do deserve the full precision. Pharmaceutical and laboratory work uses the exact factor because doses and concentrations chain through several calculations, and rounding early propagates. Precious metals use it because the money is large: at typical bullion prices, mistaking a troy ounce for an avoirdupois ounce misstates the metal by 9.7%, which dwarfs any dealer's spread.
For food and cooking, the more useful discipline is to weigh rather than to measure by volume in the first place. A cup of flour can vary by 20% depending on whether it was scooped or spooned, while 120 g of flour is 120 g of flour every time. If your recipe gives volumes and you would rather weigh, the cups to grams ingredient calculator applies each ingredient's density; this page handles the case where the recipe already gives you a weight in the other system.
Ounces to grams for common recipe and package quantities
| Ounces | Grams (exact) | Rounded to 5 g | Also equals |
|---|---|---|---|
| 1 | 28.350 | 30 | 437.5 grains |
| 2 | 56.699 | 55 | ⅛ lb |
| 3 | 85.049 | 85 | |
| 4 | 113.398 | 115 | ¼ lb |
| 5 | 141.748 | 140 | |
| 6 | 170.097 | 170 | ⅜ lb |
| 8 | 226.796 | 225 | ½ lb |
| 10 | 283.495 | 285 | |
| 12 | 340.194 | 340 | ¾ lb |
| 16 | 453.592 | 455 | 1 lb |
The rounded column is nearest-5 g, so it drifts most at small quantities: 30 g against an exact 28.350 g is 5.8% high at 1 oz, while 455 g against 453.592 g is only 0.31% high at 16 oz. Use the exact column whenever the ratio between ingredients matters more than convenience.
Mistakes that make a weight conversion wrong
- Using a troy ounce for anything but precious metal. At 31.1035 g it is 9.71% heavier than the avoirdupois ounce. Gold, silver, platinum and palladium are troy; food, freight and body weight are avoirdupois.
- Confusing fluid ounces with weight ounces. A fluid ounce is 29.5735 mL of volume. For water the two are within about 4%, and for oil, honey or flour they are not close at all. See the fluid ounces to millilitres calculator.
- Reading “3 lb 5” as 3.5 lb. It means 3 pounds and 5 ounces, which is 3.3125 lb. The mixed-radix format traps the same people that stones and pounds does.
- Rounding every ingredient in the same direction. Rounding each of ten ingredients up to the nearest 5 g shifts the batch size and, worse, shifts the ratios. Round to the nearest value, and where a ratio matters, work from the exact figures.
- Assuming a package's net weight is its drained weight. A tin labelled 15 oz is 425 g of contents including liquid; the drained solid can be materially less. The label tells you which is declared.
- Weighing on an uncalibrated scale and blaming the conversion. The factor is exact. If two scales disagree, the instruments differ — check with a known reference mass before assuming an arithmetic error.
Related weight tools and where they take over
Above a few pounds, the natural pairing shifts from ounces-and-grams to pounds-and-kilograms, and the kilograms to pounds calculator is the better tool — it also handles stones for UK body weight. Below a gram, grains and milligrams take over, which is the territory of ammunition loads and pharmacy.
For precious metals, use the troy system throughout rather than converting into it at the end. The troy ounce to grams calculator covers troy ounces, pennyweights and grains, and the gold karat alloy calculator handles the further question of how much of a given mass is actually gold.
In the kitchen, the deeper problem this page only touches is that recipes mix systems freely: weights in ounces, volumes in cups, temperatures in Fahrenheit. Converting them piecemeal is where errors creep in. The cooking measurement conversion calculator covers the volume side across four national systems, and the recipe scaling calculator resizes a whole formula without letting the ratios drift.