Article — Ounce to Kg Converter
Ounce to Kg: an Exact Conversion, Defined by Treaty
One avoirdupois ounce equals exactly 0.028349523125 kilograms, or 28.349523125 grams. The number is not a measurement, it is a treaty value fixed by the 1959 International Yard and Pound Agreement. 16 ounces make one pound, and 1 pound is exactly 0.45359237 kg. Dividing by 16 gives the ounce value used in this calculator.
Most people who land on an ounce to kg converter are working through a recipe, a US shipping label, or a food package designed for the American market. The math is bidirectional and exact, so the rounded answer depends entirely on how many decimals you ask for. The defaults below match food-label and pharmacy standards.
One ounce to kg, the exact number
The avoirdupois ounce, the version used in US commerce and on food labels, equals 0.028349523125 kg. Sixteen of them make 0.45359237 kg, which is one international pound. The factor is exact to the last digit shown, because the underlying pound is defined and not measured.
For mental arithmetic, the shortcut is to divide ounces by 35.27 to get kilograms. 100 oz comes out at 2.835 kg, which matches the exact answer to four figures. Going the other way, multiply kilograms by 35.27 to get ounces. 1 kg is 35.27 oz, or 2 lb 3.27 oz in pound-ounce form.
The word avoirdupois comes from the medieval French phrase aveir de peis, meaning goods of weight. It was the system English merchants used in the wool and spice trade to distinguish bulk goods sold by mass from precious metals sold in troy ounces. The two systems have coexisted since the 14th century and still do.
Why the ounce to kg factor is exact
Before 1959 the United States and United Kingdom used pounds that were slightly different from each other. The US pound was 0.4535924277 kg, the UK pound was 0.45359243 kg. The discrepancy was tiny but real, and it showed up in scientific work and international trade as a fractional milligram drift over time.
In 1959 the US, UK, Canada, Australia, New Zealand, and South Africa signed the International Yard and Pound Agreement, which fixed both units at a single value: 1 pound = 0.45359237 kg, 1 yard = 0.9144 m, both exact. The treaty made every derived unit exact too: the ounce became 0.028349523125 kg, the dram became 1.7718451953125 g, the hundredweight became 50.80234544 kg. Today every ounce to kg converter on the Internet ultimately traces back to that one treaty.
1 oz = 0.0283 kg4 oz = 0.1134 kg8 oz = 0.2268 kg16 oz (1 lb) = 0.4536 kg32 oz (2 lb) = 0.9072 kg128 oz (8 lb) = 3.6288 kgKitchen ounce to kg
American recipes use ounces and pounds. European recipes use grams and kilograms. The ounce to kg converter sits between them when a recipe travels. A 4-ounce stick of butter is 113 g. An 8-ounce cup of liquid is 227 g if it is water, more if it is heavier ingredients. A 12-ounce soda can is 340 mL or grams of water, which is where the round metric value of 330 mL comes from on European cans.
The pound-ounce notation that US recipes still use needs a second step. A 1 lb 6 oz turkey breast is (16 + 6) = 22 oz total, or 22 × 0.0283 = 0.624 kg. Most kitchen scales now show both grams and ounces and do the conversion internally, but old paper recipe books and US food blogs have not caught up.
If a recipe gives weights in ounces and you are halving or doubling it, convert to grams first. Halving 7 oz gives 3.5 oz, which is harder to measure than the equivalent 99 g. Most kitchen scales read precisely to 1 g; few read to 0.1 oz. Working in grams reduces compounding rounding error in scaled-down or scaled-up recipes.
Postal ounce to kg and how rates round
The US Postal Service prices First-Class Mail in 1-ounce increments up to 3.5 oz for letters, then jumps to 13 oz as the upper boundary for First-Class Package Service. Priority Mail switches to pounds at 16 oz. Domestic parcels cap at 70 lb (31.75 kg) for most services.
International shipping inverts the convention. The Universal Postal Union and most foreign postal services price in 100 g or 500 g brackets. A 14-ounce package that fits comfortably in the US Priority bracket is 397 g, which sits in the 400 g international bracket. Converting between the two is the single most common reason an ounce to kg converter gets opened by US-based eBay and Etsy sellers.
When shipping internationally, weigh the parcel in grams on a digital scale, then convert to ounces only if a US service needs the value. The metric reading is always the more precise one because international rate brackets are tighter than US ones.
Troy vs. avoirdupois ounce
The troy ounce equals 31.1034768 g, the avoirdupois ounce 28.349523125 g. The troy ounce is heavier by about 9.7%. Trade convention has hardened over centuries: gold, silver, platinum, and palladium are always quoted in troy ounces; everything else, including pharmaceuticals and food, uses avoirdupois.
This calculator uses the avoirdupois ounce, which is the right choice for shipping, cooking, and consumer goods. A gram of gold weighs the same whichever ounce you choose to express it in — the gram is the same gram — but the ounce value will differ. A 100 g gold bar is 3.527 avoirdupois oz or 3.215 troy oz. Quoting it in the wrong unit overstates or understates the weight by 10%, which becomes a real financial error very quickly.
Fluid ounce vs. weight ounce
The fluid ounce is a unit of volume; the weight ounce is a unit of mass. They share the name but measure different things. The US fluid ounce is 29.5735 mL. The avoirdupois ounce is 28.3495 g. Water at room temperature happens to weigh almost exactly one weight ounce per fluid ounce, which is why the confusion persists in casual cookery.
For any other substance the two diverge. 1 fluid ounce of honey weighs 1.42 oz on a kitchen scale. 1 fluid ounce of olive oil weighs 0.92 oz. The calculator above handles only the weight ounce. For volume conversions, use the dedicated fluid ounce to mL or fluid ounce to L converter.
Common ounce to kg mistakes
The most frequent ounce to kg mistake is using the troy factor on consumer goods, which inflates the answer by 10%. Always check the substance: if it is not a precious metal, use the avoirdupois factor.
The second is rounding the conversion factor too aggressively. Truncating 0.028349523125 to 0.028 introduces 1.2% error per ounce; over a 50 lb shipment that is almost 10 oz of phantom weight. The calculator carries the full factor internally and rounds only on display.
The third is confusing avoirdupois ounces with fluid ounces, which is volume not mass. A US recipe that says 8 oz of milk usually means 8 fluid oz (about 237 mL), not 8 weight oz of milk. Context decides which one is meant.