Four ounces is 0.1133980925 kilograms, digit for digit

Four ounces weighs 113.3980925 grams, which is 0.1133980925 kilograms. The arithmetic takes two steps: multiply 4 by 28.349523125 to reach grams, then divide by 1,000 to reach kilograms. Both factors are fixed by definition, so nothing rounds and no digit is lost along the way.

The ounce figure comes from NIST Special Publication 811, Appendix B.8, which lists the avoirdupois ounce at 28.349 523 125 g. That long decimal is not the result of a measurement. It is what falls out when a pound of 0.45359237 kg is cut into sixteen equal parts.

Run it backwards as a check. 0.1133980925 kg times 1,000 is 113.3980925 g, and 113.3980925 divided by 28.349523125 returns 4 exactly. A conversion that survives the return trip is one you can put on a customs form.

Two readings are worth memorizing because they turn up in places where a calculator is not handy. Four ounces is a quarter of a pound, so 0.25 lb, and it is 1,750 grains, because a pound carries 7,000 of them and an ounce carries 437.5.

The ounce is a sixteenth of a pound, and the pound is 0.45359237 kg

In 1959 the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa settled on one value for the yard and the pound. The pound became 0.45359237 kg exactly. NIST Handbook 44, Appendix C carries that figure as a legal definition rather than a laboratory result, which is why the digits have not moved since.

Divide by sixteen and the ounce appears: 0.45359237 / 16 = 0.028349523125. Divide by 1,000 for the kilogram and the chain closes: 4 x 0.028349523125 = 0.1133980925 kg. Sixteen ounces rebuild the pound, and 1,000 grams rebuild the kilogram, so the whole path is reversible.

The grain sits under both units and ties them together. A pound holds 7,000 grains, an ounce holds 437.5, and a grain is 64.79891 mg exactly. Four ounces is therefore 1,750 grains, the figure a bow scale or a handloading chart will show you when the same arrow or charge is weighed twice in two systems.

The troy ounce is a separate animal, 480 grains instead of 437.5. It weighs 31.1034768 g, so four troy ounces is 124.4139072 g, or 0.1244139 kg. That is 11.02 g heavier than four avoirdupois ounces, a gap of 9.7 percent that appears on every gold and silver invoice.

None of this is imperial guesswork. The old British ounce, the apothecary ounce and the Spanish ounce all weighed something different, and the 1959 agreement is what ended the argument for trade between the six countries that signed it. The modern ounce you buy coffee in is the avoirdupois one, 28.349523125 g, every time.

A stick of butter, a cup of flour, a scoop of coffee

113.4 g is small enough that cooks meet it daily without ever naming it. A US butter stick is a half cup and weighs 4 oz, so four ounces of butter is one stick, eight tablespoons, 113.4 g. Check it against a cup: a cup of butter is 227 g, and 113.398 / 227 = 0.4996.

Dry ingredients refuse to cooperate, because a cup measures volume while four ounces measures mass. All-purpose flour at about 125 g per cup puts 4 oz at 0.907 cup, near 14.5 tablespoons. Granulated sugar at 200 g per cup puts the same 4 oz at 0.567 cup. Identical weight, far different scoop, and that is the whole reason recipes list flour in grams once they get serious.

Coffee makes the point in the other direction. A level scoop of ground coffee holds about 10 g, so 4 oz of grounds brews roughly 11 cups at 10 g each: 113.398 / 10 = 11.34. Buy the same coffee in a 12 oz bag and you have 340.2 g, or 34 cups at that strength.

Chocolate is the usual flashpoint. A recipe calling for 4 oz of baking chocolate means 113.4 g, and most bars are sold in 100 g or 113 g sizes. A 100 g bar is 3.53 oz, so it comes up 13.4 g short, and that is a tenth of the chocolate in the recipe.

A nutrition label prints 113 g, and the serving math follows

The FDA requires a serving size in a household measure followed by the metric weight, and its guide to the Nutrition Facts label walks through the panel in that order. A four ounce portion therefore prints as 113 g on most packages, and as 112 g on the ones that rounded an ounce down to 28 g first.

The rounding is small but it is not free. Printing 113.398 g as 113 g drops 0.398 g, about 0.35 percent. Printing it as 112 g drops 1.398 g, about 1.2 percent, which is enough to shift a 200 calorie serving by two or three calories and enough to matter to someone counting a macro target across four servings a day.

Portion counting is where the number earns its keep. A 16 oz package holds 453.6 g, so at 4 oz (113.4 g) per serving it holds exactly 4 servings. A 24 oz family pack is 680.4 g and yields 6. A 12 oz box of frozen shrimp gives 4 portions at 3 oz each and 3 portions at 4 oz, with nothing left in the box either way.

Restaurant plates are a different scale. A 6 oz chicken breast is 170.1 g, and an 8 oz steak is 226.8 g. The 4 oz figure is the one dietitians use for a card deck of meat, and it is 113.4 g, not the 100 g that a metric recipe would call a portion.

Postage: four ounces is past the letter limit and into flats

A US First-Class Mail letter tops out at 3.5 oz, and a large envelope, called a flat, tops out at 13 oz on the USPS First-Class Mail page. Four ounces is 113.4 g, half an ounce over the letter ceiling and 30.8 percent of the flat ceiling. Work the percentage: 4 / 13 = 0.3077.

That half ounce costs a stamp. A padded mailer with a paperback, a phone case or a jar of jam usually lands between 4 and 6 oz, which pushes it into flat pricing or a Ground Advantage parcel, and the price difference is larger than the postage on a letter by a wide margin. Weigh before you seal, not after.

Postal scales in the US report ounces to 0.1 oz. A 4.0 oz reading is somewhere between 3.95 and 4.05 oz, which in grams is 112.0 to 114.8. A kitchen scale with 1 g resolution calls the same envelope 113 g, and 113 g converts back to 3.985 oz, so the two instruments agree to within a fiftieth of an ounce.

International mail uses grams outright. Many postal operators price in 100 g bands, so 113.4 g lands in the second band, not the first, and a shipper who guessed 100 g gets a surcharge at the counter. If you are filling out a customs form by hand, write 0.113 kg and 113 g, both of which are correct, and skip the ounce box entirely.

Four fluid ounces is 118.29 mL, a volume and not a weight

Fluid ounces measure space, not matter. The US fluid ounce is 29.5735295625 mL, so four of them fill 118.29411825 mL. A four ounce scoop of ice cream is a volume claim, a four ounce chicken breast is a weight claim, and the two only meet when a liquid has the density of water.

Water at 4 C comes close, at 0.99997 g/mL, so four fluid ounces of water weighs 118.3 g. That is 4.9 g heavier than 113.4 g. Milk at about 1.03 g/mL weighs 121.8 g, orange juice at 1.04 g/mL weighs 123.0 g, and honey at 1.42 g/mL weighs 168.0 g. The same 4 fl oz line on the label, four different masses.

The trap is a recipe that says 4 oz of milk. If it means fluid ounces, weigh 122 g. If it means weight ounces, weigh 113.4 g and you have measured 3.7 fl oz, a tenth of a cup less than the recipe author pictured. British recipes written before 1970 sometimes mean the imperial fluid ounce of 28.413 mL, which makes 4 fl oz 113.65 mL, close enough to the weight figure that the confusion survives another generation.

Troy ounces, carats and drams: four ounces in other trades

Bullion and coin dealers work in troy ounces. One troy ounce is 31.1034768 g, and its ratio to the food ounce is 1.097142857, so a customer who asks for four ounces of silver by weight gets an invoice reading 3.6458 troy ounces. Work: 113.3980925 / 31.1034768 = 3.645833.

Four troy ounces the other way is 124.4139072 g, or 0.1244139 kg. That is the number to use when a jeweler says four ounces and means the troy kind, and it is 9.7 percent more metal than the grocery ounce. On a 4 oz silver round the difference between the two readings is 11 g, which at any spot price is real money.

Carats divide the same mass a third way. A carat is 0.2 g, so four ounces is 566.99 carats, which is how a parcel of rough gemstone gets described before it is cut. A single one carat stone is 200 mg, so four ounces of rough yields 567 carats of material in theory and far fewer finished stones in practice.

Drams come in two flavors and both still appear on antique scales. The avoirdupois dram is 1.771845195 g, which makes four ounces exactly 64 drams. The apothecary dram is 3.8879346 g, which makes the same four ounces 29.17 drams. A vintage prescription bottle marked 4 oz may be using either, and the two differ by more than a factor of two.

Reading 0.113 kg back to ounces, and what the shortcuts cost

Going the other way, multiply kilograms by 35.27396195. For four ounces: 0.1133980925 x 35.27396195 = 4.0000. A kitchen scale reading 0.113 kg is telling you 3.985 oz, fifteen thousandths of an ounce under target, which is inside the tolerance of most 1 g scales.

Shipping desks round harder. A form that accepts 0.113 kg has already dropped 0.398 g, and a form that takes whole grams has dropped nothing much at all. Over a thousand parcels the 0.113 kg rounding loses 398 g in total, which only matters if a carrier bills by the gram, and almost none do.

The 28 g shortcut is the one to watch. It is fine for a shopping list and one and a quarter percent light on a customs declaration, which is exactly the kind of error that gets a parcel held. The 0.03 kg shortcut is worse, five and a half percent heavy, because 30 g per ounce is nearly a whole gram too much. NIST's table of metric prefixes explains why the kilo step is 1,000 and not 1,024, a mix-up that shows up on computer storage and almost never on weight.