The American kitchen weighs flour in ounces and the rest of the world weighs it in grams, so every imported recipe, every nutrition label, and every package from abroad becomes a small translation exercise. The exchange rate is 28.3495 — one avoirdupois ounce equals that many grams — and it is the single number that makes the two systems talk to each other. Enter 10 in the Ounces (oz) box and the Grams (g) readout shows 283.495; the bonus boxes translate the same weight into 0.283495 Kilograms (kg) and 0.625 Pounds (lb), the everyday triple every American cook learns by accident. Multiply ounces by 28.3495 and you have grams. That is the whole operation, and this page runs it for every box at once.

Why 28.349523125, down to the ninth decimal? Because the ounce is not defined directly — the pound is. In 1959, the United States, the United Kingdom, and other English-speaking nations signed the International Yard and Pound Agreement, defining the avoirdupois pound as exactly 453.59237 grams. A pound holds exactly 16 ounces, so one ounce is 453.59237 divided by 16, which is 28.349523125 grams. That definition is exact by international treaty, which is why every converter on earth agrees to the tenth decimal. For cooking and mailing, 28.35 is plenty. For legal metrology, the full nine decimals stand ready.

The ounce is older than the United States by about two thousand years. It descends from the Roman uncia, one-twelfth of the Roman libra — the same libra that gives the pound its "lb" abbreviation. The avoirdupois ounce, the one on your kitchen scale, came through medieval French "aveir de pois," meaning "goods of weight," the system merchants used for everything except precious metals and medicine. English kings standardized it repeatedly over the centuries before the 1959 treaty finally fixed it at 28.349523125 grams worldwide. When you convert ounces to grams, you are translating between ancient Rome and the metric system in a single multiplication.

Is an ounce exactly 28 grams? Not quite — 28 grams runs about 1.2 percent short of a true ounce, a gap that matters in some places and not at all in others. Rounding to 28 is fine when you are eyeballing a recipe or estimating luggage weight, because a 1.2 percent error disappears into the noise of every other measurement in the kitchen. It is not fine when precision is the product: bakers scaling a commercial formula, pharmacies compounding doses, and shippers paying per ounce should use 28.3495 or better. The honest rule is to match your precision to the stakes, and this converter keeps the full precision so you never have to think about it.

A ladder of anchor conversions covers nearly everything you will ever need, and it is worth walking once so the numbers feel familiar. An eighth of an ounce is 3.54 grams. A quarter ounce is 7.09 grams. A half ounce is 14.17 grams. One ounce is 28.35 grams. Two ounces is 56.70 grams. Four ounces is 113.40 grams. Six ounces is 170.10 grams. Eight ounces is 226.80 grams. Twelve ounces is 340.19 grams. Sixteen ounces is 453.59 grams — exactly one pound. Thirty-two ounces is 907.18 grams, two full pounds. The quarter-pound and half-pound marks are the ones to memorize: 4 oz = 113.4 g and 8 oz = 226.8 g show up on butter wrappers, chocolate bars, and coffee bags across the country.

The mental-math trick everyone should know is multiply by 28, then nudge. For everyday estimates, ounces times 28 lands within 1.2 percent of truth: 8 ounces is roughly 224 grams against an actual 226.8, and 12 ounces is roughly 336 grams against an actual 340.19. When you need closer, use the times-30-minus-5-percent route: multiply ounces by 30, then subtract a twentieth. Ten ounces times 30 is 300, minus 15, gives 285 grams — within a gram and a half of the true 283.5. Both tricks beat reaching for a phone in the grocery aisle, and both lose honestly at scale, so run anything over a pound through the converter.

In commercial spice, tea, and botanical trades, a quarter ounce is standardized to exactly 7 grams, a half ounce to 14 grams, and a full ounce to 28 grams. The true values are 7.087, 14.175, and 28.35, but the trade rounds down for packaging simplicity and scale calibration. If you buy bulk tea or spices, the bag marked "1 oz" holds 28 grams by trade convention — close enough that no recipe will notice, and honest enough that everyone in the trade knows the rounding exists.

The 16-ounces-to-a-pound identity is the mental anchor worth memorizing because packaging on both sides of the Atlantic revolves around it. A pound of coffee is 453.59 grams, which is why American bags say 16 oz and European bags say 500 grams — the European bag is about 10 percent heavier, a real difference when you are comparing per-bag prices. A two-pound bag of flour is 907.18 grams, just under a kilogram. Sugar sold in 4-pound bags is 1,814.37 grams. Whenever a price comparison crosses the Atlantic, convert to grams per dollar first; the packaging sizes are designed to look equivalent when they are not.

The kitchen is where this conversion earns its keep daily. A stick of butter is 4 ounces, which is 113.4 grams — European recipes that call for 115 grams of butter are asking for a stick, close enough that no one will notice. Eight ounces of chocolate, the classic baking quantity, is 226.8 grams, usually rounded to 227 on the package. Cream cheese comes in 8-ounce blocks at 226.8 grams. A cup of all-purpose flour weighs about 4.25 ounces, or 120.5 grams, which is exactly why weight beats volume: a scooped cup can vary by 20 percent depending on technique, but 120 grams is 120 grams everywhere on earth. Flour, cocoa, and powdered sugar are the ingredients where the gram side wins outright, and every serious baker eventually defects to the metric side of the scale.

Mailing is the other daily use, and it has its own rules. The US Postal Service prices First-Class letters by the ounce: one ounce covers the standard stamp, and every additional ounce costs more. One ounce is 28.35 grams, so a letter weighing 30 grams — a common European paper-weight benchmark — is just over an ounce and tips into the next price tier. The professional habit is to round up to the next whole ounce before pricing, because the scale at the post office rounds against you. For packages the same math scales up: a 16-ounce parcel is 453.59 grams, just under the half-kilogram mark that international shippers use as their own first tier.

Pediatric medicine lives in ounces and grams simultaneously. A newborn weighing 7 pounds 8 ounces is 120 ounces, which is 3,401.94 grams — and hospital charts record the grams, because medication doses are calculated per kilogram of body weight. Parents hear ounces; the chart says grams; the conversion between them is 28.3495, and it is one of the few places where the full precision genuinely matters. A 10 percent dosing error on an infant is dangerous, which is why hospitals weigh in grams and convert for the parents, never the reverse.

Airline weight limits are posted in pounds and kilograms, but the ounce math hides underneath. A 50-pound checked-bag limit is 800 ounces, which is 22,679.62 grams — 22.68 kilograms. Travelers with a kitchen scale and no luggage scale can weigh a bag's contents in ounces and multiply by 28.35 to check against the kilogram limit. A 15-pound carry-on allowance is 240 ounces, or 6,803.89 grams. The arithmetic is trivial; the overweight-bag fee it avoids is not.

The trap that catches everyone once is the fluid ounce. Ounces measure weight; fluid ounces measure volume, and they are different units that happen to share a name. One US fluid ounce is 29.5735 milliliters — a volume — and it only converts to grams once you know the density of what you are measuring. A fluid ounce of water weighs about 29.57 grams, which is suspiciously close to one dry ounce's 28.35 grams, and that coincidence is exactly why the confusion persists. But a fluid ounce of honey weighs about 42 grams, and a fluid ounce of olive oil weighs about 27 grams. Recipes that say "8 ounces of flour" mean weight; recipes that say "8 fluid ounces of milk" mean volume. When a recipe is ambiguous, the ingredient decides: flour, sugar, and chocolate are weight; milk, water, and oil are volume.

Jewelers and pharmacists use a different ounce entirely — the troy ounce, at 31.1035 grams, about 9.7 percent heavier than the avoirdupois ounce. Gold, silver, platinum, and palladium are priced per troy ounce worldwide, so if a gold price looks wrong by ten percent, you are probably converting with the wrong ounce. The troy ounce comes from the medieval French town of Troyes, once home to a great trading fair, and it survived because precious-metals markets never adopted the avoirdupois system. For food, mail, luggage, and body weight, the troy ounce never appears; for anything that glitters, it is the only ounce that counts.

Nutrition labels add one more wrinkle worth knowing. The FDA lets serving sizes round to convenient numbers, so a label that says "1 oz (28 g)" is rounding the true 28.35 grams down slightly. Protein powders, nuts, and snack bags all do this, and the rounding is harmless at single-serving scale. It only matters when you scale up: thirty servings at 28 grams is 840 grams, but thirty true ounces is 850.49 grams — a 10.5-gram drift that dietitians tracking macros should know exists even if they rarely need to correct for it.

For students, the conversion is a clean dimensional-analysis exercise. Ounces times grams-per-ounce gives grams, and the units cancel the way they should: 5 oz times 28.3495 g/oz equals 141.75 g, with the ounces canceling out. Working backward, grams divided by grams-per-ounce gives ounces: 141.75 g divided by 28.3495 g/oz equals 5 oz. Teachers use this pair to drill the habit of writing units at every step, because the moment you drop the units is the moment fluid ounces sneak in and corrupt the answer.

A final practical note: most digital kitchen scales toggle between ounces and grams with one button, and the gram side is the more honest display. Ounce readouts often round to the nearest tenth or even quarter ounce — a quarter-ounce rounding step hides 7 grams of error — while gram readouts resolve to single grams. When a recipe gives both units, weigh in grams. When it gives only ounces, convert here first, then weigh in grams. The scale does not care which system you grew up with; it only cares which one you asked it to use.

The pocket reference to keep is small. One ounce is 28.35 grams. Sixteen ounces is a pound, 453.59 grams. Multiply ounces by 28 for the grocery aisle and by 28.3495 for the recipe. Keep fluid ounces and troy ounces in their own lanes, round up for the post office, and weigh your flour. The ounce is an old unit with a modern definition, frozen by treaty in 1959, and it is not going anywhere — but with one number memorized, it never has to slow you down.

With the pre-filled defaults — Ounces (oz) 10 — the Ounce to Gram returns Grams (g) 283.4950, Kilograms (kg) 0.283495, Pounds (lb) 0.6250. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.