Three units for the same thing, and only one of them is defined in terms of the other two by law rather than by measurement.
The exact numbers
An avoirdupois ounce — the ounce used for weighing food, mail and almost everything else in everyday English-language use — is defined as exactly 28.349523125 grams. A pound is exactly 453.59237 grams, and because a pound is defined as 16 ounces, the ounce figure above follows directly from the pound rather than the other way around. Both numbers have been fixed by international agreement since 1959; nothing about them is measured or approximate.
A practical table
For everyday reading, the numbers round cleanly enough to work from:
| Grams | Ounces | Pounds |
|---|---|---|
| 10 g | 0.35 oz | 0.02 lb |
| 28.35 g | 1.00 oz | 0.06 lb |
| 50 g | 1.76 oz | 0.11 lb |
| 100 g | 3.53 oz | 0.22 lb |
| 250 g | 8.82 oz | 0.55 lb |
| 453.6 g | 16.00 oz | 1.00 lb |
| 500 g | 17.64 oz | 1.10 lb |
| 1000 g | 35.27 oz | 2.20 lb |
A large egg, about 57 grams, is close to 2 ounces. A banana, about 118 grams, is a little over 4 ounces. Heavier objects read more naturally in pounds: a grapefruit at 468 grams is just over a pound, a closed umbrella at 651 grams is about 1.4 pounds, and a cordless drill with its battery attached, at 1208 grams, comes to roughly 2.7 pounds.
Ounce is not fluid ounce
A weight ounce and a fluid ounce share a name and nothing else. A fluid ounce measures volume — about 29.57 millilitres in the US system, 28.41 in the UK — and what it weighs depends entirely on what fills it: a fluid ounce of water weighs close to an ounce, a fluid ounce of honey weighs nearly half again as much, and a fluid ounce of flour weighs a good deal less. A recipe that says “8 oz” without the word “fluid” almost always means weight; a measuring cup marked in fluid ounces is measuring the other thing.
Why gram recipes are easier to follow
A recipe written in cups asks the cook to also get packing right — a cup of flour spooned in weighs about 125 grams, the same cup scooped from the bag can weigh 150 — so two cooks measuring “one cup” from the same bag can end up with different amounts of flour before either touches a mixing bowl. A recipe written in grams removes that step entirely: 125 grams of flour is 125 grams of flour, on any scale, regardless of how it got there. That is the real advantage behind the shift from cups to grams that many recipe sites have made — not that grams are a foreign unit, but that a weight leaves no room for how loosely something was packed into a cup.