How many ounces are in a cup, and why do US and UK recipes never agree?

Baking from an American blog with a British measuring set and it went wrong twice. How many ounces in a cup, really, and why is this so confusing?

Because there are three different cups and two different ounces, and recipes rarely say which.

Volume

• A US cup is 8 US fluid ounces, about 237 ml.
• A metric cup (Australia, most modern UK recipes, most measuring sets sold in Europe) is 250 ml.
• The old imperial cup was 10 imperial fluid ounces, about 284 ml; you still meet it in older British books.
• A US fluid ounce is 29.6 ml; an imperial fluid ounce is 28.4 ml. Close, but they add up over a recipe.

Weight is the other trap: "ounces" in an American recipe for flour or chocolate usually means weight (1 oz = 28.35 g), while "fluid ounces" means volume. A cup of flour weighs about 120 to 130 g depending on how you scoop; a cup of sugar about 200 g. Same cup, very different weights.

The fix is a kitchen scale and a conversion once: find the recipe's cup size (blog from the US: 237 ml), convert each ingredient to grams using a standard table, write the grams in the margin, and never think about it again. Bakers weigh; it is the only measurement that survives the Atlantic.