The trap: two kinds of ounce
This is the single most common conversion mistake in baking. A fluid ounce is a volume: 1 US cup is defined as 8 US fluid ounces, full stop. A weight ounce (avoirdupois) is a mass: 28.3495 g. The two coincide only for water, where 1 fluid ounce weighs about 1.04 weight ounces. For everything else they diverge — sometimes wildly — because density decides how much mass fits in a cup.
One cup, five weights
| 1 US cup of | Typical weight | In ounces |
|---|---|---|
| Water | 236.6 g | 8.35 oz |
| Butter | 227 g | 8 oz |
| Granulated sugar | 200 g | 7.05 oz |
| Honey | 340 g | 11.99 oz |
| All-purpose flour (spooned & levelled) | 120 g | 4.23 oz |
The spread is the point: the same cup holds anywhere from 4.23 oz to nearly 12 oz depending on what is in it. Any guide that answers “8 ounces” without asking what is in the cup is answering the fluid-ounce question and hoping you meant water.
Why butter is the exception that proves the rule
Butter is the one dry-ish ingredient where 1 cup really does equal 8 oz, because a US stick of butter is 4 oz and two sticks fill a cup almost exactly. That coincidence is why American recipes so often interchange cups and ounces for butter — and why the same habit fails for flour, where a cup is barely over 4 oz.
How to convert a recipe safely
First, decide which ounce the recipe means. If it is a liquid measured in a jug, 1 cup = 8 fluid ounces and you are done. If it is a dry ingredient given in ounces, use a weight-per-cup figure for that ingredient — the ingredient density table lists the common ones — or weigh the ingredient directly. And if a recipe gives dry ingredients in cups and you own a scale, convert once with the grams to cups converter and write the gram weight on the recipe card: you will never need the cup again.
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