The label says grams, the recipe says cups

Pick up any packet in a supermarket and the numbers on it are weights. Grams per pack, grams per serving, grams of sugar. Then open a recipe and the numbers are volumes: a cup of this, two tablespoons of that.

Those two systems do not convert cleanly, and the reason is not arithmetic. It is that a volume is a description of a container, not of a substance. A cup of flour is however much flour happens to be in a cup, which depends on whether it was spooned in or the cup was pushed into the bag.

Where it actually bites

For most ingredients this does not matter much. Sugar crystals are hard and free-flowing, so a cup of them is a cup of them whichever way you fill it — our own sources agree on granulated sugar to within one percent.

Flour is the opposite case. Published figures for a cup of all-purpose flour run from 118.3 g to 139.7 g, and both ends are correct measurements of different filling methods. That is an eighteen percent spread on the single most common ingredient in baking, and no amount of care with the cup itself will close it.

What to do about it

• If the recipe gives grams, use grams. There is nothing to convert and nothing to get wrong.
• If it only gives cups, find out whose cup. A King Arthur recipe means a King Arthur cup.
• If you are converting, pick the method you actually use and stay with it.

A note on the cup itself

There is a second, quieter problem underneath the first. There is more than one cup. The cup on a nutrition label is a round 240 ml; the cup a measuring cup actually holds is the US customary cup, 236.5882365 ml. The difference is one and a half percent, which is small enough to ignore in a stew and large enough to matter in bread.

Every figure on this site is derived from the customary cup, and the check is that our generated table for olive oil reproduces USDA's own published numbers exactly — 216 g per cup, 13.5 g per tablespoon, 4.5 g per teaspoon. Divide by 240 instead and none of the three agree.

The other half of the problem

Knowing what a cup of something weighs still does not tell you what is in it. That is the part a scale cannot help with, and it is why I also built SwapGenie — it reads the barcode, checks the ingredients against what you personally avoid, and suggests something better when the packet does not measure up.

Measuring and reading labels are the same problem seen from two ends: knowing exactly what you are putting in.