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. The three best-sourced figures for a cup of all-purpose flour are 141.7 g, 125 g and 120 g, and all three are correct measurements of different filling methods. That is an eighteen percent spread on the single most common ingredient in baking.

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.

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.