Converting a recipe from cups to grams is worth doing once for anything you cook regularly. After that you weigh straight into the bowl, wash fewer things, and get the same result every time.
The mistake people make is trying to convert everything at once. Most of a recipe does not need it.
Convert these first
Flour, and anything flour-like. This is where nearly all the benefit is. A cup of flour ranges from about 106 g to 150 g depending on how it was measured, so it is the single ingredient where the cup figure is least trustworthy. Cocoa powder and powdered sugar have the same problem.
Anything sticky. Honey, maple syrup and peanut butter leave a meaningful amount of themselves in the measuring cup. Weighing them straight into the bowl is both more accurate and less washing up.
Anything packed. Brown sugar is 213 g packed and about 145 g loose, a third of a difference that comes down to how hard you pressed.
Do not bother with these
Granulated sugar, butter and liquids convert reliably from cups because they cannot be compressed much. If you are short of time, leave them in cups and convert only the list above; you will have captured most of the accuracy for a fraction of the work.
Salt, baking powder and spices should be left in spoons. They are measured in amounts too small for a kitchen scale to read reliably, and a teaspoon of each is far more repeatable than a cup of flour ever is. The exception is salt, where the brand matters enormously, but the fix there is knowing your salt, not converting to grams.
The method
- Write out the recipe in one column. Cups and spoons as given.
- Look up the grams-per-cup for each ingredient you are converting. The grams in a cup table covers 22 ingredients with the state each figure assumes.
- Multiply. Grams equals cups times grams-per-cup. Three quarters of a cup of flour is 0.75 × 120 = 90 g.
- Round sensibly. Round to the nearest 5 g on anything over 100 g. Precision past that is false: your flour's moisture content varies more than 2 g.
- Note the assumptions. Write "brown sugar, packed" rather than just "brown sugar". In six months you will not remember.
- Cook it once and adjust. If the recipe was written by someone who scooped their flour, your spooned conversion will run slightly dry. Add liquid and write down what you added.
Where it goes wrong
The recipe was not American. An Australian or British cup is 250 ml, not 236.6 ml, so every US-cup figure runs about 5% low for that recipe. Check the source before you convert.
The recipe says "sifted flour" rather than "flour, sifted". Word order sets the order of operations. Sifted first means less flour by weight; sifted after means the weight is unchanged.
You converted a bad measurement faithfully. If the original recipe was developed by someone scooping flour from the bag, converting their cups at 120 g per cup gives you a recipe with less flour than they used. Cook it, taste it, adjust, and keep your version.
The cups to grams converter handles the arithmetic if you would rather not.