Flour by weight:
why a cup is not a fixed number of grams

The short answer and why it moves

For ordinary all-purpose flour, a standard US cup holds about 120 grams when the flour is spooned lightly into the cup and levelled off. Two cups is therefore 240 grams. That number is a convention, though, not a physical constant.

Flour is a powder full of air. Pack it down, scoop it directly from the bag, or sift it, and the weight in the same cup can change by 15 to 20 percent. A water-filled cup is the same every time, but a flour-filled cup depends on who is holding the spoon.

This is the reason baking recipes written by weight are more consistent than ones written by cups, and why conversions need a stated assumption rather than a single magic figure.

The conversion formula

Going the other way, divide: a recipe that lists 375 g of flour needs 375 ÷ 120 = 3.125 cups, which is 3 cups plus 2 tablespoons. Using the cup measure for such a quantity is not very accurate, which is the reason to weigh it instead.

Remember that the factor belongs to the flour, not to the recipe. If you change from all-purpose to cake flour, or to a different brand, the grams per cup shift slightly, and a recipe converted once should be converted again.

How the way you fill the cup changes the weight

Three common techniques give three different answers. Fluffing the flour with a spoon and sweeping the top flat gives the lowest figure. Scooping the cup into the bag packs the flour and gives the highest. Sifting first, then spooning, is the lightest of all.

Take a recipe for 3 cups. With spooned flour you use 360 g. With scooped flour at 140 g per cup you use 420 g, which is 60 g extra, or 16.7 percent more. In a cake that excess makes it dense and dry, in a bread dough it makes it stiff and slow to rise.

Different flours, different weights

Flours differ in particle size and in how much protein, bran and moisture they carry. Typical published values for a US cup vary by brand, so treat the table as a starting point and check the packet for the supplier's own figure.

Whole wheat flour is coarser and contains bran, so it behaves differently when measured by volume and absorbs more water in the dough. When swapping flours in a recipe, swap by weight, not by cup, and adjust the liquid by a spoonful at a time.

Which cup does the recipe mean?

The US cup is 236.6 millilitres. Many metric recipes use a 250 mL cup, which holds about 5.7 percent more. At a density of roughly 0.507 g/mL, one metric cup of spooned flour weighs about 127 g, against 120 g for the US cup.

Indian kitchens commonly use steel cups or measuring sets of 200, 240 or 250 mL, so a cup in a handwritten family recipe may not be a US cup at all. Before converting, find out the cup's volume by filling it with water and pouring it into a jug.

• Check whether the recipe is American, Canadian, Australian or British.
• Look for a weight given in brackets, which is the best source of truth.
• Note whether the recipe says sifted, spooned or packed.
• If none is given, use 120 g per US cup and 127 g per metric cup as a working assumption.

Making recipes repeatable

The surest way to get a consistent loaf or sponge is to weigh the flour. A kitchen scale costs little and removes the whole cup debate. Weigh into the bowl, set it to zero, add the next ingredient and weigh again, which also reduces the washing-up.

If you only have cups, build your own reference. Measure out a cup the way you normally do, weigh it three times, and use the average for your flour. Write that gram figure next to the recipe so you do not need to remeasure, and use it consistently whenever you convert.

The conversion is only as dependable as the starting assumption, so state it. A note such as 240 g, spooned and levelled, on a recipe card saves arguments and ruined batches.

Bread shows how much this matters. A dough of 3 cups of flour at 360 g and 1 cup of water at about 237 g has a hydration of 237 ÷ 360 = 66 percent. Scoop the flour to 420 g and the same cup of water gives 237 ÷ 420 = 56 percent, a much stiffer dough that rises slowly and bakes into a tight crumb. Bakers describe this relationship as a percentage of the flour weight, which is why they weigh.