A cup of flour can weigh anywhere from 120 to 155 grams — same cup, same flour, same recipe. The only variable is how you fill it. Dip the cup straight into the bag and the flour packs down; spoon it in loosely and level it off and it stays airy. That's a 30% swing hiding inside one word: "cup."
For a batch of cookies, 30% more flour is the gap between chewy and dry-and-crumbly. For bread, it's the difference between a dough that comes together and one you keep adding water to, wondering why the recipe lied. This is the whole case for weighing, and it's why nearly every serious baking recipe published outside the US now lists grams.
Why a cup isn't a cup
Volume measures how much space something takes up. Weight measures how much of the ingredient is actually there. For anything that compresses, settles, or clumps, those two numbers drift apart.
Flour is the worst offender. Freshly sifted flour is fluffy and full of air; flour that's sat in the bag for a month has settled and packs denser. Scoop that settled flour with the measuring cup itself and you compress it further still. King Arthur, whose recipes assume 120g per cup, notes that a straight "dip and sweep" scoop can push a single cup past 140g without anything looking wrong.
Brown sugar has the same problem in reverse — recipes tell you to pack it, because loose brown sugar in a cup is almost meaningless as a measurement. Powdered sugar, cocoa, and shredded cheese all behave the same way.
A scale sidesteps all of it. 120 grams is 120 grams whether the flour is sifted, settled, or six months old.
The conversions that matter, and why you can't reuse them
Here's the trap: people learn "1 cup = 237 grams" — that's water — and try to apply it to everything. Density makes that useless. A cup is a fixed volume, about 237 milliliters, but what that volume weighs depends entirely on the ingredient sitting in it.
The common ones, per US cup:
- All-purpose flour — 120g (spooned and leveled)
- Granulated sugar — 200g
- Brown sugar, packed — 213g
- Butter — 227g (easy to remember: 2 sticks, half a pound)
- Powdered sugar — 120g
- Water — 237g
- Milk — 240g
- Honey — 340g
Look at flour against sugar: same cup, but 120g versus 200g. Sugar is denser and packs tighter, so it weighs two-thirds more for the same volume. Butter is denser again. This is why you can never swap the numbers between ingredients — a recipe calling for "1 cup flour, 1 cup sugar" is asking for 120g and 200g, not two equal weights.
If you're converting a recipe rather than memorizing it, the Cups to Grams Converter has the per-ingredient densities built in, so you pick "flour" or "sugar" and get the right number instead of applying water's ratio to everything. It runs in the browser — nothing to install, no account, nothing uploaded.
When volume is completely fine
Weighing isn't a religion. For liquids, a measuring cup is genuinely accurate. Water, milk, oil, and stock don't compress — a cup of water is a cup of water every time, and 1 milliliter of water weighs almost exactly 1 gram, so the volume marking and the weight agree. Measuring liquids by volume is fast and carries no penalty.
Volume is also fine when precision doesn't matter. A cup of chopped vegetables for a soup, a handful of chocolate chips, a splash of vanilla — no bake fails because the walnuts came in at 95g instead of 110g. Cooking, broadly, forgives you. Baking doesn't, because baking is chemistry: the ratio of flour to liquid to fat to leavening decides whether the thing rises, spreads, or seizes.
The rough rule: weigh the dry structural ingredients — flour, sugar, cocoa, and leavening in large batches — and measure liquids by volume if you prefer. That's roughly how professional kitchens split it too.
Why European recipes already do this
Open a British or German baking blog and the ingredients are in grams, full stop. It isn't that Europe is more precise by temperament — the metric system made weighing the obvious default, and once a whole baking culture writes in grams, the ambiguity of "a cup" never gets a foothold. There isn't even a single metric cup everyone agrees on the way the US settled on roughly 237ml.
This is why converting an American recipe for a European kitchen, or the reverse, goes wrong so often when you only convert volume to volume. Translating "2 cups flour" into "473 milliliters of flour" is technically correct and practically worthless — nobody measures dry flour by milliliters. Converting it to 240g is what the recipe actually meant.
Honest limits of converting cups to grams
A converter gets you close, not perfect. Four gaps to plan around:
- The source recipe's own cup was a guess. If the original author scooped and packed their flour, their "2 cups" might have been 300g, not 240g. Your clean 240g conversion is more consistent, but it won't be identical to what they baked. For most recipes that's fine; for a finicky one, expect to adjust once.
- "Flour" isn't one thing. All-purpose, bread, cake, and whole wheat flours have different densities. 120g per cup is the all-purpose figure — cake flour tends to run lighter, whole wheat heavier.
- Charts disagree by a few grams. You'll see flour listed as 120g, 125g, even 130g depending on the source. Any of them beats an unmeasured scoop, but don't treat a single number as gospel to the gram.
- A scale is the real fix. A converter turns a volume recipe into weights. To stop the variance at the source, weigh directly — set the bowl on the scale, hit tare, pour to the gram. A $12 kitchen scale pays for itself in the flour you stop wasting.
The first time you weigh a recipe you've always eyeballed, write the gram figures in the margin next to the cups. From then on you skip the cup entirely — and the batch-to-batch drift you could never quite explain, why these cookies spread and last month's didn't, just stops.