A recipe says “1 cup of flour” and you scoop your measuring cup straight into the bag, level it off, and move on. That single scoop can weigh anywhere from 120 to 150 grams depending on how it was packed, and if the recipe was written in Australia or the UK instead of the US, the cup itself is a different size to begin with. Here’s the actual math behind converting cups to grams, so your baking stops being a guessing game.
Why “a cup” doesn’t mean one fixed thing
A cup is a volume measurement, not a weight, and volume only tells you the weight of an ingredient once you know how densely it’s packed in. On top of that, the cup itself isn’t standardized worldwide: a US cup holds 236.588 mL, a metric cup (used in Australia and much of Europe) holds 250 mL, and the old British “breakfast cup” holds 227.3 mL. Three different containers, all called “a cup,” in three different recipes.
The formula: turning volume into weight
Once you know the volume of the cup being used and the density of the ingredient, the conversion is simple:
weight (g) = volume (mL) x density (g/mL)
Density varies by ingredient because different foods pack differently into the same space. Useful approximate densities: flour is about 0.7 g/mL, granulated sugar about 0.8 g/mL, table salt about 1.2 g/mL, and butter about 0.9 g/mL. Water is the one exception where the math is trivial, since 1 mL of water weighs almost exactly 1 gram.
A real example: converting flour and sugar
Take a US recipe calling for 1 cup of flour and 1 cup of sugar. A US cup is 236.588 mL, so:
Flour: 236.588 mL x 0.7 g/mL = about 165.6 grams
Sugar: 236.588 mL x 0.8 g/mL = about 189.3 grams
Now swap in a metric cup instead, which holds 250 mL rather than 236.588 mL, and that same “1 cup of flour” becomes 250 x 0.7 = 175 grams, nearly 10 grams more than the US version. For a delicate cake or bread recipe where ratios matter, that difference is big enough to change how the final result rises and sets.
Common mistakes that wreck a bake
The biggest source of error isn’t even the cup size, it’s how the ingredient gets into the cup in the first place. Scooping a measuring cup directly into a bag of flour and packing it down can add roughly 20 percent more flour than gently spooning it in and leveling the top, which is the technique most baking recipes actually assume. That single habit explains a lot of “why did my cake turn out dense” questions that have nothing to do with the recipe itself.
The second mistake is applying one conversion factor to every ingredient. 100 mL of flour and 100 mL of sugar do not weigh the same, because their densities are different, so a single “cups to grams” number that ignores which ingredient is being measured will always be an approximation at best. And the third mistake is not checking which country’s recipe you’re following: a “cup” in an Australian recipe, a US recipe, and an old British one are three different volumes before density even enters the picture.
When precision actually matters
For a stew or a salad, being a few grams off per cup changes nothing. For baking, where the ratio of flour to liquid to fat controls texture and rise, it can be the difference between a recipe that works and one that doesn’t. That’s why professional bakers weigh ingredients on a kitchen scale instead of using cups at all: a gram of flour is a gram of flour everywhere in the world, no cup size or packing technique involved.
