The formula
grams = millilitres × density of the ingredient
Where it comes from
A US cup is 237 ml of volume, always the same. But that volume weighs 120 g filled with flour and 340 g filled with honey. That is why converting American recipes with one single factor gives disastrous results, above all in baking, where the proportions are strict.
How to work it out by hand
- Find the volume measure and convert it to millilitres
- Look up the density of the ingredient in grams per millilitre
- Multiply the volume by the density
What is worth knowing
The weight of a cup of flour also varies with how you fill it: spooned in it comes to about 120 g, but scooped straight from the bag and packed down it can reach 150, 25 % more. The correct method is to spoon the flour into the cup and level it with a knife, without tapping. It is one of the reasons professional baking always weighs in grams rather than measuring in cups.