Why a Cup of Flour and a Cup of Honey Are Different Problems
A cup is a volume and a gram is a mass, and converting between them requires knowing what is in the cup. That is the whole reason a converter needs a density table rather than a single factor.
Why one factor cannot work
The measurement converter carries per-ingredient densities for the common ones: water, milk, oil, honey, several sugars, all-purpose and rice flour, cornstarch, butter, oats, uncooked rice, cocoa, salt, shredded cheese, chopped nuts and chili powder.
They differ enormously. Honey is dense and syrupy; flour is light and full of air; cocoa is lighter again. A cup of each is a completely different mass, so a recipe converted with a single factor is wrong for everything except whatever that factor was derived from.
Flour is where volume measuring fails
The classic problem. How a cup of flour is filled — scooped, spooned, sifted, tapped down — changes the amount by a margin large enough to change a dough.
Which is why baking recipes increasingly give weights, and why a converted volume figure for flour should be treated as an approximation even when the density is right. Weighing removes the variable entirely.
Where conversion matters most in street food
Batters and doughs. A tempura batter, a churro dough or a fritter mix is a ratio problem, and getting the flour-to-liquid ratio wrong by the margin volume measurement introduces is the difference between a coating that adheres and one that slides off.
Marinades and sauces are more forgiving — a little more or less soy sauce is a taste adjustment rather than a failure — which is a useful thing to know when deciding where to be careful.
Salt deserves its own note
Salt is the ingredient where volume conversion misleads most, because crystal size varies so much between types. The same volume of fine table salt and coarse flake salt carries very different amounts of salt, since one packs far more densely than the other.
Recipes that specify a salt type are doing so for this reason. Where one does not, weight is safer than volume, and tasting is safer than either.
The practical habit
Convert once, write it on the recipe, and work in weight from then on. Scales are inexpensive, they remove the largest source of variation in home cooking, and they make scaling with the portion scaler trivially reliable — multiplying grams is exact in a way multiplying cups is not.
And keep the original units on the page. A recipe converted and then converted back accumulates error, and the source figures are worth preserving.
Ingredients where the difference is largest
The extremes in the density table are the ones to be careful with. Honey and syrups are dense; flours, cocoa and powdered sugar are light and full of air. A cup of each is a very different mass, and a recipe converted with the wrong density is wrong by a wide margin rather than a small one.
Chopped nuts and shredded cheese are the awkward cases, because how finely they are cut changes how they pack — a cup of coarsely chopped and a cup of finely chopped are not the same weight.
Temperature and state matter too
Melted butter and solid butter occupy different volumes for the same mass. So do packed and loose brown sugar, which is why recipes specify "packed" — it is a density instruction rather than a technique one.
Where a recipe is specific about state, the conversion depends on it, and ignoring that is a common source of quiet error.