Why a cup of one thing is not a cup of another
Volume and mass are different quantities, and nothing converts between them without knowing what is being measured. A US cup holds 236.6 ml of whatever you put in it. What that weighs depends entirely on the ingredient:
1 US cup of flour ≈ 125 g
1 US cup of sugar ≈ 200 g
1 US cup of water ≈ 237 g
1 US cup of honey ≈ 339 g
Nearly a threefold spread, and every one of those is "one cup". The bridging quantity is density — grams per millilitre — which is why this converter asks for an ingredient before it will give you a weight. Any chart offering a single cups-to-grams factor is quietly assuming one, usually flour, and is wrong for everything else.
The arithmetic itself is unremarkable:
grams = millilitres × density
millilitres = grams ÷ density
The work is in knowing the density, and in knowing how much confidence it deserves.
Four cups, and they differ by 20%
Before any ingredient is involved, there is a problem with the cup itself. At least four are in current use:
| Cup | Volume | Where |
|---|---|---|
| US customary | 236.5882365 ml | US recipes |
| US legal | 240 ml | US nutrition labels |
| Metric | 250 ml | Australia, New Zealand |
| Imperial | 284.130625 ml | Older British recipes |
The US customary cup is defined as half a US pint, which makes it that unwieldy exact figure. The legal cup exists because food labelling needed a round number and 240 ml was close enough. The metric cup is a clean quarter litre. The imperial cup is ten imperial fluid ounces, and mostly survives in books published before Britain went metric in the kitchen.
Smallest to largest is a 20% difference. In a cake that is the gap between the recipe working and not.
Spoons are worse, because the discrepancy is larger relative to the amount and the ingredients involved are the potent ones:
US tablespoon 14.79 ml
Metric / UK 15 ml
Australian 20 ml ← four teaspoons, not three
The Australian tablespoon is the trap. Measure an Australian recipe's tablespoon of baking powder with a US spoon and you have put in a quarter less than intended; go the other way and you have added a third too much. Neither is recoverable once it is in the bowl. Teaspoons, mercifully, are 5 ml everywhere except the US, where they are 4.93 ml — a difference too small to matter.
Flour does not have a fixed cup weight
The single most useful thing to understand about volume measuring is that for some ingredients it does not have one right answer.
Take flour. Published figures for a cup range from about 120 g to about 145 g, and none of them are careless. They describe different filling methods:
Scoop and sweep. Push the cup into the bag, drag it out full, level the top. This compacts the flour on the way in. Around 140-145 g.
Spoon and level. Spoon flour into the cup, then level with a knife. Almost no compaction. Around 120-125 g.
Sifted before measuring. Aerated, so less fits. Around 115 g.
That is a 20% swing between two people following the same written instruction correctly. Bread is fairly forgiving of it; a delicate cake batter is not, and the outcome is the dry, dense result that gets blamed on the oven.
The same applies, less dramatically, to anything that packs or settles: brown sugar, cocoa, oats, desiccated coconut, grated cheese, chopped herbs. For those the tool shows a warning rather than pretending its figure is precise, because the honest answer is a range.
This is the reason professional and European recipes give weights. A scale removes the entire problem — not by being more precise about the conversion, but by making the conversion unnecessary.
The ingredients where volume is fine
Volume measuring is not wrong everywhere, and it is worth knowing where it holds up.
Liquids. Water, milk, stock, oil, wine. A liquid takes the shape of the container, so there is no packing variable at all. Measuring 250 ml of milk in a jug is as accurate as weighing it.
Free-flowing granulated sugar. The crystals are hard, uniform and do not compress meaningfully. A cup of granulated sugar is reliably about 200 g.
Small amounts of potent things. Half a teaspoon of vanilla, a teaspoon of baking powder. The absolute quantities are so small that a scale reading to one gram is the less accurate instrument. Spoons win here.
Butter, in North America. A stick is a quarter pound — 113 g, half a cup, eight tablespoons — and the wrapper is printed with the divisions. It is the one solid fat that measures accurately by volume, because the packaging did the work.
Where volume falls down is bulk dry goods, and that is exactly where the quantities are large enough for the error to matter.
Salt is its own problem
Salt deserves separating out, because the variation between types is larger than for almost any other ingredient and the tolerance for error is lower.
Fine table salt is dense: a cup of it weighs about 292 g. Coarse kosher and flaked salts have far more air between the crystals, and the same cup can hold barely half that. So a teaspoon of one is emphatically not a teaspoon of the other, and the difference between correctly seasoned and ruined is well inside a factor of two.
Two rules cover it. If a recipe names a salt, use that salt, or convert by weight rather than by volume. If it does not name one, and the quantity is more than a pinch, assume the type the author's cuisine and country would default to and taste as you go.
Anything cured, brined or fermented has to be by weight regardless. Those are ratios by mass and the volume of the salt is irrelevant to how they behave.
Converting a recipe, in practice
A workflow that avoids most of the trouble:
Find out whose cups. Australian and New Zealand sources use 250 ml cups and 20 ml tablespoons. American sources use 236.6 ml cups and 14.8 ml spoons. British sources published since the 1970s almost always give weights, and if they give cups they may mean imperial ones. The site or the book usually says somewhere.
Convert the bulk dry ingredients to weight once, then write the weights on the recipe. Flour, sugar, oats, rice. This is where volume error concentrates and where a scale pays for itself in a single bake.
Leave the small measures alone. Spoons of raising agent, spices, extracts and salt are fine as spoons — unless the source is Australian, in which case the tablespoons need scaling by 20/15.
Keep ratios in mind rather than absolutes. Baking is proportional. If every measurement in a recipe drifts up by 5% together, the result is a slightly larger cake. If the flour drifts up and the liquid does not, the result is a different cake. Consistent method matters more than being right to the gram.
Weigh your own cup once. For an ingredient used often that is not on the list here, fill the cup the way you always fill it, weigh it, and enter that figure as the custom density. It will be more accurate for your kitchen than any published chart, because it encodes how you actually measure.