100 grams of water in cups
Water weighs about a gram to the millilitre, so 100 grams of it is close to 100 ml and comes to about 0.42 of a US cup. It is the one thing a cup gets nearly exactly right, and even here two charts disagree by 10 grams.
The one cup with a physical answer
Water is the reference the whole system rests on: a millilitre of it weighs a gram. So 100 g is very nearly 100 ml, 99.8 to be exact, and that comes to 0.42 of a US cup. Every other ingredient borrows this coincidence and bends it a little, which is why their cups need a density and water's barely does.
An ounce is not a fluid ounce
Even water splits the two US authorities. King Arthur prints 227 g for a cup, the weight of eight avoirdupois ounces. The USDA prints 237 g, the weight of eight US fluid ounces of water. A fluid ounce is a volume and an ounce is a mass, and they are not the same size. Read 100 g through 227 g and you get 0.44; through the USDA's 237 g you get 0.42. This is where the eight-ounce cup rule was born, and where it starts to fail for anything denser.
| A cup of water at | In US cups |
|---|---|
| USDA, 237 g | 0.42 |
| King Arthur, 227 g | 0.44 |
The water master page works through the ounce-versus-fluid-ounce split and why the USDA's 237 g is the right one.
Other water weights in cups
The same reading for common amounts:
| Water | US cups |
|---|---|
| 50 g | 0.21 cups |
| 100 g | 0.42 cups |
| 150 g | 0.63 cups |
| 200 g | 0.84 cups |
| 250 g | 1.1 cups |
The same 100 grams fills more as things get lighter: 0.47 of a packed cup of brown sugar, and 0.54 of a cup of rice. Double it and 200 grams of water is exactly one Japanese cup.
Questions about 100 grams of water
- Is 100 grams of water half a cup?
- Not quite. Half a US cup of water is about 118 g, so 100 g is a little less, about 0.42 of a cup.
- How many cups is 100 grams of water?
- About 0.42 of a US cup, on the USDA's 237 g cup. Because water runs about a gram to the millilitre, 100 g is also close to 99.8 ml.
- Why do charts say a cup of water is 8 ounces?
- They mean fluid ounces, a volume: a US cup is 8 fl oz. King Arthur reads that as 8 ounces of weight, 227 g, lighter than the 237 g the USDA measured. That mix-up is where the eight-ounce rule comes from.
Sources
USDA FoodData Central SR Legacy for the 237 gram cup, King Arthur Baking for the 227 gram eight-ounce figure. Full citations on the sources page.