100 ml of water weighs 100 g. Water is the one ingredient where the millilitre-to-gram conversion is exact: 1 ml of water weighs 1 g, at room temperature, because the gram was originally defined by the weight of water. So 100 ml is 100 g, 250 ml is 250 g, 500 ml is 500 g, and 1 litre is 1,000 g — which is 1 kilogram. No other ingredient works this neatly. Milk is slightly denser than water, oil is lighter, honey is far denser. Water is the baseline, and every other conversion is measured against it.
This is the most useful conversion in the kitchen for anyone who cooks by weight. If a recipe calls for 100 g of water and you have a measuring jug, pour 100 ml. If a recipe calls for 100 ml of water and you have a scale, weigh 100 g. The two are interchangeable, and that interchangeability is what makes a kitchen scale so powerful: you can measure water by weight, which is more accurate than measuring by volume, and you never need to wash a jug. It is also the reason bread bakers work in grams — water, flour and salt are all weighed, and the ratios are exact.
Water volume to weight table
The table below shows the volume-to-weight conversion for water at the common amounts. Because 1 ml of water weighs 1 g, the two columns are identical — which is the point.
| Water | Millilitres | Grams | US cups |
|---|---|---|---|
| Small amount | 50 ml | 50 g | 0.21 cups |
| Standard | 100 ml | 100 g | 0.42 cups |
| One cup | 237 ml | 237 g | 1 cup |
| Two cups | 473 ml | 473 g | 2 cups |
| Half a litre | 500 ml | 500 g | 2.11 cups |
| One litre | 1,000 ml | 1,000 g | 4.23 cups |
One US cup of water weighs 237 g (USDA), because a US cup is 236.6 ml and 1 ml of water weighs 1 g. King Arthur Baking rounds water to 227 g per cup; the exact figure is 237 g.
Worked examples
A bread dough that needs 100 g of water
A small bread dough for one loaf uses 100 g of water, weighed on a kitchen scale. Put the bowl on the scale, tare it to zero, and pour water until the scale reads 100 g. That is more accurate than measuring 100 ml in a jug, because you are not relying on reading a meniscus at eye level. The water temperature matters too: lukewarm water, around 35°C, activates the yeast without killing it. If the water is too hot, the yeast dies and the dough will not rise; if it is too cold, the rise will be very slow.
Scaling a soup recipe by weight
A soup recipe calls for 500 ml of water, and you are scaling it down to a single serving that needs 100 ml. Weigh 100 g of water on a scale instead of measuring 100 ml in a jug — it is faster, more accurate, and you do not have to wash a jug. The soup needs the water to be exact, because the seasoning is calculated for that amount: too much water makes a thin, under-seasoned soup, and too little makes a thick, over-salted one. Weighing the water is the professional way to get it right.
A sourdough starter feeding
A sourdough starter feeding uses 100 g of water and 100 g of flour, mixed into the starter. Weigh both on a scale — do not measure them by volume, because a cup of flour weighs 120 g and a cup of water weighs 237 g, and the ratio would be completely wrong if you used cups. The 100 g of water is 100 ml, and the 100 g of flour is about 5/6 of a cup. Feed the starter at the same time every day, and keep it in a warm place so the yeast stays active.
How the figure is worked out
There is no weighing involved in the headline answer — it is arithmetic on two published numbers. The package weight is 100 millilitres of water. The cup weight comes from the ingredient table, and dividing one by the other gives the cups in the package. That is also why the answer is approximate: scoop a cup differently and the grams per cup change, so the cups per bag move with it. For anything where the amount truly matters, weigh the ingredient instead of counting cups. The cup weight used here is 237 g per cup, poured, sourced from USDA. One US cup is 236.6 ml, so water weighs 237 g per cup. King Arthur Baking rounds water to 227 g per cup; we use the exact 237 g.
Practical guidance
The 1 ml = 1 g rule applies to water only. Do not use it for other liquids: milk is slightly denser than water, so 100 ml of milk weighs about 103 g; oil is lighter, so 100 ml of olive oil weighs about 91 g; honey is far denser, so 100 ml of honey weighs about 142 g. Using the water conversion for another liquid will give you a wrong weight, and in baking that matters. For water, though, the conversion is exact, and it is the reason a kitchen scale is the most accurate way to measure water: pour until the scale reads the number you want, and you are done.
The temperature of the water makes a tiny difference to its density — cold water is very slightly denser than hot water — but the difference is less than half a gram per 100 ml across the whole range from fridge-cold to boiling, which is far too small to matter in cooking. Use the 1 ml = 1 g rule at any temperature. The one thing to watch is the scale itself: put the bowl on the scale, tare it to zero, and then pour. If you forget to tare, you will be measuring the bowl as well as the water, and your 100 g will be short by the weight of the bowl.
Frequently asked questions
Does 100 ml of milk weigh 100 g?
No — about 103 g. Milk is slightly denser than water, so 100 ml of milk weighs a little more than 100 g. The 1 ml = 1 g rule applies to water only. For milk, use the USDA figure of 244 g per cup, which works out to about 103 g per 100 ml.
Does 100 ml of oil weigh 100 g?
No — about 91 g for olive oil. Oil is lighter than water, so 100 ml of oil weighs less than 100 g. The exact figure depends on the oil: olive oil is about 216 g per cup, which is about 91 g per 100 ml. Do not use the water conversion for oil.
How many grams are in a litre of water?
1,000 g, which is 1 kilogram. A litre of water is 1,000 ml, and 1 ml weighs 1 g, so a litre weighs exactly 1 kilogram. This is the definition of the kilogram in practical terms, and it is why water is the reference for every other density.
Why is water 237 g per cup and not 227 g?
Because a US cup is 236.6 ml, and 1 ml of water weighs 1 g, so a cup of water weighs 236.6 g — rounded to 237 g. King Arthur Baking rounds water to 227 g per cup, which is a rounding on their chart, not a different measurement. The exact figure is 237 g.
Should I measure water by weight or by volume?
By weight, if you have a scale — it is more accurate, because you are not relying on reading a meniscus. Put the bowl on the scale, tare it, and pour until the scale reads the number you want. By volume, if you do not have a scale — use a jug with millilitre markings and read it at eye level.