Grams and litres only line up if you know the density
Grams are mass and litres are volume, so you cannot convert one to the other without knowing how heavy the substance is per unit of volume. That is its density. Water is the easy case at 1 g/mL, which is exactly why "1 gram = 1 millilitre" feels universal but is not.
Density of common substances
| Substance | Density (g/mL) | g per litre | 1000 g equals |
|---|---|---|---|
| Water (20 °C) | 1.00 | 1000 | 1.00 L |
| Milk (whole) | 1.03 | 1030 | 0.97 L |
| Granulated sugar | 0.80 | 800 | 1.25 L |
| Brown sugar | 0.88 | 875 | 1.14 L |
| Vegetable oil | 0.92 | 920 | 1.09 L |
| All-purpose flour | 0.59 | 593 | 1.69 L |
Flour density swings with packing (0.53 loose to 0.85 packed), so weighing beats measuring by volume for baking.
The same weight fills very different volumes
This is the trap. 100 g of water, sugar and flour weigh the same but take up completely different amounts of space, because the lighter a substance is for its bulk, the more room it needs.
Identical mass, different volume: 100 g of flour fills nearly 1.7 times the space of 100 g of water.
Common questions
How many litres is 1000 grams?
For water, 1000 g is exactly 1 litre, because water has a density of about 1 g/mL. For anything else it depends on the density: 1000 g of oil is about 1.09 litres, and 1000 g of flour is closer to 1.7 litres.
Why can't I convert grams to litres without knowing the substance?
Grams measure mass and litres measure volume, which are different things. The link between them is density. Without a density you have no way to turn one into the other, the same way you cannot convert kilometres into hours.
Is a gram the same as a millilitre?
Only for water. Water is defined so that 1 g is very close to 1 mL, so people assume it holds everywhere. It does not: 1 g of flour takes up about 1.7 mL, and 1 g of oil about 1.09 mL.
