You need the density — kg and liters measure different things
Kilograms measure mass; liters measure volume. To go from one to the other you have to know how heavy the substance is per liter. For water that number is 1, so 1 kg is 1 liter. For everything else it is not.
How much a kilogram fills, by substance
| Substance | Density (kg/L) | 1 kg = ? L | 1 L = ? kg |
|---|---|---|---|
| Water (4°C) | 1.000 | 1.00 L | 1.00 kg |
| Whole milk | 1.030 | 0.97 L | 1.03 kg |
| Seawater | 1.025 | 0.98 L | 1.03 kg |
| Olive oil | 0.911 | 1.10 L | 0.91 kg |
| Cooking oil | 0.920 | 1.09 L | 0.92 kg |
| Diesel | 0.832 | 1.20 L | 0.83 kg |
| Ethanol | 0.789 | 1.27 L | 0.79 kg |
| Gasoline | 0.748 | 1.34 L | 0.75 kg |
| Honey | 1.420 | 0.70 L | 1.42 kg |
Flour, sugar and other powders use bulk density (they trap air), so a "cup of flour" weighs far less than a cup of water. Those are looser and depend on how much the powder is packed.
Lighter fills more, denser fills less
One kilogram is the same weight in every case, but the volume it fills swings widely with density. Oil floats on water because it is lighter, so a kilo of it spreads into more liters; honey is dense, so a kilo barely fills two-thirds of a liter.
The same one kilogram fills nearly twice as many liters as gasoline than it does as honey.
Rules of thumb worth memorizing
- Water. 1 kg = 1 L exactly. This is the reference point everything else is measured against.
- Oils. 1 kg ≈ 1.1 L. Oil is lighter than water, so it takes up a little more room.
- Milk. 1 kg ≈ 0.97 L. Almost water, just a touch denser.
- Honey and syrups. 1 kg ≈ 0.7 L. Much denser, so far less volume.
Common questions
Is 1 kg always 1 liter?
Only for water. One kilogram of water is one liter because water has a density of 1 kg per liter. Anything lighter than water, like oil or gasoline, takes up more than a liter per kilogram; anything denser, like honey or milk, takes up less.
How do you convert kg to liters?
Divide the mass in kilograms by the density in kg per liter. For example, 5 kg of cooking oil at 0.92 kg per liter is 5 divided by 0.92, which is about 5.43 liters.
How much does 1 liter of milk weigh?
About 1.03 kg. Whole milk is slightly denser than water because of its fat, protein and sugar content, so a liter weighs a little over a kilogram.
