You cannot convert kilograms to gallons without a density
Kilograms are mass; gallons are volume. To cross between them you divide by the substance's density (kg per litre), then divide by the litres in a gallon. Only water lands on the tidy "1 kg ≈ 1 litre" that most people assume.
Litres per gallon is 3.78541 (US) or 4.54609 (Imperial). So 1 kg of water is 1 ÷ 1.0 ÷ 3.78541 = 0.264 US gal.
Same mass, different volume — density is the whole story
Denser substances take up less volume, so a kilogram of honey is far fewer gallons than a kilogram of gasoline. This is why the same 1 kg gives a different gallon figure for every liquid.
One kilogram of gasoline fills nearly twice the volume of one kilogram of honey, because honey is roughly twice as dense.
Density and gallons for common liquids
| Substance | Density (kg/L) | 1 kg → US gal | 1 kg → Imperial gal |
|---|---|---|---|
| Water | 1.00 | 0.264 | 0.220 |
| Milk (whole) | 1.03 | 0.257 | 0.214 |
| Vegetable oil | 0.92 | 0.288 | 0.239 |
| Gasoline | 0.75 | 0.352 | 0.293 |
| Honey | 1.42 | 0.186 | 0.155 |
To go the other way (gallons to kg), multiply: 5 US gal of water = 5 × 3.78541 × 1.0 = 18.93 kg.
The mistakes to avoid
- Assuming 1 kg = 1 litre for everything. That only holds for water. Oil takes more volume per kg, honey takes less.
- Mixing up US and Imperial. The Imperial gallon is 20% larger. Convert a US figure to Imperial by multiplying by about 0.833.
- Wrong density units. Density here is kg/L (same as g/cm³), not kg/m³. If a table gives 750 kg/m³, that is 0.75 kg/L.
- Ignoring temperature. Water is 1.000 kg/L at 4°C but 0.998 at 20°C. For most jobs the difference is negligible; for precise work, state the temperature.
Common questions
Why do I need the substance to convert kg to gallons?
Kilograms measure mass and gallons measure volume, so there is no fixed conversion between them. You need the density, which is how much mass fits in a given volume. One kilogram of water fills about a litre, but one kilogram of honey fills only 0.7 litres because it is heavier.
How many gallons is 1 kg of water?
1 kilogram of water is 0.264 US gallons, or 0.220 Imperial gallons. This works because a kilogram of water is very close to one litre.
What is the difference between US and Imperial gallons here?
A US gallon is 3.78541 litres and an Imperial (UK) gallon is 4.54609 litres, so the Imperial one is about 20% bigger. The same mass gives you a smaller gallon number in Imperial than in US.
