You cannot convert gallons to kilograms without density
Gallons measure how much space a liquid takes; kilograms measure how heavy it is. Density links the two. For water, density is essentially 1 kg/L, so a US gallon comes out at 3.785 kg. Change the liquid and the weight changes even though the gallon is the same size — that is why one factor cannot cover every case.
What a US gallon weighs, by liquid
| Liquid | Density (kg/L) | 1 US gal | 5 US gal |
|---|---|---|---|
| Gasoline | 0.755 | 2.86 kg | 14.30 kg |
| Diesel | 0.850 | 3.22 kg | 16.09 kg |
| Olive oil | 0.920 | 3.48 kg | 17.40 kg |
| Water | 1.000 | 3.79 kg | 18.93 kg |
| Milk (whole) | 1.032 | 3.91 kg | 19.54 kg |
| Honey | 1.420 | 5.38 kg | 26.88 kg |
Multiply by the density in kg/L, then by 3.785 for US gallons. For an Imperial gallon use 4.546 instead.
US and Imperial gallons are not the same size
An Imperial gallon holds about 20% more than a US gallon, so it weighs about 20% more for any given liquid. The gap is big enough to matter for fuel tanks and shipping.
Same word, different volume: an Imperial gallon of water is about 0.76 kg heavier than a US gallon of the same water.
Common questions
How much does a gallon of water weigh in kilograms?
A US gallon of water is about 3.785 kg. That comes straight from the definition: a US gallon is 3.78541 liters and water is close to 1 kg per liter. An Imperial (UK) gallon of water is larger, about 4.546 kg.
Why do I need the liquid's density to convert gallons to kilograms?
Gallons measure volume and kilograms measure mass, so the bridge between them is density. A gallon of gasoline is much lighter than a gallon of water, and a gallon of honey is much heavier, even though the volume is identical. Without density there is no single answer.
Is a US gallon the same as a UK gallon?
No. An Imperial gallon is about 20% bigger than a US gallon (4.546 L versus 3.785 L), so it weighs about 20% more for the same liquid. Always confirm which gallon a figure refers to.
