A US gallon of water weighs about 8.34 pounds, or 3.785 kilograms. The figure shifts a little with temperature — from 8.345 lb near freezing down to 8.0 lb at boiling — but for any practical purpose 8.34 lb is the number to use.
The metric version is the reason the conversion is so clean: the original definition of the kilogram was the mass of a litre of water. So a litre of water weighs a kilogram, and everything else follows from the size of the gallon.
| Volume | Pounds | Kilograms |
|---|---|---|
| 1 cup | 0.52 lb | 0.24 kg |
| 1 pint | 1.04 lb | 0.47 kg |
| 1 quart | 2.09 lb | 0.95 kg |
| 1 litre | 2.20 lb | 1.00 kg |
| 1 US gallon | 8.34 lb | 3.79 kg |
| 5 US gallons | 41.7 lb | 18.9 kg |
| 10 US gallons | 83.5 lb | 37.9 kg |
| 20 US gallons | 167 lb | 75.7 kg |
| 55 US gallons (drum) | 459 lb | 208 kg |
| Temperature | Density (kg/L) | Weight per US gallon |
|---|---|---|
| 0°C / 32°F (ice point) | 0.99984 | 8.344 lb |
| 4°C / 39°F (densest) | 0.99997 | 8.345 lb |
| 20°C / 68°F (room temp) | 0.99820 | 8.330 lb |
| 25°C / 77°F | 0.99705 | 8.321 lb |
| 37°C / 99°F (body temp) | 0.99333 | 8.290 lb |
| 100°C / 212°F (boiling) | 0.95840 | 7.998 lb |
Water is at its densest at about 4°C, not at freezing — which is why ice floats and why lakes freeze from the top down. Across the whole range from ice to boiling the weight varies by only about 4%.
Why the imperial gallon weighs exactly 10 pounds
The US and imperial gallons weigh different amounts, and only one of them was designed that way.
The imperial gallon was created by Britain’s Weights and Measures Act of 1824 and defined as the volume occupied by 10 pounds of distilled water at a specified temperature and pressure. The weight came first and the volume was derived from it — so an imperial gallon of water weighs 10 pounds essentially by construction. Fixed at exactly 4.54609 L today, it works out to 10.02 lb.
The US gallon has no such tidiness. It is the old English wine gallon of exactly 231 cubic inches, a volume chosen for reasons that had nothing to do with the weight of water, and it happens to hold 8.34 lb.
That 20% difference matters for anything shipped or metered by the gallon: a UK gallon of fuel is a fifth more liquid, and a fifth more weight, than a US one.
Practical loads
Water is heavy, and the arithmetic catches people out on projects:
- A 5-gallon water-cooler bottle holds 41.7 lb of water — close to 45 lb with the container
- A 55-gallon drum holds 459 lb, which is why full drums are moved with equipment rather than by hand
- A 40-gallon domestic water heater holds 334 lb of water on top of its own weight
- A 55-gallon aquarium holds about 459 lb of water alone; with substrate, rock and glass the total commonly exceeds 600 lb, which is more than most floors are designed to carry over a small footprint
- A standard bathtub holds roughly 40–50 gallons, or 330–420 lb
For anything structural, work from the full loaded weight and check the floor rating. Water’s weight is the part people forget because the container looks manageable when it is empty.
Other liquids by the gallon
Water is the reference point, and most household liquids are close to it because they are mostly water:
- Water: 8.34 lb per US gallon
- Milk: about 8.6 lb — slightly denser than water because of dissolved solids
- Seawater: about 8.56 lb, roughly 2.5% denser due to dissolved salt
- Gasoline: about 6.1 lb — noticeably lighter, which is why it floats on water
- Diesel: about 7.1 lb
- Honey: about 12 lb
Gasoline is the useful one to remember: a 15-gallon tank of fuel adds around 92 lb to a car, against 125 lb if it were water.
Water weight in metric
The metric version needs no conversion factor at all, which is the whole point of the system:
- 1 millilitre of water = 1 gram
- 1 litre of water = 1 kilogram
- 1 cubic metre of water = 1,000 kilograms (1 tonne)
So any volume of water in litres gives you its weight in kilograms directly. A 20-litre jerrycan holds 20 kg; a 200-litre drum holds 200 kg.
This was deliberate. When the metric system was established, the gram was defined as the mass of a cubic centimetre of water. Modern definitions are based on physical constants rather than water, so the relationship is now very slightly approximate — water at its densest is 0.999972 kg/L rather than exactly 1 — but the error is 28 parts per million, which is irrelevant outside a laboratory.
Frequently Asked Questions
How much does a gallon of water weigh?
A US gallon of water weighs about 8.34 pounds, or 3.785 kilograms. At water’s maximum density (4°C) it is 8.345 lb; at room temperature it is closer to 8.33 lb.
How much does 5 gallons of water weigh?
41.7 pounds, or 18.9 kilograms. A 5-gallon water-cooler bottle weighs around 45 lb once you include the container.
Why does an imperial gallon of water weigh 10 pounds?
Because that is how it was defined. Britain’s 1824 Weights and Measures Act set the imperial gallon as the volume of 10 pounds of water, so the weight came first. The US gallon is the older wine gallon of 231 cubic inches and holds 8.34 lb by coincidence.
Does temperature change how much water weighs?
Yes, but not much. A gallon weighs 8.345 lb at 4°C where water is densest, 8.33 lb at room temperature, and 8.0 lb at boiling — a total variation of about 4% across the entire liquid range.
How much does a gallon of milk weigh?
About 8.6 pounds — slightly more than water’s 8.34 lb, because milk contains dissolved sugars, proteins and minerals that make it denser.
How much does a litre of water weigh?
Exactly one kilogram, for all practical purposes — that relationship was the original definition of the kilogram. Precisely, water at its densest is 0.999972 kg per litre.