A US gallon of water weighs 8.345 pounds (3.785 kilograms). A UK (imperial) gallon weighs 10.022 pounds (4.546 kilograms). The difference comes from the different gallon sizes: the US gallon is 3.785 liters while the UK gallon is 4.546 liters (about 20 percent larger).
Water weight reference table
| Volume | Pounds (US gal) | Kilograms | Pounds (UK gal) |
|---|---|---|---|
| 1 fl oz | 0.065 lb | 30 g | 0.063 lb |
| 1 cup (US, 237 mL) | 0.52 lb | 237 g | - |
| 1 pint | 1.04 lb | 473 g | 1.25 lb |
| 1 quart | 2.09 lb | 946 g | 2.51 lb |
| 1 gallon | 8.345 lb | 3.785 kg | 10.022 lb |
| 5 gallons | 41.72 lb | 18.93 kg | 50.11 lb |
| 10 gallons | 83.45 lb | 37.85 kg | 100.22 lb |
| 50 gallons | 417.25 lb | 189.25 kg | 501.10 lb |
| 100 gallons | 834.5 lb | 378.5 kg | 1,002.2 lb |
Why exactly 8.345 lb per US gallon
The US gallon is defined as 3.785411784 liters exactly. Water at 4°C has a density of exactly 1.000 g/mL by definition. So a US gallon of water at 4°C weighs 3,785.411784 grams = 3.7854 kg = 8.3454 pounds (when converted using the standard 0.45359237 kg per pound). For everyday use, 8.34 lb is the standard rounded figure.
Temperature matters (a little)
Water expands as it warms. At 4°C (40°F), density is 1.000 g/mL exactly. At 20°C (68°F, room temperature), density drops to 0.998 g/mL. At 100°C (212°F, boiling), density is 0.958 g/mL. The practical implication: a gallon of hot water weighs about 4 percent less than a gallon of cold water. For everyday math this doesn't matter. For engineering precision (boiler design, pool heating), it does.
Practical weight calculations
5-gallon water jug: 41.7 lb (about 19 kg) of water plus a pound for the plastic. About what a 4-year-old child weighs. Often kept in office and home water coolers.
Bathtub full of water: A standard bathtub holds 40-80 gallons (150-300 liters). That's 334-668 lb (151-303 kg) of water. The bathroom floor needs to support this load plus the bather and the tub itself.
Aquarium: A 55-gallon fish tank holds 458 lb of water (208 kg). Add about 50 lb for the tank, gravel, and stand. The combined load is 500+ lb, often more than what an unreinforced floor can handle. Always check structural capacity before setting up large aquariums.
RV fresh water tank: A typical 40-gallon tank holds 334 lb of water (151 kg). This affects vehicle handling and fuel consumption; many RV owners travel with tanks empty and fill at the campsite to save fuel.
Water vs other common liquids
Knowing water's weight helps you estimate other liquids by their density relative to water (specific gravity):
- Petrol/gasoline: 0.74 g/mL, a gallon weighs about 6.2 lb (US) or 7.4 lb (UK).
- Diesel fuel: 0.84 g/mL, about 7.0 lb (US) or 8.4 lb (UK) per gallon.
- Heating oil: 0.85 g/mL, similar to diesel.
- Milk (whole): 1.03 g/mL, about 8.6 lb (US) per gallon.
- Honey: 1.42 g/mL, nearly 12 lb (US) per gallon.
- Cooking oil: 0.92 g/mL, about 7.7 lb (US) per gallon.
- Maple syrup: 1.32 g/mL, about 11 lb (US) per gallon.
The pattern: anything with significant dissolved sugar (honey, syrup) is heavier than water. Anything based on hydrocarbons (fuel, oil) is lighter. Milk and most household liquids are close to water.
Why "a pint a pound" only works in the US
The American rhyme "a pint's a pound the world around" is true in the US (a US pint weighs 1.04 lb, close enough to 1 lb to be a useful mnemonic). In the UK, a pint is 568 mL (vs 473 mL US), so a UK pint of water weighs about 1.25 lb, noticeably more than a pound. The rhyme is a US-only convenience.