How Much Does a Gallon of Water Weigh
A gallon-of-water-weight page calculates water mass based on volume. At room temperature, a US liquid gallon weighs approximately 8.34 pounds (3.78 kilograms). An Imperial (UK) gallon weighs about 10.02 pounds (4.55 kilograms). The Imperial gallon was defined in 1824 as the volume filled by 10 pounds of distilled water, which is where the round number comes from, but since 1985 it has been defined as exactly 4.54609 cubic decimetres and the water it holds no longer weighs a round 10 pounds. Water weight varies slightly depending on its temperature and density.
Find out the exact weight of water in pounds or kilograms. Use our instant weight-to-volume converter and explore how temperature alters liquid density.
Quick Answer
Determine the exact weight of water in pounds or kilograms for US and Imperial liquid gallons.
How Much Does a Gallon of Water Weigh?
A US gallon of water weighs about 8.35 pounds (3.79 kg) at its densest, and 8.34 pounds at 62°F.
An Imperial gallon (UK) of water weighs about 10.02 pounds (4.55 kg). It weighed exactly ten by definition until 1985, which is where the round number people remember comes from.
Liquid System
Note: Based on water at its maximum density, 39°F / 4°C. Warmer water weighs slightly less.
US Customary Water Weight Table
| Volume (US Gallons) | Weight in Pounds (lbs) | Weight in Kilograms (kg) |
|---|---|---|
| 1/2 Gallon | 4.17 lbs | 1.89 kg |
| 1 Gallon | 8.34 lbs | 3.78 kg |
| 2 Gallons | 16.69 lbs | 7.57 kg |
| 5 Gallons | 41.73 lbs | 18.92 kg |
| 10 Gallons | 83.45 lbs | 37.85 kg |
Examples
1 Gallon of Water (US)
8.35 lbs / 3.79 kg
1 Gallon of Water (Imperial)
10.02 lbs / 4.55 kg
5 Gallons of Water (US)
41.73 lbs / 18.93 kg
How it works
The weight of water depends on volume and density. The calculator above uses water at its densest, 39°F / 4°C, where the density is 0.999972 g/mL. Water is lighter at every other temperature, so these are the heaviest a gallon gets:
US Gallon Weight
1 US Gallon ≈ 8.35 pounds (3.79 kg)
Imperial Gallon Weight
1 Imperial Gallon ≈ 10.02 pounds (4.55 kg)
How Temperature and Pressure Affect Water Weight
Water is unique because its density peaks at 39.2°F (4°C). As it cools below this temperature toward freezing, or as it heats up, its molecules spread out, meaning it takes up more space for the same amount of mass.
For everyday home, pool, and construction calculations, using the standard factor of 8.34 lbs per gallon is highly accurate and widely accepted.
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