One US gallon of water weighs about 3.785 kilograms near room temperature. One imperial gallon weighs about 4.546 kilograms. These are practical estimates based on water density close to 1 kilogram per litre; the exact value changes slightly with temperature and water composition.
Water weight by gallon
| Volume | Approximate mass |
|---|---|
| 1 US gallon | 3.785 kg |
| 5 US gallons | 18.927 kg |
| 10 US gallons | 37.854 kg |
| 20 US gallons | 75.708 kg |
| 1 imperial gallon | 4.546 kg |
These figures describe the water only, not the container.
Why US and imperial gallons differ
A US liquid gallon contains 3.785411784 litres. An imperial gallon contains 4.54609 litres, so it holds about 20% more water. Always state the gallon standard when recording a measurement or comparing a recipe, tank, or shipment.
How to calculate water weight
For a known volume:
water mass (kg) = liters × water density (kg/L)
For a known mass:
gallons = kilograms ÷ density ÷ gallon size in liters
Use the water kg-to-gallons calculator for a result using the selected gallon standard.
Does temperature matter?
Yes, but usually only slightly for everyday estimates. Water density changes with temperature, so a fixed gallon can contain a little more or less mass. For laboratory, custody-transfer, or engineering work, use the measured density and reference temperature rather than rounding water to exactly 1 kg/L.
Bottom line
Use 3.785 kg per US gallon and 4.546 kg per imperial gallon as practical room-temperature estimates. Include the gallon standard and temperature whenever the result needs to be reproducible.
Sources & References
- NIST: SI Units and Volume Measurement Standards
- BIPM: The International System of Units (SI Brochure)
- The Engineering ToolBox: Liquids Densities Reference
- USDA FoodData Central: Food and Beverage Reference Values
- For product-specific density, use the manufacturer’s verified Technical Data Sheet (TDS) or Safety Data Sheet (SDS) at 20°C (68°F).
