How Much Does a Gallon of Water Weigh? Temperature Explained
One US gallon of water weighs approximately 8.34 pounds (3.78 kg). This measurement applies to water at cold or room temperature. The weight changes as the temperature varies; at 4°C-the temperature at which water reaches its maximum density-it weighs 8.345 pounds (3.785 kg), whereas at the boiling point, the weight drops to approximately 8.0 pounds (3.63 kg). The UK, or 'Imperial', gallon is a larger unit, holding approximately 10 pounds (4.54 kg) of water. For general purposes, you can consider the weight to be 8.34 pounds (or 3.78 kg) per US gallon; however, when temperature is a significant factor, please refer to the table provided below.
Weight of a US Gallon of Water at Different Temperatures
A US gallon is a fixed volume: exactly 231 cubic inches, or about 3.785 litres . The table converts the density data published by the USGS Water Science School into gallons of water weight. Our calculated values match the pounds-per-gallon figures in Milwaukee Valve’s water properties sheet to within about 0.01 lb.
| Water temperature | Pounds per US gallon | Kilograms per US gallon |
|---|---|---|
| 0°C (32°F) | 8.344 | 3.785 |
| 4°C (39.2°F), maximum density | 8.345 | 3.785 |
| 15.6°C (60°F) | 8.337 | 3.782 |
| 21°C (70°F) | 8.328 | 3.778 |
| 32.2°C (90°F) | 8.304 | 3.767 |
| 60°C (140°F) | 8.206 | 3.722 |
| 100°C (212°F) | 8.000 | 3.629 |
Between 0°C and 21°C the weight changes by only about 0.2%. From 4°C to boiling, a gallon loses about 4% of its weight. In an Indian summer, tap water at around 32°C weighs roughly 3.77 kg per US gallon, which is why 3.78 kg is a safe working number for most days.
Why Does Temperature Change Weight?
A gallon measures volume, not weight. The weight of a gallon depends on how much water it holds, which in turn depends on density-the mass packed into a given unit of volume.
As water heats up, its molecules spread further apart; consequently, the same 3.785 liters contain less water and weigh less. Pure water reaches its maximum density near 4°C. Below this temperature, it begins to expand again; ice is about 9% less dense than liquid water, which is why it floats.
US Gallon vs Imperial Gallon
The term "gallon" refers to two different volumes. A US gallon is 3.785 liters. An Imperial gallon is approximately 4.546 liters-about 20% larger-and was historically defined as the volume of 10 pounds of water under specific conditions. Therefore, an Imperial gallon of water weighs approximately 10 pounds (4.54 kg), while a US gallon weighs approximately 8.34 pounds (3.78 kg).
Check which "gallon" is used in the manual or product specification sheet. In India, liquids are measured in liters; thus, the term "gallon" typically appears in specifications for imported items, such as US-made aquariums or tanks. A 50-gallon tank (rated in US gallons) holds approximately 189 kg of water, whereas a 50-gallon tank (rated in Imperial gallons) holds approximately 227 kg of water.
Does Salt or Anything Else in the Water Change It?
Yes. Dissolved material makes water denser, so a gallon weighs more . Using typical densities, which are approximate:
- Seawater (about 1.025 g/mL): roughly 8.55 lb (3.88 kg) per US gallon, about 2.5% more than fresh water.
- Saturated salt solution (about 26% salt by weight, roughly 1.19 g/mL at room temperature, per a Mississippi State chemistry text): roughly 10 lb (4.5 kg) per US gallon.
- Tap, filtered and RO water: the minerals they carry are far too small to matter, well under 1%, for lifting or load planning.
How to Calculate the Weight of Any Amount of Water?
Multiply the volume by the weight per gallon:
- Pounds = US gallons × 8.34
- Kilograms = US gallons × 3.78
| US gallons | Pounds | Kilograms |
|---|---|---|
| 1 | 8.34 | 3.78 |
| 5 | 41.7 | 18.9 |
| 10 | 83.4 | 37.8 |
| 20 | 166.8 | 75.6 |
| 50 | 417 | 189 |
| 55 (drum) | 458.7 | 208 |
| 100 | 834 | 378 |
These figures cover the water only. Add the weight of the container, tank or jug. If your volume is in litres, the rule is simpler: a litre of water weighs about 1 kg (0.998 kg at 21°C), so a 20-litre can hold close to 20 kg. To switch between pounds and kilograms for other values, Online Calculator Tools offers a free weight converter.
Common Mistakes When Estimating Water Weight
- Using 8.34 lb for an imperial gallon. The result will be about 17% too low.
- Treating a gallon as 4 litres. The real figure is 3.785 litres, so this overestimates by nearly 6%.
- Ignoring the container. A full jug, barrel or tank weighs more than the water inside it.
- Worrying about altitude. Its effect on a gallon’s weight is much smaller than temperature’s, so ignore it for household use.
Frequently Asked Questions
Question 1. What is the weight of one gallon of water in kilograms?
At room temperature, one US gallon of water weighs approximately 3.78 kilograms, and at a temperature of 4°C, it weighs 3.785 kilograms. One Imperial gallon weighs approximately 4.54 kilograms.
Question 2. What is the weight of a 5-gallon water jug?
Based on US gallons, the weight of the water alone is approximately 41.7 pounds (lb) or 18.9 kilograms. The weight of the empty jug itself must also be added; therefore, check the label on the jug or weigh it to find out.
Question 3. Does hot water weigh less than cold water?
Yes, for the same volume, this is the case. At a temperature of 60°C, one gallon of water weighs approximately 3.72 kilograms, whereas at room temperature, it weighs 3.78 kilograms. For everyday use, this difference is negligible.
Question 4. Why do some websites show the weight as 8.33, 8.34, or 8.35 pounds?
They are displaying density data-calculated at different temperatures-that has been rounded off. At a temperature of 70°F, one gallon of water weighs approximately 8.33 pounds, and at 4°C, it weighs about 8.35 pounds. A commonly used figure is 8.34, which falls between these two values.
Question 5. Does a gallon of ice weigh the same as a gallon of water?
No. The density of ice is about 10% lower; therefore, a gallon of ice weighs approximately 7.5 pounds (3.4 kilograms). The exact figure may vary because air can get trapped inside the ice.
Question 6. How many US gallons are there in 1,000 kilograms of water?
Approximately 264. One cubic meter (1,000 liters) of water weighs roughly 1,000 kilograms and is equivalent to about 264 US gallons.