Quick answer: measure the inside dimensions and the actual water depth, then apply the shape's formula. Convert cubic feet to US gallons by multiplying by 7.48. Water weighs 8.34 lb per US gallon, or in metric, 1 liter = 1 kilogram — the tidiest conversion in all of measurement.
Water volume and weight, side by side
Water is the reference substance for the metric system, which makes its conversions unusually memorable:
| Volume of water | Weight (lb) | Weight (kg) |
|---|---|---|
| 1 US gallon | 8.34 | 3.79 |
| 1 imperial gallon | 10.02 | 4.55 |
| 1 liter | 2.20 | 1.00 |
| 1 cubic foot | 62.4 | 28.3 |
| 1 cubic meter | 2,205 | 1,000 |
| 1 cubic yard | 1,685 | 764 |
| 55-gallon drum | 459 | 208 |
| 100 gallons | 834 | 378 |
| 1,000 gallons | 8,340 | 3,785 |
Those numbers add up fast, and they are the reason water weight is a structural question rather than a trivia question. A modest 100-gallon aquarium on a stand puts more than 800 lb of water on one small footprint — before glass, substrate and rock. A 275-gallon IBC tote holds well over a ton.
Formulas by container shape
Round: V = π × (diameter/2)² × water depth
Oval: V = (π/4) × length × width × water depth
Worked example — a stock trough 8 ft long, 3 ft wide, filled to 20 inches:
- Convert the depth: 20 in ÷ 12 = 1.667 ft
- Volume: 8 × 3 × 1.667 = 40 ft³
- Gallons: 40 × 7.48 = 299 US gallons
- Weight: 299 × 8.34 ≈ 2,494 lb — over a ton sitting on that ground
The single most common error here is measuring the container's height rather than the water's depth. Containers are almost never full to the rim, and the difference is pure error.
Where the number matters
Floor loading
Residential floors are typically designed for around 40 lb per square foot of live load. A 125-gallon aquarium with a 2 ft × 6 ft footprint puts about 1,040 lb of water over 12 ft² — roughly 87 lb per square foot before the tank itself. That is why large aquariums are placed against load-bearing walls and perpendicular to joists.
Water storage and emergency planning
Standard emergency guidance is one gallon per person per day. A family of four planning for two weeks needs 56 gallons, which is one 55-gallon drum weighing 459 lb when full — worth knowing before deciding where it lives.
Heating and treating water
Heating water takes 8.34 BTU per gallon per degree Fahrenheit. Raising a 40-gallon water heater by 70 °F requires about 23,350 BTU. Chemical treatment likewise depends entirely on knowing the volume — see the pool volume calculator for dosing tables.
Common mistakes & pro tips
- Container height instead of water depth. Measure to the waterline.
- Outside dimensions on a thick-walled tub. Capacity lives inside the walls.
- Ignoring displacement. Rocks, substrate, equipment and fish all take up space. In a planted aquarium, substrate and hardscape commonly displace 10–15% — the aquarium calculator handles that subtraction.
- Pro tip — verify with a meter. Water meters usually read in cubic feet or gallons. Fill a container while watching the meter and you have both a volume check and a leak test.
- Pro tip — the metric shortcut. Work in centimeters and the numbers fall out for free: volume in cm³ ÷ 1,000 = liters = kilograms.
Frequently asked questions
How much does a gallon of water weigh?
A US gallon of fresh water weighs about 8.34 pounds (3.785 kg) at room temperature. An imperial gallon weighs about 10.02 pounds because it is roughly 20% larger. In metric the relationship is cleaner still: one liter of water weighs almost exactly one kilogram.
How do I calculate the volume of water in a container?
Measure the inside dimensions and the actual water depth, not the container height. For a rectangular container, volume = length × width × water depth. For a round one, volume = π × (diameter ÷ 2)² × water depth. Convert cubic feet to US gallons by multiplying by 7.48.
Does water weight change with temperature?
Slightly. Water is densest at 4 °C, where a liter weighs 1.000 kg. At 20 °C it is 0.998 kg and near boiling about 0.958 kg — roughly a 4% swing across the liquid range. For everyday calculations 8.34 lb per gallon is accurate enough; for laboratory work, use the density at your actual temperature.
How many liters are in a cubic meter of water?
Exactly 1,000 liters, weighing 1,000 kg — one metric tonne. This is the definition the metric system is built around, and it makes water calculations unusually easy: a cubic meter of water is a tonne, and a 10 cm cube is a liter is a kilogram.
Is salt water heavier than fresh water?
Yes. Seawater averages about 1.025 kg per liter against fresh water's 1.000, so it is roughly 2.5% heavier — about 8.55 lb per US gallon. That difference is what makes floating easier in the ocean, and it matters when sizing pumps or calculating loads for marine aquariums.