Water Weight Converter
Convert between water volume and weight in any unit
Water Weight Formulas
Understanding Water Weight
Water's weight is fundamental to many calculations—from filling aquariums and pools to shipping liquids and calculating structural loads. Unlike most substances, water has a beautifully simple relationship between volume and mass in the metric system: 1 liter of water weighs exactly 1 kilogram (at 4°C, where water is densest).
This convenient 1:1 relationship is by design—the kilogram was originally defined as the mass of one liter of water. In imperial units, the relationship is less elegant: a US gallon weighs 8.34 pounds, and a cubic foot weighs 62.4 pounds.
Temperature affects water's density slightly (water is densest at 4°C/39°F), but for practical purposes at room temperature, these conversions are accurate within 1%. This converter is invaluable for aquarium enthusiasts, pool owners, engineers, and anyone transporting or storing water.
Water Weight by Container
Gallon Jug
1 US gallon = 8.34 lbs (3.78 kg). A full 5-gallon water jug weighs 41.7 lbs.
Aquarium
A 50-gallon aquarium holds 417 lbs of water. Add 10% for gravel/décor.
Swimming Pool
Average 15,000-gallon pool = 125,000 lbs (62.5 tons) of water.
Bathtub
Full bathtub (80 gallons) = 667 lbs of water, important for floor load calculations.
Water Weight Reference
| Volume | US Weight | Metric Weight | Common Use |
|---|---|---|---|
| 1 Liter | 2.2 lbs | 1 kg | Water bottles |
| 1 Gallon (US) | 8.34 lbs | 3.78 kg | Milk jugs |
| 1 Cubic Foot | 62.4 lbs | 28.3 kg | Aquariums |
| 5 Gallons | 41.7 lbs | 18.9 kg | Water coolers |
| 55 Gallons (drum) | 459 lbs | 208 kg | Rain barrels |
| 1 Cubic Meter | 2,205 lbs | 1,000 kg | 1 metric ton |
Practical Applications
Floor Load Capacity
Before placing a large aquarium, check floor capacity. A 100-gallon tank exceeds 800 lbs—too heavy for some floors.
Vehicle Loading
Carrying water adds up fast. 10 gallons = 83 lbs. Know your vehicle's payload capacity.
Emergency Storage
FEMA recommends 1 gallon/person/day. A week's supply for 4 people = 28 gallons = 234 lbs.
Temperature Effect
Hot water is slightly lighter than cold. Boiling water is ~4% less dense than 4°C water.
Frequently Asked Questions
Why does a gallon of water weigh 8.34 pounds specifically?
A US gallon equals 231 cubic inches (by definition). Water weighs 0.036127 pounds per cubic inch at room temperature. Thus: 231 × 0.036127 = 8.34 lbs. It's not a round number because the gallon wasn't defined with weight in mind.
Does salt water weigh more than fresh water?
Yes. Seawater (3.5% salt) weighs about 2.5% more than fresh water. A gallon of seawater weighs 8.56 lbs vs. 8.34 lbs for fresh water. This is why it's easier to float in the ocean.
How much does ice weigh compared to water?
Ice weighs the same as the water it came from—freezing doesn't change mass. However, ice is about 9% less dense, so it takes up more volume. That's why ice floats and pipes burst when water freezes.
How do I calculate water weight for an irregularly shaped container?
Measure the volume using the water displacement method or calculate using geometry. Then convert: multiply liters by 1 for kg, or gallons by 8.34 for pounds. For aquariums: L × W × H (in inches) ÷ 231 = gallons.
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