Aquarium Tank Volume Calculator
Enter your tank's inside dimensions to get its volume in gallons and liters â plus the filled weight, so you know whether your stand and floor can handle it.
Calculate tank volume
Measure the inside dimensions for the most accurate result. Filled weight includes ~12% extra for glass, substrate and equipment.
The formula
Gallons = (Length à Width à Height in inches) Ãˇ 231. One US gallon occupies exactly 231 cubic inches. For metric, divide cubic centimeters by 1,000 to get liters.
Example: a standard 40-gallon breeder measures 36 à 18 à 16 inches inside. 36 à 18 à 16 = 10,368 cubic inches Ãˇ 231 â 44.9 gallons of theoretical volume. Real tanks hold slightly less once you account for glass thickness and the fact that you never fill to the very brim.
Common tank sizes
| Nominal size | Typical dimensions (in) | Actual gallons |
|---|---|---|
| 10 gallon | 20 Ã 10 Ã 12 | ~10.4 |
| 20 gallon long | 30 Ã 12 Ã 12 | ~18.7 |
| 29 gallon | 30 Ã 12 Ã 18 | ~28.0 |
| 40 gallon breeder | 36 Ã 18 Ã 16 | ~44.9 |
| 55 gallon | 48 Ã 13 Ã 21 | ~56.7 |
| 75 gallon | 48 Ã 18 Ã 21 | ~78.6 |
Why filled weight matters
Water weighs 8.34 lb per gallon. A 55-gallon setup easily exceeds 500 lbs once you add glass, substrate, rocks and equipment. That's heavier than a refrigerator â so verify your stand is rated for aquariums (not just furniture) and, for large tanks, that your floor can take the load. When in doubt, place big tanks against a load-bearing wall.
Frequently asked questions
How do you calculate the volume of a fish tank?
Multiply length à width à height in inches and divide by 231 for US gallons. In metric, multiply the dimensions in centimeters and divide by 1,000 for liters.
How much does a filled fish tank weigh?
About 8.34 lbs per gallon of water, plus 10â15% for the tank itself, substrate and equipment. A 55-gallon tank totals roughly 520 lbs.
Should I measure inside or outside dimensions?
Inside dimensions give the true water volume. Outside dimensions are fine for a rough estimate but will read slightly high because of glass thickness.