Aquarium Volume & Stocking Calculator
Enter your aquarium's dimensions to calculate its volume in gallons, plus a general fish stocking estimate using the commonly cited (though simplified) 1-inch-of-fish-per-gallon guideline.
How the Aquarium Volume & Stocking formula works
Tank volume from dimensions, then a general stocking guideline:
Volume (gallons) = Length × Width × Height (inches) / 231 General stocking estimate = Volume (gallons) inches of adult fish
Step-by-step calculation
- Multiply tank length, width, and height in inches to get cubic inches.
- Divide by 231 (cubic inches per US gallon) to get tank volume in gallons.
- Use the 1-inch-per-gallon guideline as a rough starting point for stocking, recognizing it's a simplification.
Worked example
A tank measuring 36 × 18 × 16 inches: Volume = 36×18×16/231 ≈ 44.9 gallons — a general stocking guideline would suggest up to about 45 inches of adult fish, combined across however many fish are kept.
Frequently asked questions
Is the 1-inch-per-gallon rule accurate?
It's a widely known, simplified starting guideline, but it doesn't account for fish shape, waste production, swimming behavior, or filtration capacity — a slim, active fish and a bulky, waste-heavy fish of the same length have very different real stocking needs, so this should be treated as a rough starting point rather than a precise rule.
Does the calculated volume match the water you'll actually add?
No — actual water volume is somewhat less than total tank volume due to substrate, decorations, and the water line typically sitting below the very top of the tank, so real usable water volume runs a bit lower than this calculation.