Aquarium Stocking Calculator
Check whether your fish tank is understocked, balanced or overstocked using the inch-per-gallon guideline, with remaining capacity in inches.
How it works
Capacity (inches) = tank volume in gallons · Stocking % = total fish inches ÷ gallons × 100 · Remaining = gallons − fish inches
The classic aquarium guideline allows about one inch of slim, adult fish per gallon of water, so a 20-gallon tank targets roughly 20 inches of fish. Stocking percentage compares your total adult fish length to that ceiling, and staying at or below 100% leaves margin for the waste and oxygen demand real fish create. Treat it as a rough starting point, not a law: it assumes small community species and ignores body mass, so it badly overstates capacity for chunky or messy fish like goldfish and cichlids. A complementary check is surface area — roughly one inch of fish per 12 square inches of water surface — because oxygen exchange happens at the top, which is why a wide, shallow tank holds more fish than a tall, narrow one of the same volume.
Worked example
A 20-gallon tank stocked with 12 inches of small tetras and rasboras is 12 ÷ 20 × 100 = 60% of the guideline — understocked, with about 8 more inches of slim fish before reaching the one-inch-per-gallon ceiling.
Frequently asked questions
Is the inch-per-gallon rule reliable?
Only as a rough guide for small, slim community fish. It ignores body mass and waste output, so it overstates capacity for large or heavy-bodied species like goldfish, which can need 10–20 gallons each despite their length.
Should I use adult or current size?
Always use the full adult length. Fish keep growing, and a tank that fits juveniles can become badly overstocked once they mature — plan for the size they will reach, not the size they are.
What is the surface-area rule?
It allows about one inch of fish per 12 square inches of water surface, because oxygen enters at the top. It rewards wide, shallow tanks and is a useful sanity check alongside the volume guideline.
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This calculator is for educational purposes. Double-check important results before acting on them.