Unlike the old "inch per gallon" rule, this works from metabolic fish weight and filtration, the way aquariumscience.org argues stocking really works. It also checks something most calculators ignore: whether your fish suit your water hardness.
Enter dimensions and the volume fills in (about 85% of the raw box, for substrate and gaps), or type a volume directly. Not sure of your hardness? See keeping fish in hard water.
Flow is only a rough proxy. What actually limits stocking is biomedia surface area (aquariumscience.org puts it at ~100 sq ft of good media per 454 g of fish), so more and better media beats raw flow. Aim for at least 4 to 6 times the tank volume per hour.
No fish added yet. Pick a species above and press Add.
It depends on adult size and filtration, not a length rule. This calculator uses a metabolic-weight model and also flags group sizes, minimum tank size, temperament and hardness.
Waste scales with body mass, not length. A 30 cm fish produces far more than ten 3 cm fish of the same total length, and the rule ignores filtration. A metabolic-weight approach tracks bioload far better.
The total waste your fish produce for the filter to process. This tool estimates it from fish weight scaled by turnover, but the honest gauge is your water: if it goes cloudy, stop adding fish.
Want something simpler? The quick stocking calculator works from combined body length. Then read how many fish you can keep and size the filter with choosing an aquarium filter.
An estimate to help you plan, not a guarantee. It uses David Bogert's metabolic-weight model from aquariumscience.org (fish weight in grams ≈ 0.02 × length³, healthy stocking ≈ 900 g per 100 US gallons with good filtration), scaled by your turnover as a rough stand-in for biofiltration. Real capacity depends on media surface area, aeration, planting and maintenance, and the honest gauge is your water: if it goes cloudy, stop adding fish. Not affiliated with aquariumscience.org or AqAdvisor. Aggression and compatibility vary between individuals.