Aquarium Stocking Density Calculator

About this calculator

The inch-per-gallon rule is a rough aquarium stocking guideline: 1 inch of adult fish length per gallon of water for smaller freshwater tanks (sometimes 1 inch per 2 gallons for larger or more waste-heavy species). This calculator multiplies your tank's gallon capacity by the rule ratio to estimate total stockable “fish-inches,” then subtracts what you've already added to show remaining capacity.

The rule is intentionally conservative and ignores body mass, bioload, and swimming behavior — a deep-bodied goldfish or an active schooling fish needs far more space than a slim inch-for-inch comparison suggests, so most experienced keepers treat it only as a starting sanity check. Beginner aquarists and pet-store staff use it to avoid the most common overstocking mistakes before doing species-specific research or testing water parameters after adding new fish.