Aquarium Stocking Calculator
The aquarium stocking calculator helps fishkeepers estimate how many fish a tank can safely support. Two common methods are the inch-per-gallon rule (one inch of adult fish body length per US gallon of water) and the surface area method (based on oxygen exchange at the water surface). Both are simplified guidelines that ignore bioload, fish temperament, and filtration quality, but they provide a useful starting point. Enter your tank dimensions and fish details to get a stocking estimate using both methods. Always prioritise water quality testing over any numerical rule.
Stocking formulas
Tank volume (US gal) = Length x Width x Depth (in) / 231
Max inches (1 in/gal) = Tank volume (gallons)
Surface area (sq in) = Length x Width
Max inches (surface) = Surface area / sq in per fish inch
Tropical: 24 sq in per inch; Cold water: 12 sq in per inch
Frequently asked questions
What is the one-inch-per-gallon rule?
The one-inch-per-gallon rule suggests one inch of adult fish body length per US gallon of tank water. For example, a 20-gallon tank can hold 20 inches of fish: 4 fish that are 5 inches each, or 10 fish that are 2 inches each. This is a rough guide only, not a scientific formula.
Is the inch-per-gallon rule accurate?
The inch-per-gallon rule is a simplified starting point, not a precise rule. It ignores fish body shape (a fat goldfish needs more space than a thin tetra of the same length), bioload (goldfish produce more waste than tetras), behaviour (territorial fish need more territory), and filtration quality. Use it as a minimum guide only.
What is the surface area method for stocking?
The surface area method calculates stocking based on the water surface area, which determines oxygen exchange. One rule of thumb is 12 square inches of surface area per inch of fish for cold water, or 24 square inches per inch for tropical fish. A 24x12 inch tank has 288 sq in of surface, allowing 24 inches of tropical fish.
Does filtration affect how many fish I can keep?
Yes, significantly. A high-quality filter with adequate biological filtration allows a higher stocking density than the simple formulas suggest. Many experienced aquarists with excellent filtration and regular maintenance keep 2 to 3 times the inch-per-gallon guideline without water quality issues.
Do I count the full tank volume?
Use net water volume, not gross tank capacity. Subtract volume occupied by substrate, decorations, and any internal equipment. A 20-gallon tank typically holds 16 to 18 gallons of water after accounting for gravel and decorations.
Sources
- USDA APHIS: USDA Animal Welfare Act - Fish and Aquatic Animal Care.
- Smithsonian National Zoo: Smithsonian National Zoo - Fish Conservation and Husbandry.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.