Fish Tank Stocking Calculator
Calculate how many fish your tank can hold using the classic one-inch-per-gallon rule. Enter tank volume and average fish length.
How to use this tool
- Enter your tank volume in US gallons (use the Aquarium Volume Calculator if needed).
- Enter the typical adult length of the fish species in inches.
- The result is the approximate maximum number of fish.
- Always research specific species needs — filtration, territory, and water chemistry matter more than this rule alone.
The one-inch-per-gallon rule is a simple starting point for stocking a freshwater aquarium. It works best for small, slim-bodied community fish. For large, round-bodied, or predatory species use more conservative stocking.
⚠ This tool provides general estimates for education only and is not financial, tax or legal advice. Figures may not reflect your situation — verify with a qualified professional.
Formula
Total stocking inches = tank volume (US gal)
Maximum fish = ⌊ tank volume (US gal) ÷ average fish length (inches) ⌋
This implements the classic one-inch-of-fish-per-gallon rule.
How it works
The calculator applies the one-inch-per-gallon guideline: total allowable fish inches equals the tank volume in US gallons, and the number of fish is that total divided by the average adult fish length, rounded down to a whole fish.
This rule is a rough starting point only; it ignores bioload differences between species, filtration capacity, fish body shape, and territorial behaviour. Heavily bodied or waste-producing fish (such as goldfish) typically require more space per inch than slim tropical species.
Worked example
- Tank volume: 20 US gallons; average fish length: 2 inches.
- Total stocking inches allowed = 20 (one inch per gallon).
- Maximum fish = floor(20 ÷ 2) = floor(10) = 10 fish.
Maximum fish = 10; total stocking inches = 20
Common mistakes to avoid
- Applying the one-inch-per-gallon rule to large fish such as goldfish or cichlids -- the rule was designed for small community fish and dramatically understates the bioload of bigger species.
- Measuring fish at purchase size rather than adult length -- a 2-inch common pleco grows to 18+ inches; stocking based on juvenile size fills the tank to unsafe levels within months.
- Ignoring tank footprint and counting only total gallons -- a tall, narrow 20-gallon tank has far less surface area and swimming space than a standard 20-gallon long, so the maximum fish count should be lower.
Key terms
- One-inch-per-gallon rule
- A widely cited aquarium guideline stating that a tank can support one inch of adult fish body length per US gallon of water.
- Bioload
- The total biological waste produced by fish and other inhabitants; high-bioload species require more filtration and water volume than the inch-per-gallon rule suggests.
- Stocking density
- The total fish biomass or length per unit volume of water; keeping this within safe limits is essential for water quality and fish health.
- Adult fish length
- The full-grown body length of a fish species, which should be used (not juvenile size) when calculating stocking capacity.
Frequently asked questions
- Is the one-inch-per-gallon rule accurate?
- It is a rough guideline only. Larger, messy fish (e.g. goldfish, cichlids) produce more waste per inch than small tropical fish. Always consider filtration capacity and species behaviour.
- How many fish can I put in a 10-gallon tank?
- Using the 1-inch-per-gallon rule with 1.5-inch fish: 10 ÷ 1.5 ≈ 6 fish. In practice, a 10-gallon is better suited to 5–6 small fish such as neon tetras.