Inch Per Gallon Calculator
Calculate your aquarium’s stocking level using the inch per gallon rule. Enter your tank size, fish length, and number of fish to determine if your tank is overstocked, at capacity, or understocked.
How the inch per gallon calculator works
This calculator applies the inch per gallon rule, a simple aquarium stocking guideline: for every gallon of water in your tank, you can safely keep up to 1 inch of adult fish. Enter your tank volume, the length of your fish (or select a species), and how many you have — the calculator will show your total fish length, your inches-per-gallon ratio, and whether your tank is understocked, at capacity, or overstocked.
The inch per gallon rule works best for small, peaceful freshwater fish. For larger or more active species, or for saltwater tanks, more nuanced stocking advice is recommended. This tool gives you a quick, easy starting point for responsible aquarium planning.
Step 1: Tank Information
Understanding the Inch Per Gallon Rule
The rule suggests 1 inch of adult fish per gallon of water. For a 20-gallon tank, that’s 20 inches of fish total. This is a rough guideline — always consider fish behavior, filtration, and tank mates.
Step 2: Fish Information
Stocking Tips
Always use the adult size of your fish, not their current size. Overstocking can lead to poor water quality, stress, and disease. When in doubt, stock less.
Stocking Results
Stocking Status
Stocking Scenarios
| Scenario | Total Fish (in) | Inches per Gallon | Status |
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Stocking Breakdown
About the Inch Per Gallon Rule
How it works
The rule is simple: 1 inch of fish per gallon of water. For example, a 20-gallon tank can hold up to 20 inches of adult fish. This calculator multiplies your fish length by the number of fish, then compares that total to your tank’s capacity in inches (which equals the number of gallons).
Note: This rule is a general guideline, not a strict law. Factors like swimming space, aggression, filtration, and waste production also matter. Always research your specific fish species.
Frequently Asked Questions
What is the inch per gallon rule?
The inch per gallon rule is a simple aquarium stocking guideline that suggests you should have no more than 1 inch of fish (adult size) per gallon of water. For example, a 20-gallon tank can safely hold up to 20 inches of fish.
Is the inch per gallon rule accurate?
The inch per gallon rule is a rough guideline, not a precise scientific formula. It works best for small, peaceful fish but doesn’t account for fish behavior, swimming space needs, filtration, or waste production. Larger, more active fish need more space than this rule suggests.
How do I calculate inches per gallon for my aquarium?
To calculate inches per gallon, divide the total length of all fish (in inches) by the tank volume (in gallons). A ratio of 1:1 is considered at capacity, below 1 is understocked, and above 1 is overstocked.
What happens if my tank is overstocked?
Overstocking can lead to poor water quality (high ammonia, nitrite, nitrate), stress, disease outbreaks, aggression, and stunted growth. It also makes the tank harder to maintain. Always aim to stock below the maximum capacity.
Does the inch per gallon rule apply to saltwater tanks?
The inch per gallon rule is less accurate for saltwater tanks, where fish tend to be more active and have higher metabolic rates. Saltwater aquariums often require more conservative stocking — aim for 0.5 to 0.75 inches per gallon as a safer starting point.
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