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Fish Tank Stocking Calculator: Inch-per-Gallon, Surface Area & Bioload

Calculate how many fish your aquarium can hold using 3 methods: inch-per-gallon, surface area, and bioload. Fresh and saltwater. 13 preset species.

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How to Use This Calculator

  1. 1

    Set water type and choose your stocking method

    Select freshwater or saltwater. Choose inch-per-gallon for a quick estimate, surface area for tanks with lots of aeration, or bioload for the most accurate result — especially important for goldfish, cichlids, or any heavy-bodied species.

  2. 2

    Enter your tank dimensions or volume

    Choose rectangular, bowfront, or cylindrical shape and enter the inside dimensions in inches. Or switch to 'Enter gallons directly' if you already know your tank's stated volume.

  3. 3

    Add your fish

    Select a species from the dropdown or choose 'Custom fish' to enter any species. Set the quantity and click Add. Repeat for each species in your planned community.

  4. 4

    Check stocking percentage

    Click Calculate to see your tank's stocking level as a percentage of safe capacity. Green (under 80%) means good. Yellow (80–100%) means near capacity — monitor water quality closely. Red means overstocked.

  5. 5

    Compare all three methods

    The results show all three stocking methods side by side. If inch-per-gallon says 60% but bioload says 95%, trust the bioload figure — especially for heavy-bodied fish like goldfish or large cichlids.

What Each Value Means

Bioload Score (bioload units)
A weighted measure of the waste a fish produces, calculated as: fish length × body-type multiplier. Slim-bodied fish (tetras, danios) = 1.0×. Medium-bodied (platys, mollies) = 1.8×. Heavy-bodied (goldfish, cichlids, oscars) = 3.0×. A tank's safe bioload capacity is approximately 0.80 × tank volume in gallons.
Surface Area Capacity (inches of fish per surface area)
Calculated as tank length × width (in square inches) divided by 12. Based on the guideline that 12 sq in of surface area supports 1 inch of slim-bodied fish. More relevant in shallow tanks or tanks with high surface agitation (airstone, strong filter output) that maximizes oxygen exchange.
Stocking Percentage (percent)
Total fish inches (or bioload score) divided by the tank's method-specific capacity, expressed as a percent. A healthy community tank should run at 70–80% to maintain a safety margin for filter performance and water quality. Above 100% indicates overstocking.

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Frequently Asked Questions

How many fish can I put in a 55-gallon tank?
Using the inch-per-gallon rule, a 55-gallon tank holds 55 inches of slim-bodied fish — about 36 neon tetras or 15 mollies. However, the bioload method is more accurate: a 55-gallon tank can sustain about 10–12 slim fish averaging 2–3 inches each, once filtration, waste production, and water change frequency are factored in. Avoid filling to 100% capacity — 70–80% is healthier and allows buffer for fish growth.
Is the inch-per-gallon rule accurate?
The inch-per-gallon rule is a rough beginner guideline that only works reliably for small, slim-bodied fish under 3 inches. It significantly underestimates waste for goldfish, cichlids, and any heavy-bodied species. A single 8-inch goldfish produces as much waste as 24 neon tetras despite using the same 8 'inches' in the calculation. The bioload method is far more accurate for mixed or heavy-bodied communities.
What is the bioload method for fish stocking?
The bioload method scores each fish based on body size and body type — heavy-bodied fish like goldfish score 3.0× their length in bioload units, medium-bodied fish score 1.8×, and slim fish score 1.0×. A 55-gallon tank can safely support about 44 bioload units (55 × 0.80), so three 8-inch goldfish (3 × 8 × 3.0 = 72 units) would actually overstock a 55-gallon tank despite the inch-per-gallon rule suggesting they fit.
How do you stock a saltwater aquarium?
Saltwater tanks support 25% fewer fish than freshwater tanks of the same volume because saltwater fish are generally more sensitive to water quality, have stricter territorial needs, and the biological filtration is more complex. Apply a 0.75× multiplier: a 75-gallon saltwater tank has the effective stocking capacity of a 56-gallon freshwater tank. Live rock and a quality protein skimmer can increase capacity somewhat.
How do I calculate my fish tank volume in gallons?
For a rectangular tank: Volume (gallons) = Length (inches) × Width (inches) × Height (inches) ÷ 231. A standard 48×13×21 inch tank = 48 × 13 × 21 ÷ 231 = 56.7 gallons. For cylindrical tanks: gallons = π × radius² × height ÷ 231. The calculator accepts tank dimensions directly and computes volume for you — or you can enter gallons directly if you know your tank size.
Should I stock my tank to 100% capacity?
No — 70–80% of calculated capacity is the recommended target. This buffer accounts for filter maturity, seasonal temperature fluctuations, unexpected ammonia spikes, and future fish growth. A tank running at 100% capacity has no room for error: one missed water change or filter issue can trigger an ammonia spike that kills fish. Stock to 75% and maintain a consistent water change schedule.