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Aquarium Calculator

Calculate tank volume and fish stocking levels

Units

Tank Volume

20.0 gal

Max Fish

17"

Stocking

48%

Heater

80W

e.g., Tetras, Guppies, Corys

Add up adult sizes of all fish (e.g., 5 tetras × 2" = 10")

Tank Volume

20.0 gal

Actual Volume

16.5 gal

Max Fish

17"

Stocking

48%

understocked

Room for more fish. Consider adding compatible species.

8 inmax 17 in

Equipment

Heater

76-80°F

80W

Filter

5x turnover

100 GPH

Water Change

20% weekly

3.3 gal

Volume Breakdown

Total Volume20.0 gal
Actual (after substrate)16.5 gal

Stocking Capacity

8 of 17 inches used48%
UnderstockedIdeal (50-75%)Overstocked

Frequently Asked Questions

Q

How many fish can I put in my aquarium?

The general rule is 1 inch of fish per gallon for small fish, but this varies. A better approach: 1" per 2 gallons for active swimmers, and consider adult size. A 20-gallon tank might hold 10-12 small fish (1-2") or 4-5 medium fish (3-4").

Q

How do I calculate aquarium volume?

For rectangular tanks: Length × Width × Height (in inches) ÷ 231 = US gallons. For example: 24" × 12" × 16" = 4,608 ÷ 231 = 19.9 gallons. Subtract 10% for gravel and decorations.

Q

What size heater do I need for my fish tank?

Use 3-5 watts per gallon. 10-gallon tank: 50W heater. 29-gallon: 100W. 55-gallon: 200W. For rooms colder than 70°F, use 5 watts/gallon. Consider two smaller heaters for tanks over 50 gallons.

Q

What GPH filter do I need?

Filter should cycle tank 4-6 times per hour. A 20-gallon tank needs 80-120 GPH filter. Heavily stocked tanks need higher turnover. Planted tanks can use lower flow (3-4x). Canister filters are best for tanks over 30 gallons.

Q

How often should I change aquarium water?

Change 10-25% weekly for most tanks. Heavily stocked tanks need 25-30% weekly. Lightly stocked planted tanks can do 15% every 2 weeks. Never change more than 50% at once to avoid shock.

Example Calculations

120-Gallon Tropical Community Tank

Inputs

Dimensions24" × 12" × 16" (rectangular)
Substrate Depth2 inches
Fish TypeTropical Community
Current Fish8 inches total

Result

Tank Volume19.9 gallons
Actual Volume16.5 gal (after substrate/decor)
Max Fish Capacity16 inches
Stocking Level49%
Heater80W
Filter100 GPH

Volume = (24 × 12 × 16) / 231 = 19.9 gal. Substrate loss = (2/16) × 19.9 = 2.49 gal. Decor loss = 19.9 × 0.05 = 1.0 gal. Actual = 16.5 gal. Max fish = 16.5 × 1.0 = 16 inches. Heater = 80W (4W/gal). Filter = 100 GPH (5x turnover).

255-Gallon Goldfish Tank

Inputs

Tank Size55 gallons (preset)
Substrate Depth2 inches
Fish TypeColdwater (Goldfish)
Current Fish12 inches total

Result

Tank Volume55.0 gallons
Actual Volume45.2 gal (after substrate/decor)
Max Fish Capacity23 inches
Stocking Level53%
Heater220W
Filter275 GPH

Preset 55 gal tank (standard dims ~48" × 12" × 16"). Substrate and decor reduce to ~45.2 gal usable. Goldfish rule = 0.5"/gal, max = 23 inches. With 12" of fish, stocking is 53% (ideal range). Filter = 275 GPH for 5x turnover.

Formulas Used

Rectangular Tank Volume

Gallons = (Length × Width × Height) / 231

Calculates tank volume in gallons from dimensions in inches. 231 cubic inches equals 1 US gallon.

Where:

Length= Tank length in inches
Width= Tank width in inches
Height= Tank height in inches
231= Cubic inches per US gallon

Actual Volume (after substrate)

Actual Gallons = Volume - (Substrate Depth / Height × Volume) - (Volume × 0.05)

Subtracts volume lost to substrate and decorations (estimated 5%) from total volume.

Where:

Substrate Depth= Gravel/sand depth in inches
Height= Tank height in inches
0.05= 5% volume loss for decorations

Max Fish Capacity

Max Fish Inches = Actual Gallons × Inches per Gallon

Uses the inches-per-gallon rule for the selected fish type to determine total fish capacity.

Where:

Actual Gallons= Usable water volume
Inches per Gallon= Stocking rule (tropical = 1, coldwater = 0.5)

Guide to Aquarium Stocking and Equipment

Proper aquarium stocking directly affects fish health. Overstocking leads to poor water quality, stress, aggression, and disease. Always research each species' adult size, temperament, and tank requirements before purchasing.

Equipment sizing affects your success. An undersized heater causes temperature swings. A weak filter allows ammonia buildup. Invest in quality equipment rated for your tank size or slightly larger.

The nitrogen cycle is essential - never add fish to a new tank. Cycle for 4-6 weeks, test for zero ammonia and nitrite, then add fish gradually over several weeks to allow beneficial bacteria to adapt.

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Last Updated: Feb 12, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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