Aquarium Forge

Aquarium Volume Calculator

Enter your tank's dimensions to calculate its capacity in US gallons and liters —plus the estimated actual water volume once the tank is filled to a realistic level.

Most tanks are filled to the frame line, about 90–95% of nominal height.

Fill in the fields above to calculate.

How It Works

Aquarium volume is the product of the tank's internal length, width, and height. Measure the inside glass-to-glass dimensions rather than the exterior, because glass thickness reduces capacity —especially on smaller tanks. The calculator converts your measurements to cubic inches (or cubic centimeters), then applies the standard conversion: 231 cubic inches equals one US gallon, and 1,000 cubic centimeters equals one liter.

Formula

Volume = Length × Width × Height

US Gallons = Cubic Inches ÷ 231

Liters = Cubic Centimeters ÷ 1000

Actual Water Volume = Nominal Volume × Fill %

Example

A standard 20 gallon long tank measures 30 × 12 × 12 inches internally. That is 4,320 cubic inches, and 4,320 ÷ 231 = 18.7 US gallons (70.8 liters). Filled to 95%, it holds about 17.8 gallons of actual water.

Important Limitations

  • Results assume a rectangular tank; bowfront, hex, and cylindrical tanks need different formulas.
  • Measuring external dimensions overestimates capacity.
  • Displacement from substrate, rocks, and driftwood is not subtracted —use the Stocking Calculator for a planning-level assessment.

FAQ

How do I calculate aquarium gallons?

Measure the inside length, width, and height in inches, multiply them together to get cubic inches, then divide by 231. For example, a 24 × 12 × 16 inch tank holds 4,608 cubic inches, which is about 19.9 US gallons.

How many gallons is my fish tank?

Use the calculator above with your tank's dimensions. Common nominal sizes: a standard 10 gallon measures 20 × 10 × 12 inches, a 20 gallon long is 30 × 12 × 12 inches, and a 55 gallon is 48 × 13 × 21 inches.

How many liters are in a 20 gallon tank?

20 US gallons equals about 75.7 liters. To convert any volume, multiply gallons by 3.785. A nominal 20 gallon tank usually holds slightly less in practice.

Does aquarium size mean actual water volume?

No. Nominal tank size assumes the tank is filled to the very top. In reality the water level sits below the frame line, and substrate, rocks, and equipment displace water, so actual volume is typically 5–5% lower.

How much water does a fish tank actually hold?

It depends on the fill percentage and decoration. Set the Water Fill Percentage in the calculator above (90–5% is typical) to estimate real water volume. Subtract displacement from substrate and hardscape for a more precise figure.

Why do glass thickness and rimmed tanks matter?

Thick glass and braced (rimmed) tanks displace a small additional amount of water. The effect is usually under 1% and is not subtracted in this calculator, but it contributes to why nominal and actual volumes differ.

Sources

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