Aquarium Volume Calculator

Find out how many gallons your fish tank holds. Enter your aquarium's dimensions and we'll calculate the volume in gallons and liters, the water weight when full, and the real volume after substrate.

Measure in:
0 US gallons

    Results are based on internal dimensions and assume the tank is filled to the rim. Real volume is a little lower after rim trim, rock, and equipment displacement.

    How to calculate your aquarium volume

    For a standard rectangular tank, volume is simply length × width × height. Measure in the same unit, then convert:

    • US gallons = cubic inches ÷ 231
    • Liters = cubic centimeters ÷ 1000

    For example, a tank measuring 48 × 12 × 18 inches holds about 44.9 US gallons. Bow-front, cylinder and hexagon tanks use different formulas — just choose your shape above and the calculator handles the math. Always measure the inside dimensions, not the outside, because glass thickness can add a surprising amount of error.

    Why your real water volume is lower than the label

    A "20 gallon" tank rarely holds 20 gallons of water. The number on the box is the brim-full glass volume. In a running aquarium the actual water volume is reduced by:

    • The gap between the water line and the rim.
    • Substrate — sand or gravel can take up 10–20% of the tank.
    • Rocks, driftwood, and equipment.

    Enter your substrate depth above to see your true water volume, and use that figure when you plan stock with our Aquarium Stocking Calculator and dose treatments.

    How much does a full aquarium weigh?

    Water alone weighs about 8.34 lb per US gallon (1 kg per liter), before you add glass, substrate and rock. A filled 40 gallon tank weighs over 400 lb — heavy enough that the stand and floor matter. The calculator shows the filled weight in both pounds and kilograms so you can check your setup is properly supported.

    Aquarium Volume — FAQs

    How many gallons is my fish tank?

    Measure the inside length, width and height, multiply them together, and divide cubic inches by 231 for US gallons. Or just enter your dimensions above and we'll do it instantly, including liters and water weight.

    How many gallons is a 24 x 12 x 16 inch tank?

    24 × 12 × 16 = 4,608 cubic inches, divided by 231 ≈ 20 US gallons. That's the classic "20 gallon high" footprint. Remember the real water volume will be a little lower once substrate and the rim gap are accounted for.

    Should I measure the inside or outside of the tank?

    Measure the inside. Glass or acrylic thickness adds to the outside dimensions but holds no water, so using outside measurements overestimates your volume — sometimes by a gallon or more on smaller tanks.

    How much does a full 20 or 40 gallon tank weigh?

    Water is about 8.34 lb per gallon, so a 20 gallon holds roughly 167 lb of water and a 40 gallon about 334 lb — plus the weight of glass, substrate and decor. Always place larger tanks on a stand rated for the load.

    How do I convert liters to gallons?

    Divide liters by 3.785 to get US gallons (1 US gallon = 3.785 liters). The calculator shows both automatically, so you can switch between inches/gallons and centimeters/liters with one click.