Aquarium Volume Calculator: Real Gallons in Your Tank

The formulas

• Rectangle: length × width × height.
• Cylinder: 3.1416 × (diameter ÷ 2)² × height.
• Quarter-circle corner tank: 3.1416 × radius² ÷ 4 × height, where the radius is the length of one straight back side.

In inches, divide cubic inches by 231 to get U.S. gallons (NIST defines the U.S. gallon as 231 cubic inches). In centimeters, divide cubic centimeters by 1,000 to get liters. One U.S. gallon is 3.785 liters.

Glass volume vs water volume

The calculator shows both. Full glass volume is the inside of the tank, rim to bottom. Water volume subtracts:

• Fill gap: most tanks run with the water an inch or more below the rim, under the lid or trim.
• Substrate: gravel or sand takes up most of its own volume. The calculator subtracts its full depth, which slightly underestimates water because some water sits between the grains, but it's the safer figure for dosing.

Rock, driftwood and decorations displace more. If you have a lot of hardscape, your true water volume can be noticeably lower again.

Worked example

A tank measuring 36 × 18 × 20 inches inside:

• Glass volume: 36 × 18 × 20 = 12,960 cubic inches ÷ 231 = 56.1 gallons (212 L).
• Water 1 inch below the rim, 2 inches of substrate: 36 × 18 × 17 = 11,016 cubic inches = 47.7 gallons (180 L).
• Water weight: 47.7 × 8.34 ≈ 398 lb.

That's 15% less water than the glass volume, which matters when dosing a medication.

Why the real volume matters

• Dosing. Conditioners, fertilizers and medications are labeled per gallon or liter. Dosing a 48 gallon water volume as 56 gallons gives about 17% more than intended. Use the dosing calculator with your real volume.
• Water changes. A 25% change on 47.7 gallons is about 12 gallons, not 14. See the water change calculator.
• Weight. USGS gives the density of water as about 8.34 lb per gallon. Water is only part of a full tank's weight; glass, substrate, rock and the stand add more.

Measuring tips

• Measure inside dimensions with a tape or ruler; if you can only measure outside, subtract twice the glass thickness from length and width.
• Measure substrate depth in a few places and average it, since it's often sloped.
• Measure the fill gap from the waterline to the top of the glass.
• For odd shapes, fill the empty tank with a measured bucket and count, or note how much water it takes during a refill.

Weight and stands

A full aquarium is heavy, and the weight is concentrated on the stand's footprint. Use a stand rated for the tank, place it level, and for large tanks consider where floor joists run. If you are unsure about a floor's capacity, ask a qualified contractor.

Common questions

How many gallons is a 36 × 18 × 20 inch tank?

The glass holds 12,960 cubic inches, or 56.1 gallons. With the water 1 inch below the rim and 2 inches of substrate, the water is about 47.7 gallons.

Should I measure inside or outside the glass?

Inside, if you can. Outside measurements include the glass thickness, which makes a noticeable difference on large tanks with thick glass.

How do I measure a bow-front tank?

Approximate it as a rectangle using the average front-to-back depth, or fill it with a measured container. For dosing, err toward the lower estimate if the treatment label warns about overdosing.

How heavy is a full aquarium?

Water alone is about 8.34 lb per gallon. Glass, substrate, rock and the stand add more. Check that your stand is rated for the tank and that the floor is suitable for the total weight.