Tank Volume Calculator

What is a tank volume calculator?

A tank volume calculator works out how much a tank can hold based on its inside dimensions. Whether you are sizing a water storage tank, an aquarium, a fuel container, or a process vessel, knowing the volume tells you the maximum amount of liquid it can hold. This calculator supports the two most common tank geometries — rectangular (box-shaped) tanks and vertical cylindrical tanks — and reports the result in three practical units: US gallons, liters, and cubic feet.

All inputs are entered in inches. Because the calculator converts a single internal volume (in cubic inches) into every output unit, you never have to juggle conversion factors by hand.

How does it work?

First, select the tank shape. The calculator then asks only for the dimensions that shape needs and computes the interior volume in cubic inches.

For a rectangular tank, multiply length, width, and height:

$V_{rect} = l \times w \times h$

For a vertical cylindrical tank, the cross-section is a circle of diameter $d$. The radius is $r = d/2$, and the volume is the circular area multiplied by the height:

$V_{cyl} = \pi r^2 h$

Once the volume in cubic inches is known, it is converted to each output unit:

• US gallons: divide by 231 (there are exactly 231 cubic inches in a US gallon).
• Liters: multiply by 0.0163871 (1 cubic inch equals approximately 0.0163871 liters).
• Cubic feet: divide by 1728 (there are $12^3 = 1728$ cubic inches in a cubic foot).

Worked examples

Example 1: Rectangular tank

Suppose a rectangular tank measures 24 inches long, 12 inches wide, and 12 inches high.

$V_{rect} = 24 \times 12 \times 12 = 3456 \text{ cubic inches}$

Converting to each unit:

$\frac{3456}{231} \approx 14.96 \text{ US gallons}$

$3456 \times 0.0163871 \approx 56.6 \text{ liters}$

$\frac{3456}{1728} = 2.0 \text{ cubic feet}$

So this tank holds about 14.96 US gallons, or 56.6 liters.

Example 2: Cylindrical tank

Consider a vertical cylindrical tank with a diameter of 24 inches and a height of 36 inches. The radius is $r = 24/2 = 12$ inches.

$V_{cyl} = \pi \times 12^2 \times 36 = \pi \times 5184 \approx 16286 \text{ cubic inches}$

Converting to US gallons:

$\frac{16286}{231} \approx 70.5 \text{ US gallons}$

A cylinder this size therefore holds roughly 70.5 US gallons of liquid.

Practical notes

• Measure the inside, not the outside. Wall thickness reduces the usable interior, so for an accurate capacity you should measure interior dimensions.
• This is the brim-full volume. Real tanks are rarely filled to the very top; subtract the freeboard (the space left above the liquid line) to get the working capacity.
• Keep your units consistent. Enter every dimension in inches. The calculator handles the conversion to gallons, liters, and cubic feet for you.
• For a horizontal cylinder filled only partway, the geometry differs from a full vertical cylinder, and a dedicated partial-fill calculation is required.

If you are sizing a fish tank specifically, the aquarium volume calculator supports additional shapes such as bow-front and hexagonal designs. For a general look at cylinder geometry in any unit, see the cylinder volume calculator.

Frequently asked questions

Why is the result the same liquid measured in gallons, liters, and cubic feet?

These are simply different units for the same physical volume. The calculator computes one internal volume and expresses it three ways so you can use whichever unit suits your task.

How many cubic inches are in a US gallon?

Exactly 231 cubic inches. That is why the gallon result is the cubic-inch volume divided by 231. Note that this is the US liquid gallon; the UK (imperial) gallon is larger.

Can I use this for an aquarium?

Yes. A standard rectangular aquarium uses the rectangular option, and a column-style cylindrical aquarium uses the cylinder option. Keep in mind that gravel, rocks, and decorations displace water, so the actual water volume will be a little less than the calculated tank volume.