Aquarium Volume Calculator for Cylinder Tanks

This calculator works out the water volume of a cylindrical aquarium in US gallons and liters. You enter the tank’s diameter and height (in inches or centimeters), and it returns the volume in both units, along with the surface area. The result is the volume of the tank if filled to the brim; for a real aquarium, you’ll want to subtract the space taken by substrate, decorations, and the gap at the top.

Formula

The volume of a cylinder is the area of its circular base times its height. In symbols:

V = π × (d/2)² × h

Where:

• V = volume in cubic inches (if d and h are in inches)
• d = diameter of the cylinder (in inches, cm, etc.)
• h = height of the cylinder (same unit as d)
• Ï€ ≈ 3.14159

To get gallons, divide the cubic inches by 231 (since 1 US gallon = 231 cubic inches). To get liters, multiply the cubic inches by 0.0163871 (since 1 cubic inch = 0.0163871 liters). If you measure in centimeters, the volume is in cubic centimeters (mL); divide by 1000 for liters, or by 3785.41 for US gallons.

Worked Example

Using the default values: diameter = 24 inches, height = 18 inches.

1. Radius = 24 / 2 = 12 inches
2. Base area = π × 12² = 3.14159 × 144 = 452.389 in²
3. Volume in cubic inches = 452.389 × 18 = 8143.008 in³
4. Volume in US gallons = 8143.008 / 231 = 35.25 gallons
5. Volume in liters = 8143.008 × 0.0163871 = 133.5 liters

The calculator shows 35.3 gallons and 133.5 liters for these defaults.

Why Cylinder Tanks Are Different

Most aquarium volume calculators assume a rectangular box (length × width × height). A cylinder is a different shape – it has a circular footprint. The formula is the same as for any cylinder: area of the circle times height. The key difference is that you need the diameter (or radius), not length and width. Some people mistakenly use the circumference or the diameter as if it were a side length, which gives a wrong answer. For example, a cylinder with a 24-inch diameter has a base area of about 452 square inches, while a square with 24-inch sides has an area of 576 square inches – that’s 27% more. So you can’t just treat the diameter as a side length.

How to Measure Your Tank

To get an accurate volume, measure the inside dimensions of the tank, not the outside. Glass thickness matters – a 0.5-inch thick glass wall adds an inch to the outside diameter. Use a tape measure or ruler and measure from inner edge to inner edge across the widest point of the circle. For the height, measure from the bottom inside surface to the top edge where the water would reach. If you have a rim or brace, measure below that. Also, if the tank is not a perfect cylinder (some have a slight taper or a rounded bottom), the formula still gives a good approximation, but the actual volume may differ slightly.

What the Result Really Means

The calculated volume is the total capacity of the cylinder if filled to the very top. In practice, you never fill an aquarium to the brim – you leave an air gap (usually 1-2 inches) to prevent fish jumping out and to allow for water movement. Also, substrate (gravel or sand), rocks, driftwood, and equipment (filter, heater) displace water. A typical rule of thumb is that substrate and decor can take up 10-20% of the volume. So the actual water volume will be less than the calculated number. For stocking decisions, you should use the actual water volume, not the total volume. For example, a 35-gallon cylinder with 2 inches of substrate and a few rocks might hold only 28-30 gallons of water.

Common Mistakes and How to Avoid Them

• Mixing units: If you measure diameter in inches and height in centimeters, the result is wrong. Always use the same unit for both.
• Using radius instead of diameter: The formula uses radius (d/2). If you mistakenly use diameter as radius, the volume is 4 times larger.
• Forgetting to convert to gallons: The volume in cubic inches is not gallons. You must divide by 231.
• Measuring outside dimensions: Glass thickness adds volume that isn’t water. Always measure inside.
• Ignoring the air gap: The calculator gives total capacity, not water volume. Subtract the gap and displacement.

When to Use This Calculator

Use this calculator if you have a cylindrical tank – these are less common than rectangular tanks but are popular for shrimp, bettas, or as display tanks. You might also use it for a cylindrical sump or a refugium. If you have a hexagonal or bow-front tank, this calculator won’t work – you need a different formula. For a rectangular tank, use a standard volume calculator. For an irregular shape, you can estimate by measuring water added with a bucket (a practical method).

Safety and Practical Tips

Knowing the volume is crucial for dosing medications, fertilizers, and water conditioners. Overdosing can harm fish, so always use the actual water volume, not the total capacity. Also, weight matters: 1 US gallon of freshwater weighs about 8.34 pounds (3.78 kg). A 35-gallon tank weighs about 292 pounds (132 kg) just for the water, plus the tank and stand. Make sure your floor and stand can support that. When filling, always leave a gap – water expands slightly with temperature, and if you fill to the brim, water can splash out or overflow. For saltwater, the volume is the same, but the weight is slightly higher (about 8.6 pounds per gallon) due to dissolved salts.

Frequently Asked Questions

How many gallons is a 24×18 cylinder tank?

Using the formula: radius = 12 in, height = 18 in. Volume = Ï€ × 12² × 18 = 8143 in³. Divide by 231 = 35.25 US gallons. That’s the total capacity. With substrate and a 1-inch air gap, you’d have roughly 30-32 gallons of water.

Do I measure the outside or inside of the tank?

Always measure the inside. Glass thickness can be significant, especially on larger tanks. For a 24-inch outside diameter with 0.5-inch glass, the inside diameter is 23 inches, which gives about 4% less volume. That difference matters for dosing and stocking.

Can I use this for a hexagon or bow-front tank?

No. This calculator is only for perfect cylinders. A hexagonal tank has a different volume formula (area of a hexagon times height). A bow-front tank is more complex – often approximated as a rectangular tank with a curved front. For those, use a dedicated calculator or a water-measuring method.

How much substrate do I need for a cylinder tank?

Substrate volume is calculated by area of the base times desired depth. For a 24-inch diameter tank, base area is about 452 in². For 2 inches of substrate, you need about 904 cubic inches (about 0.52 cubic feet or 14.8 liters) of substrate. Note that substrate volume is measured dry, and it will settle.

Why does my tank hold less than the calculator says?

Because the calculator gives total capacity. Real water volume is less due to the air gap at the top, substrate, rocks, and equipment. Also, if you measured the outside, the inside is smaller. It’s normal to have 10-20% less actual water volume.

Is the volume in US gallons or imperial gallons?

This calculator gives US gallons (231 in³). Imperial gallons are larger (277.42 in³). If you’re in the UK or elsewhere using imperial, divide the cubic inches by 277.42 instead. The difference is about 20%, so it’s important to know which you’re using.

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in d | Diameter | in | 24 | 0
in h | Height | in | 18 | 0
out gallons | Volume (US gal) | gal | (pi * (d/2)^2 * h) / 231 | 1
out liters | Volume (L) | L | (pi * (d/2)^2 * h) * 0.0163871 | 1
out area | Base area | in² | pi * (d/2)^2 | 0