Hexagonal Aquarium Volume Calculator: How It Works

This calculator works out the water volume of a hexagonal aquarium in US gallons and litres. You enter the length of one side of the hexagon, the height of the tank, and the glass thickness (if you want the inside volume), and it returns the total volume and the actual water capacity when filled to a typical level. The first result shown is the total volume in US gallons, followed by litres and a water-volume estimate.

Formula

The volume of a regular hexagon prism is the area of the hexagonal base multiplied by the height. The base area of a regular hexagon with side length s is:

Area = (3√3 / 2) × s²

So the full volume is:

V = (3√3 / 2) × s² × h

Where:

• s = length of one side of the hexagon (same unit as height)
• h = height of the tank (same unit as side)
• V = volume in cubic units (e.g., cubic inches)

To convert cubic inches to US gallons, divide by 231 (since 1 US gallon = 231 in³). To convert to litres, multiply cubic inches by 0.016387064 (1 in³ = 0.016387064 L). If you measure in centimetres, the volume is in cubic centimetres (mL); divide by 1000 for litres, and divide by 3785.41 for US gallons.

Worked Example

Using the default values: side length = 12 in, height = 18 in, glass thickness = 0.25 in, fill level = 95%.

1. Calculate base area: (3 × √3 / 2) × 12² = (2.598076) × 144 = 374.1229 in²
2. Multiply by height: 374.1229 × 18 = 6734.21 in³
3. Convert to US gallons: 6734.21 / 231 = 29.15 US gal
4. Convert to litres: 6734.21 × 0.016387064 = 110.35 L
5. Water at 95% fill: 29.15 × 0.95 = 27.69 US gal (approx.)

The calculator shows 29.2 US gal total, 110.4 L total, and 27.7 US gal water at 95% fill. Note: the glass thickness input is not used in the default calculation because the side and height are already assumed to be inside dimensions. If you measure outside, subtract twice the glass thickness from the side and height before entering.

Why Hexagonal Tanks Are Different

Most aquarium volume calculators assume a rectangular box (length × width × height). A hexagon has six sides, so you cannot just multiply three numbers. The cross-section is a six-sided polygon, and its area depends on the side length and the apothem (distance from centre to the midpoint of a side). For a regular hexagon, the apothem equals s × √3 / 2, which is why the area formula uses 3√3/2. This is the same formula used for any regular hexagon, whether it is a bolt head, a tile, or a fish tank.

Aquarium manufacturers often label hex tanks by “side length� and “height� rather than length and width, so this calculator matches the specs you see on product pages. If you have the distance across the flats (the width of the tank from one flat side to the opposite flat side), you can convert: side = (width across flats) / √3. If you have the distance across the points (corner to corner), side = (distance across points) / 2.

How to Measure Your Hex Tank Correctly

For accurate volume, measure the inside dimensions. Use a tape measure or ruler:

• Side length: measure one of the six interior glass edges from corner to corner. Do not include the glass thickness.
• Height: measure from the inside bottom to the top edge (where water would spill). Again, exclude the glass thickness if the tank has a rim or frame.
• Fill level: most tanks are not filled to the brim. Leave 1–2 inches of air space at the top. A 95% fill is a common estimate for a standard tank with a lid; 90% is safer for tanks with HOB filters or open tops.

If you only have outside measurements, subtract twice the glass thickness from the side and height. For example, if the outside side is 12.5 in and glass is 0.25 in, the inside side is 12.0 in. The same applies to height: subtract the bottom glass thickness (and top rim if present).

Common Mistakes and Misconceptions

One frequent error is using the distance across the flats as if it were the side length. That overestimates volume by about 73% because the side length is shorter than the width. Another mistake is mixing units—entering side in inches and height in centimetres. Always convert to the same unit first.

People also forget that the “volume� of a tank is not the same as the amount of water it holds. Substrate, decorations, and equipment displace water. A tank with 2 inches of gravel and a large driftwood piece will hold noticeably less water than the calculated volume. For a rough estimate, subtract 5–10% for substrate and décor, in addition to the unfilled top space.

Another misconception: a hex tank with the same side length and height as a rectangular tank does not hold the same volume. A hexagon with side 12 in has a maximum width of about 24 in (corner to corner), but a rectangle 24 in wide and 12 in deep has a much larger footprint. Hex tanks are space-efficient but hold less water than a similarly sized rectangular tank.

How to Use the Result

The total volume tells you the tank’s nominal size (e.g., “30-gallon hex�). The water volume at your fill level is what you use for:

• Dosing medications or treatments: most products give doses per gallon of actual water.
• Choosing a heater: a general rule is 3–5 watts per gallon of water volume, but check the heater’s instructions.
• Filter sizing: filters are rated for tank volume, but many hobbyists recommend a filter rated for 2–3 times the water volume.
• Stocking fish: the “inch per gallonâ€� rule is a rough guide; use the water volume, not the total volume.

If you are buying a tank, the total volume is what the seller lists. If you are setting it up, use the water volume for practical decisions.

Alternatives and Related Calculations

If your tank is not a regular hexagon (e.g., one side is longer than the others), this calculator will not be accurate. In that case, measure the area of the base using a different method, such as dividing it into triangles or using a polygon area calculator, then multiply by height. For a rectangular tank, use the simple length × width × height formula. For a cylindrical tank, use π × radius² × height.

You can also work backwards: if you know the volume you want, rearrange the formula to find the side length for a given height. For example, to get 30 US gallons (6930 in³) at 18 in height, the base area must be 385 in², so s = √(385 / 2.598) ≈ 12.2 in. This is useful when planning a custom tank.

Frequently Asked Questions

How do I measure a hexagon tank if I only have the width across the flats?

Divide the width across the flats by √3 (approximately 1.732). For example, a tank that is 20.78 in across the flats has a side length of 20.78 / 1.732 = 12 in. Then use that side length in the calculator.

Does the glass thickness matter for volume?

Yes, if you measure outside dimensions. The inside volume is smaller by the glass thickness on all sides. For a typical 0.25 in glass, the difference on a 12 in side is about 4% of the volume. If you measure inside, you can ignore glass thickness.

What is the difference between US gallons and imperial gallons?

US gallon = 231 in³ (3.785 L). Imperial gallon = 277.42 in³ (4.546 L). This calculator uses US gallons, which is standard for aquarium equipment in North America. If you need imperial gallons, multiply the US gallon result by 0.83267.

How much water do I actually put in a hex tank?

Most tanks are filled to about 90–95% of the total height, leaving an air gap at the top. With substrate and décor, the actual water volume can be 10–20% lower than the total volume. Use the fill level slider to estimate, then adjust for your specific setup.

Can I use this calculator for a bow-front or corner hex tank?

Only if the tank is a true regular hexagon (all six sides equal). Bow-front and corner tanks have curved or angled panels that do not match the regular hexagon formula. For those, use a different method or a manufacturer’s specification.

Why does my tank label say 30 gallons but the calculator says 29.2?

Manufacturers often round up to the nearest standard size, or they measure outside dimensions and include the glass in the calculation. The actual water capacity is usually slightly less than the advertised size. The calculator gives a more accurate number for dosing and stocking.

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in side | Side length (one edge) | in | 12 | 0
in h | Height (inside) | in | 18 | 0
in glass | Glass thickness | in | 0.25 | 0
in fill | Fill level | % | 95 | 0
out gal | Total volume | US gal | (3 * sqrt(3) / 2 * side^2 * h) / 231 | 1
out l | Total volume | L | (3 * sqrt(3) / 2 * side^2 * h) * 0.016387064 | 1
out gal_water | Water at fill level | US gal | gal * fill / 100 | 1