Hexagon vs Flat Back Hexagon Aquarium Volume

Quick Answer

A full hexagon aquarium has six equal sides and sits freestanding. A flat back hexagon has one flat side that rests against a wall. They use different volume formulas and different calculators.

Shape Differences

A regular hexagonal aquarium has six equal-length glass panels forming a symmetric hexagon when viewed from above. It is a freestanding design viewable from all sides. A flat back hexagon aquarium replaces the rear panel arrangement with a single wide flat panel designed to sit flush against a wall. This gives it five visible glass panels: one flat back, two angled wing panels, and two front-facing panels.

Measurement Differences

A regular hexagon requires only one side length measurement (all six sides are equal) plus the height. A flat back hexagon requires three measurements: the front panel width, the depth from front to back, and the height. The flat back version needs more measurements because its panels are not all equal length. See our guide on measuring your tank accurately for the full procedure.

Formula Differences

The regular hexagon uses the formula V = (3√3 ÷ 2) × s² × H, where s is the side length. The flat back hexagon uses a different formula based on the front panel width and depth. Using the wrong formula will produce incorrect results because the cross-sectional areas are different.

Which Calculator Should You Use?

Count the panels and check the back of the tank. If your tank has six equal panels and no flat back wall, use the hexagon calculator. If your tank has one wide flat back panel designed for wall placement, use the flat back hexagon calculator.

Example Comparison

A regular hexagon with 8-inch sides and 20-inch height: V = (3√3 ÷ 2) × 64 × 20 = 3,326 cubic inches = 14.40 gallons. A flat back hexagon with 8-inch front panel, 10-inch depth, and 20-inch height uses its own formula and produces a different volume. The two shapes are not interchangeable for volume calculations. To translate any gallon result to litres, use the unit converter.

Try the Calculators

Hexagon Aquarium Volume Calculator Flat Back Hexagon Aquarium Volume Calculator

Frequently Asked Questions

Can I use the hexagon formula on a flat back hexagon?

No. The flat back hexagon has a different cross-sectional shape than a regular hexagon. Using the regular hexagon formula will produce incorrect results. Always use the flat back hexagon calculator for tanks with a flat rear panel.

Which shape holds more water?

It depends on the specific dimensions. A regular hexagon with a given side length has a known area, while a flat back hexagon volume depends on both front width and depth. They cannot be directly compared without knowing all dimensions.

How do I tell if my tank is a regular hexagon or flat back hexagon?

Look at the back of the tank. A regular hexagon has two angled panels at the rear forming a point or edge. A flat back hexagon has one wide flat panel at the back designed to sit against a wall.

Are flat back hexagon tanks more common than full hexagon tanks?

Yes. Flat back hexagon tanks are more commonly available in retail because they are more practical for home use — they sit flat against a wall and use space more efficiently.

Do both shapes come in standard aquarium sizes?

Full hexagon tanks are available in standard sizes like 20, 35, and 55 gallons. Flat back hexagons are available in fewer standard sizes and are more often found in the 20–45 gallon range.