This calculator treats the footprint as a rectangle with two triangular corners removed. It needs five measurements, but every one is a straight tape measurement along a glass panel.
How to measure a flat-back hexagon tank
- Back length is the full inside width across the back panel.
- Overall depth is from the back panel to the front panel at the centre.
- Front panel width is the centre panel only; the angled panels are not entered directly, they are implied by the other four measurements.
- Straight side panel length is how far each short side runs forward from the back before the angled panel begins.
Always measure inside the glass. Exterior sizes include the panel thickness and any trim, which on larger tanks can remove 5 to 10 percent of the apparent volume. See the measuring guide for the full method.
Flat-Back Hexagon volume formula
V = (L × D − (L − F) × (D − S) ÷ 2) × H
Start with the full rectangle (back length × overall depth) and remove the two triangular corners cut off by the angled panels. Each triangle has legs (L − F) ÷ 2 and (D − S).
With inches, divide the cubic result by 231 to get US gallons and multiply gallons by 3.785 for litres. With centimetres, divide by 1,000 for litres directly.
Worked example
Inputs: Back Length 36 inOverall Depth 18 inFront Panel Width 18 inStraight Side Panel Length 8 inHeight 20 in
Rectangle 36 × 18 = 648 in². Corners removed: (36 − 18) × (18 − 8) ÷ 2 = 90 in². Floor 558 in² × 20 = 11,160 in³ = 48.3 US gallons (182.9 litres).
Flat back hexagon versus rectangle and regular hexagon
Against the bounding rectangle, the flat back hex loses only the two cut corners, usually 10 to 20 percent. Against a regular hexagon with the same front panel width it holds considerably more, because the back half of a regular hexagon tapers to a point while the flat back keeps the full width. That is why the shape is so popular for wall placement.
Getting to real water volume
The formula gives the geometric volume of the tank to the rim. Three things make the real water volume smaller, and the calculator above lets you account for two of them directly:
- Fill height. Most tanks run an inch or two below the rim. Enter the actual water line and the result scales to match.
- Substrate. Gravel and sand sit in the water column. The substrate option removes the solid fraction of the bed (62 percent of its depth for typical gravel) from the total.
- Rock, wood and equipment. Use the displacement calculator to subtract hardscape by weight and equipment by type.
Water weight
Fresh water weighs about 8.34 pounds per US gallon (1 kg per litre). Reef water at a specific gravity of 1.025 is about 8.55 pounds per gallon. The calculator shows both so you can check what your stand and floor will carry before adding glass, substrate and rock on top.