Quick answer
They are not the same shape. A regular hexagon has six equal panels and sits away from the wall. A flat back hexagon has a long back panel, two short sides, two angled panels and a front panel so it can sit flush against a wall. Using the regular hexagon formula on a flat back tank gives the wrong volume.Shape differences
The regular hexagon is symmetric in every direction and tapers to a point at the back. The flat back hexagon is a rectangle with its two front corners cut off at an angle. Seen from above, one looks like a honeycomb cell and the other like a stop sign cut in half.
Measurement differences
The regular hexagon needs one side length and the height. The flat back hexagon needs five measurements: back length, overall depth, front panel width, straight side panel length and height.
Formula differences
Regular hexagon: V = 2.598 × s² × H. Flat back hexagon: V = (L × D − (L − F) × (D − S) ÷ 2) × H.
Which calculator should you use?
If the tank has a flat back that is longer than the other panels, it is a flat back hexagon. If all six panels are the same width, it is a regular hexagon.
Example comparison
Regular hexagon, 12 in sides, 20 in tall
32.4 gal (122.6 L)
Flat back hex 36 × 18 in, 18 in front, 8 in sides, 20 in tall
48.3 gal (182.9 L)
Despite similar panel widths, the flat back holds about 50% more because it keeps the full width across the back.