Measure one panel and the inside height. The regular octagon area formula handles the rest.
How to measure a octagon tank
- Measure the inside width of one of the eight panels between seams.
- Face-to-face width is 2.414 × side; use that to convert if you measured across the tank.
- Confirm all eight panels match; a stretched octagon needs to be split into a rectangle and two trapezoids.
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.
Octagon volume formula
V = 2 × (1 + √2) × s² × H
A regular octagon has area 4.828 × s². Multiply by height.
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: Side Length 10 inHeight 24 in
4.828 × 10² × 24 = 11,588 in³, which is 50.2 US gallons or 189.9 litres.
Octagon versus cylinder efficiency
An octagon inscribed in a circle fills 90% of the circle's area, versus 83% for a hexagon and 64% for a square. So for a given width across the tank, an octagon gives up very little volume compared with a round tank while being far easier and cheaper to build from flat panels.
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.