Cylinder vs Half Cylinder Aquarium Volume
A full cylinder aquarium encloses a complete circular cross-section, while a half cylinder — also called a D-tank — has a flat back panel and a semicircular front. Both shapes are calculated using variants of the same formula, but a half cylinder holds exactly half the water of a full cylinder with the same diameter and height. This page explains why, shows the worked formulas side by side, and gives a reference table for common sizes.
Formula Comparison
Full Cylinder
Formula: V = π × r² × H
Where: r = interior radius (diameter ÷ 2), H = water height
Example: diameter 18 in, height 24 in
r = 9 in
V = π × 9² × 24 = π × 81 × 24 = 6,107 in³ ÷ 231 = 26.44 gal
Half Cylinder
Formula: V = (π × r² × H) ÷ 2
Where: r = interior radius (diameter ÷ 2), H = water height
Example: diameter 18 in, height 24 in
r = 9 in
V = (π × 9² × 24) ÷ 2 = 3,053 in³ ÷ 231 = 13.22 gal
A half cylinder always holds exactly 50% of the volume of a full cylinder with the same diameter and height. The flat back panel removes precisely half the circular cross-section.
Cylinder vs Half Cylinder Volume Chart
All values use interior dimensions (glass thickness reduces usable volume compared to exterior measurements) and assume the tank is filled to the full measured height. US gallons are calculated by dividing cubic inches by 231; litres by multiplying gallons by 3.785. See aquarium volume in litres for metric reference sizes, or use the unit converter for any other unit pair. The chart below covers diameters from 12 to 30 inches and heights from 18 to 36 inches in both US gallons and litres.
| Diameter | Height | Full Cyl (gal) | Half Cyl (gal) | Full Cyl (L) | Half Cyl (L) | Difference (gal) |
|---|---|---|---|---|---|---|
| 12 in | 18 in | 8.81 | 4.41 | 33.4 | 16.7 | 4.41 |
| 12 in | 24 in | 11.75 | 5.87 | 44.5 | 22.2 | 5.87 |
| 18 in | 24 in | 26.44 | 13.22 | 100.1 | 50.0 | 13.22 |
| 18 in | 36 in | 39.66 | 19.83 | 150.1 | 75.0 | 19.83 |
| 24 in | 24 in | 47.00 | 23.50 | 177.9 | 88.9 | 23.50 |
| 24 in | 36 in | 70.50 | 35.25 | 266.9 | 133.4 | 35.25 |
| 30 in | 24 in | 73.44 | 36.72 | 278.0 | 139.0 | 36.72 |
| 30 in | 36 in | 110.16 | 55.08 | 417.0 | 208.5 | 55.08 |
Why Half Cylinder Volume Is Exactly Half
The half cylinder formula is exactly half of the full cylinder formula because the flat back panel bisects the circular cross-section perfectly along the diameter. A diameter is a straight line passing through the centre of a circle, dividing it into two equal semicircles of equal area. Multiplying that semicircular area by the water height gives a volume that is, by definition, half of the full cylinder. This mathematical relationship holds regardless of size — whether the tank is 6 inches across or 60 inches across, the ratio between the two shapes is always 2:1. Try the same dimensions in both the cylinder volume calculator and the half cylinder volume calculator to see the relationship in action.
The practical implications are straightforward. A half cylinder sits flush against a wall, which makes it space-efficient and a popular choice for hallways, offices, and rooms where floor depth is constrained. The trade-off is exactly half the water volume of a full cylinder with the same diameter. For the same floor footprint, a full cylinder gives double the biological capacity, double the dilution capacity for waste, and a wider margin for error in stocking and dosing. The full set of cross-sectional area formulas behind every shape is collected on the aquarium volume formulas reference, and the correct way to measure each dimension is covered in the how to measure aquarium volume guide.
Which Shape Should You Choose
If wall placement and visual impact are the priority, the half cylinder is the practical choice. It mounts flush against the wall, requires no clearance behind the tank, and the curved front creates an attractive display from a wide range of viewing angles. The volume trade-off is predictable and exact: you always get half the water of the equivalent cylinder, so sizing the stand, lighting, and filtration is straightforward once the diameter and height are known.
If maximum water volume within a given diameter is the goal, the full cylinder wins by definition. Larger water volume means more stable water chemistry, more biological filtration capacity, and a larger margin for error in stocking and dosing. For total system volume that includes a sump, refugium, or external filter, use the aquarium sump volume calculator to estimate the combined water volume; to account for rocks, substrate, and hardscape that reduce the effective swimming volume, the aquarium displacement calculator handles the subtraction.