A half-cylinder (D-shaped) aquarium has a flat back and a semicircular front. It can sit against a wall to save floor space, and its volume is exactly half that of a full cylinder with the same diameter and height.
You enter the length of the flat back glass (the diameter) and the height. The depth is half the diameter, so you do not need to enter it.
How is the volume of a half cylinder aquarium calculated?
Gross volume (litres) = π × (diameter ÷ 2)² × height ÷ 2 ÷ 1000. For the net water volume, the thickness of the flat back glass and the curved front glass is subtracted from the inner half-circle area.
How to take the measurements
- Measure the outer length of the flat back glass: this is the diameter.
- Check that the depth (from the back glass to the farthest point of the front curve) is about half the diameter. If it is much less, the tank is not a true half circle — use the bow front calculator.
- Measure the height from the bottom to the top rim.
- Enter the glass thickness and the advanced settings.
Worked example
For a half cylinder aquarium with diameter 80 cm, height 60 cm, the gross volume is 150.8 litres. With 0.8 cm glass, no substrate and 2 cm of headroom, the net water volume is 134.5 litres.
Example sizes and their volumes
| Diameter (cm) | Height (cm) | Gross (L) | Net water (L) | Net water (US gal) |
|---|---|---|---|---|
| 40 | 40 | 25.1 | 20.4 | 5.4 |
| 60 | 50 | 70.7 | 61 | 16.1 |
| 80 | 60 | 150.8 | 134.5 | 35.5 |
| 100 | 60 | 235.6 | 213 | 56.3 |
Net water volume assumes 0.8 cm glass, 0 cm substrate and 2 cm headroom; results are approximate.
Frequently asked questions
Is a half-cylinder tank exactly half of a cylinder?
In gross volume, yes: with the same diameter and height it is exactly half. For net volume the flat back glass is an extra edge, so the figures differ slightly.
Do I need to enter the depth?
No. Because it is a half circle, the depth equals half the diameter and the calculator uses it automatically.
Which calculator should I use if the front is not a perfect half circle?
If the front is a shallower arc and the side panels are flat, the bow front calculator gives a more accurate result.