A horizontal tank 48 inches across and 10 feet long holds 940 US gallons when full. At half its depth it holds exactly half, 470 gallons — but at a quarter of its depth only 184 gallons, 20 %, because a lying cylinder is narrow at the bottom. The calculator above finds the total and filled volume of horizontal and vertical cylinders, rectangular tanks and spheres at any liquid level.
How the calculator works
Choose the tank type, enter its size and the fill level as a share of its height (its diameter for a lying tank). The result is the capacity, the liquid in it, the empty space and the weight of the water; the drawing shows the section with the level.
Lying tanks
The liquid in a horizontal cylinder is a circle segment along the tank's length. With the radius r and the liquid depth h, θ = 2 arccos((r − h) ÷ r) and the segment area is r²/2 × (θ − sin θ); times the length it gives the volume. This is what tank charts tabulate.
| Depth, share of diameter | Share of volume |
|---|---|
| 10 % | 5.2 % |
| 25 % | 19.6 % |
| 50 % | 50 % |
| 75 % | 80.4 % |
| 90 % | 94.8 % |
Other tanks
A standing cylinder and a rectangular tank fill in proportion to depth. A sphere holds a spherical cap: π h² ÷ 3 × (3r − h).
Common mistakes
- Treating a lying tank as linear. At a quarter of the depth it is a fifth full.
- Outside dimensions. Steel tanks are close; plastic and double-wall tanks lose several inches.
- Ignoring the heads. Dished ends add a little volume beyond the straight shell.
For a simple cylinder, use the Cylinder Volume Calculator; for a pool, the Pool Volume Calculator.