Water Tank Volume Calculator
A cylindrical tank 4 feet across and 6 feet tall holds about 564 gallons of water. This water tank volume calculator finds the full capacity of rectangular tanks, upright cylindrical tanks, and horizontal cylindrical tanks in US gallons, liters, and cubic feet. Pick the shape that matches your tank. A rain barrel or cistern built as a box uses length, width, and height. An upright cylindrical tank, like a vertical water storage tank or a well pressure tank, uses diameter and height. A horizontal cylindrical tank, like a propane tank, a fuel tank, or a septic tank lying on its side, uses diameter and the tank's straight length along its long axis. Every result is the tank's total (brim-full) capacity. It does not model partial fill level, wall thickness, fittings, or internal baffles.
Quick answer
Choose a shape first: rectangular, vertical cylinder, or horizontal cylinder.
What this tells you
- •Choose a shape first: rectangular, vertical cylinder, or horizontal cylinder.
- •Rectangular tanks use length x width x height.
- •Vertical cylindrical tanks use pi x radius squared x height, standing upright.
- •Horizontal cylindrical tanks use the same pi x radius squared x length formula, just lying on their side.
- •For a horizontal tank, the 'length' input is the tank's long-axis dimension, not the height off the ground.
- •Results show cubic feet, US gallons, and liters for the same tank.
- •One US gallon equals 231 cubic inches, and one cubic foot equals about 7.4805 US gallons.
- •A cylinder's total volume does not change based on orientation. An upright tank and the same tank lying on its side hold the same amount when both are completely full.
How to Use
- 1Select the shape that matches your tank: rectangular, vertical cylinder, or horizontal cylinder.
- 2For a rectangular tank, enter inside length, width, and height.
- 3For a vertical cylinder, enter the inside diameter and the inside height from bottom to top.
- 4For a horizontal cylinder, enter the inside diameter and the tank's straight length along its long axis (the direction it lies).
- 5Choose the unit your measurements are in: inches, feet, centimeters, or meters.
- 6Calculate to see total tank capacity in US gallons, liters, and cubic feet.
- 7Use inside dimensions, not outside dimensions, for the most accurate usable-capacity estimate.
How It Works
Formula
Rectangular: Volume = Length x Width x Height
Vertical cylinder: Volume = pi x (Diameter / 2)^2 x Height
Horizontal cylinder: Volume = pi x (Diameter / 2)^2 x Length
Gallons = Cubic Feet x 1728 / 231
Liters = Cubic Feet x 28.316846592The calculator converts every dimension to feet, then applies the formula that matches the selected shape. A rectangular tank multiplies length x width x height directly. Both cylinder shapes divide diameter by 2 for radius, square it, multiply by pi, then multiply by the tank's height (vertical) or length (horizontal). A tank does not gain or lose total volume by changing orientation, so a vertical and horizontal cylinder with the same diameter and same length dimension return the same total capacity. Once cubic feet is known, the calculator multiplies by 1728 / 231 (about 7.480519) for US gallons and by 28.316846592 for liters.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Rectangular cistern
Volume = 6 x 4 x 3 = 72 cubic feet. Multiplying by 7.480519 gives 538.60 US gallons, and multiplying by 28.316846592 gives 2038.81 liters.
Vertical cylindrical water storage tank
A 4-foot diameter gives a 2-foot radius. Volume = pi x 2^2 x 6 = 75.3982 cubic feet, which converts to about 564.04 US gallons and 2135.19 liters.
Horizontal propane or fuel tank
A 2-foot diameter gives a 1-foot radius. Volume = pi x 1^2 x 6 = 18.8496 cubic feet, the same total capacity as an upright cylinder with the same diameter and the same length dimension.
Small IBC-style rectangular tote
48, 40, and 46 inches convert to 4, 3.333, and 3.833 feet. Their product is 51.111 cubic feet, or about 382.28 US gallons.
Metric vertical tank
1.2 m converts to a 1.969-foot radius and 2 m converts to about 6.562 feet of height. Volume = pi x 1.969^2 x 6.562 = 79.88 cubic feet, or about 597.54 US gallons.
Common Tank Capacities
Total (brim-full) volume for typical cylindrical and rectangular tank sizes.
| Shape | Diameter or Footprint | Height or Length | US Gallons |
|---|---|---|---|
| Vertical cylinder | 2 ft diameter | 4 ft | 94.0 gal |
| Vertical cylinder | 4 ft diameter | 4 ft | 376.0 gal |
| Vertical cylinder | 4 ft diameter | 6 ft | 564.0 gal |
| Vertical cylinder | 6 ft diameter | 6 ft | 1269.1 gal |
| Horizontal cylinder | 2 ft diameter | 6 ft | 141.0 gal |
| Horizontal cylinder | 3 ft diameter | 8 ft | 423.0 gal |
| Rectangular | 4 ft x 3 ft footprint | 3 ft | 269.3 gal |
| Rectangular | 6 ft x 4 ft footprint | 3 ft | 538.6 gal |
All figures are total tank capacity when completely full, not everyday operating volume.
How to Measure Your Tank for an Accurate Volume
Use inside dimensions whenever possible. Wall thickness on a rotomolded plastic tank, fiberglass tank, or steel tank takes up space that outside dimensions include but that water never fills.
For a vertical cylindrical tank, measure diameter at the tank's widest inside point and measure height straight up from the inside bottom to the inside top, not along a sloped or domed end.
For a horizontal cylindrical tank, such as a propane tank, fuel tank, or a septic tank lying on its side, the 'length' dimension is the straight-line distance along the tank's long axis, from end to end. It is not the tank's height off the ground or its width as you view it from the side.
For a rectangular tank, measure length, width, and height between the inside faces of the walls, the same way you would measure the inside of a box.
Rounded or dished tank ends (common on horizontal cylindrical tanks) add a small amount of extra volume this calculator does not include, since it treats the tank as a plain cylinder capped by flat ends.
Common mistakes
- Entering diameter where the formula needs radius, which doubles the effective size and roughly quadruples the calculated volume
- Measuring outside dimensions instead of inside dimensions, which overstates true usable capacity
- For a horizontal cylindrical tank, entering the tank's height off the ground instead of its straight length along the long axis
- Mixing units, such as entering diameter in feet and height in inches without converting
- Assuming the calculated volume is available operating volume rather than total brim-full capacity
- Using this full-tank result to represent a partially filled horizontal tank's current volume
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