How this calculator works
A tank's capacity is its volume. A cylinder is π times the radius squared times the height (or the length, if it lies on its side), and a rectangular tank is length times width times height. A cubic foot holds 7.48 US gallons. For a horizontal cylinder that is part full, the volume is the area of the circular segment filled by the liquid times the length, which is not in proportion to the depth.
Worked example: a vertical tank 4 ft across and 6 ft high holds 564 gallons (75.4 ft³). The same tank 8 ft long on its side holds 752 gallons, and a box 4 × 3 × 6 ft holds 539. Lying down and filled to a quarter of the diameter it holds 147 gallons (19.6%), and to half the diameter 376.
Gallons in common round tanks standing up
Capacity in US gallons.
| Diameter | 4 ft high | 6 ft high | 8 ft high |
|---|---|---|---|
| 2 ft | 94 | 141 | 188 |
| 3 ft | 212 | 317 | 423 |
| 4 ft | 376 | 564 | 752 |
| 5 ft | 588 | 881 | 1,175 |
| 6 ft | 846 | 1,269 | 1,692 |
| 8 ft | 1,504 | 2,256 | 3,008 |
Tips before you buy
- Measure the inside of the tank, since the wall thickness cuts the capacity.
- A dip stick marked in gallons is simpler than doing sums each time you check the level.
- For a horizontal tank use a chart or this calculator: the volume does not go up in step with the depth.
- Do not fill a tank to the top. Liquids expand with heat, so leave an air space.
- A cubic foot holds 7.48 US gallons, and a US gallon is 3.785 liters.
Tools & materials for this project
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Frequently asked questions
How many gallons are in a round tank?
π × radius² × height × 7.48. A tank 4 ft across and 6 ft high holds 564 gallons.
How do you calculate a horizontal tank that is part full?
The volume is not linear in the depth. A horizontal tank filled to 25% of its diameter holds 19.6% of its capacity, and to 50% of its diameter exactly half. Use this calculator with the liquid depth.
How many gallons are in a cubic foot?
About 7.48 US gallons. A cubic foot of water weighs 62.4 lb.
How much does a full water tank weigh?
Water weighs 8.34 pounds a gallon, so a full 564 gallon tank holds about 4,704 lb of water, plus the tank.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.