Tank specs mix gallons, liters and cubic feet, so the calculator works in cubic feet and converts from there. One cubic foot holds 7.48 US gallons, one US gallon is 3.785 liters, and a cubic foot is also 1,728 cubic inches.
How to use this calculator
- Pick the shape. Vertical is a cylinder on its base, horizontal is a cylinder on its side, and rectangle is a box with flat sides and a flat floor.
- Measure the inside from wall to wall. For a cylinder, that means the inside diameter and, if it stands upright, the inside height. For a box, measure the inside length, width and height.
- Check the unit on each length field. Lengths start in feet and the liquid depth starts in inches, so change any field that does not match your tape.
- Skip the liquid depth for the full capacity. To see the level now, measure from the inside floor up to the surface of the liquid, and enter that depth.
- Read the headline. With no liquid depth it is the full capacity. With one, it is the gallons in the tank now, with the percent full and the room left beside it.
Worked examples
Upright rain water tank
Tank shape: Vertical · Diameter: 3 ft · Height: 4 ft
- Capacity
- 212 US gallons
- Liters
- 801 L
- Cubic feet
- 28.27 cu ft
Horizontal tank part full
Tank shape: Horizontal · Diameter: 4 ft · Length: 6 ft · Liquid depth: 12 in
- Liquid in the tank
- 110 US gallons
- Liters
- 417 L
- Full
- 19.6%
- Room left
- 454 US gallons
- Full capacity
- 564 US gallons
Rectangular stock tank
Tank shape: Rectangular · Size: 6 × 2 ft · Height: 2 ft
- Capacity
- 180 US gallons
- Liters
- 680 L
- Cubic feet
- 24 cu ft
Aquarium in inches
Tank shape: Rectangular · Size: 48 × 18 in · Height: 20 in
- Capacity
- 74.81 US gallons
- Liters
- 283 L
- Cubic feet
- 10 cu ft
Capacity of a horizontal tank in US gallons
| Shape and diameter | Length: 6 ft | Length: 8 ft | Length: 10 ft |
|---|---|---|---|
| Horizontal, 3 ft | 317 | 423 | 529 |
| Horizontal, 4 ft | 564 | 752 | 940 |
| Horizontal, 5 ft | 881 | 1,175 | 1,469 |
| Horizontal, 6 ft | 1,269 | 1,692 | 2,115 |
Each cell: Capacity (US gallons). Inputs not in the table use the calculator's starting values.
How the calculation works
An upright cylinder holds the area of its round base, pi times the radius squared, times its height. A rectangular tank holds its length times its width times its height.
Tanks with oval or flat-sided ends, which are common for heating oil, are not exact cylinders. Use the maker's chart for those tanks.
Why a lying tank is different
A tank on its side does not fill evenly. Each inch of depth holds a different volume: the most in the middle, where the tank is widest, and the least near the top and bottom. So a tank half full by depth is half full by volume, but a tank filled to a quarter of its depth holds much less than a quarter of its capacity.
Measure the depth rather than guess the level. The percent full on the result reflects that curve.
Tanks this calculator fits
Rain barrels, water tanks, stock tanks and fuel or water tanks on a trailer fit well when the shape is one of the three options. Aquariums fit the rectangle option: measure the inside of the glass. Water is rarely filled to the rim, so add a liquid depth for the level you will keep.
Well pressure tanks are not a fit. Only part of their volume is water you can draw, and that depends on the air pressure and the switch settings. For a swimming pool with a sloped floor, use the pool calculator. Gravel and mulch are ordered by the cubic yard, so for a bin of them, use the cubic yard calculator.
Mistakes to avoid
- Measuring the outside. The shell holds no liquid, so measure wall to wall.
- Trusting a rated size without checking it. A rating may be quoted to the rim, not to the level you run.
- Typing a percentage into the depth field. The field takes a depth in inches from the inside floor.
- Mixing units. Each field has its own unit, so check each one before you read the result.
How much cleaner or bleach do I need for a water tank?
For a drinking-water tank, one state health department guide calls for 1 gallon of plain unscented household bleach for every thousand gallons the tank holds, to reach 50 ppm chlorine, left in for 24 hours. Scale it to the full capacity from the calculator, and never climb inside a tank to do it.
Common questions
How do I calculate the volume of a horizontal cylindrical tank?
With the tank full, multiply pi by the radius squared, then by the length. Part full, only a slice of the round end holds liquid, so enter the depth in inches and let the calculator find that slice.
How many liters are in a tank?
The gallons and liters on the result are the same volume. One US gallon is 3.785 liters, so multiply gallons by that figure to get liters, and divide liters by it to get gallons.
What is the formula for the capacity of a round water tank?
For an upright tank, take the radius as half the inside diameter, square it, multiply by pi, then multiply by the height. That gives cubic feet, and multiplying by 7.48 gives US gallons.
Can I use this calculator to size a septic tank?
No. Septic tank size follows local rules and the home's expected water use, so this calculator only tells you what a tank holds. Check the size you need with your local permit office.
How do I calculate the gallons in a rectangular tank?
Multiply the inside length, width and height, all in feet, to get cubic feet. Then multiply by 7.48 US gallons to get US gallons. If you measured in inches, divide the cubic inches by 1,728 cubic inches first to get cubic feet.
How accurate is a tank volume calculator?
The result is only as exact as your measurements. Measure inside wall to wall, and measure the diameter carefully, because it is squared in the formula. For tanks with dished, oval or sloped ends, the maker's chart is more reliable.
How much does a full water tank weigh?
Take the gallons from the result and multiply by 8.34 lb per US gallon. Then add the weight of the empty tank. Check that your stand or deck can carry the total.