Tank Volume Calculator
A tank volume calculator uses gallons = π × (diameter ÷ 2)² × height × 7.48. A 10 ft wide, 12 ft tall upright tank holds 942.5 ft³, or about 7,050 gallons.
Tank volume calculator: gallons from diameter and height
A tank volume calculator multiplies the circular footprint of an upright tank by its height. The formula is π × (diameter ÷ 2)² × height, which gives cubic feet. Multiply by 7.48 for US gallons. A tank 10 ft across and 12 ft tall holds 942.5 cubic feet, or about 7,050 gallons.
The fill-level box scales that capacity, so you can see how much is inside at 40 percent, 75 percent, or any other level.
The formula in plain words
Picture stacking round pancakes. Each pancake has the area of the tank’s circular floor and a thickness of one foot. Stack as many as the tank is tall. Add up the pancakes and you have the volume.
- Radius = diameter ÷ 2
- Cubic feet = π × radius² × height
- Full gallons = cubic feet × 7.48
- Filled gallons = full gallons × (fill % ÷ 100)
Every extra foot of height adds the same slice of water, which is why a straight fill percentage works for upright tanks.
Worked examples
- Water tank, 10 ft diameter, 12 ft tall, partly full
- Radius: 5 ft
- Cubic feet: π × 5² × 12 = 942.5 ft³
- Full capacity: 942.5 × 7.48 = 7,050 gal
- At 40% fill: 7,050 × 0.40 = 2,820 gal
- Cistern, 6 ft diameter, 8 ft tall, 75% full
- Radius: 3 ft
- Cubic feet: π × 3² × 8 = 226.2 ft³
- Full capacity: 226.2 × 7.48 = 1,692 gal
- At 75% fill: 1,269 gal
For example 1, the water weight matters too. At 8.34 pounds per gallon, a full tank carries about 58,800 pounds, so the tank pad is a structural item.
Capacity per foot of height
| Diameter | Gallons per foot of height | Full volume at 8 ft tall |
|---|---|---|
| 4 ft | 94.0 | 752 gal |
| 6 ft | 211.5 | 1,692 gal |
| 8 ft | 376.0 | 3,008 gal |
| 10 ft | 587.5 | 4,700 gal |
| 12 ft | 846.0 | 6,768 gal |
Multiply the middle column by your water depth to read the current volume of any upright tank.
Horizontal tanks
A cylinder lying on its side holds the same total volume, so full capacity is right. The fill level is a different story. The top and bottom of the circle are narrow, so the first few inches of depth hold far less water than the middle. Only the 50 percent point is exact, because half the depth is half the volume by symmetry. At other levels, use the tank maker’s chart or a dipstick table. The calculator’s “length” label supports a horizontal tank’s full capacity, not partial fills.
How to measure
- Inside diameter: Measure across the inside, or measure the circumference and divide by π, then subtract twice the wall thickness.
- Height: Use the usable internal height, from the floor to the overflow, not the top of the dome.
- Odd bottoms: Cone or dished bottoms add volume that this simple formula ignores.
- Fill level: Measure water depth with a stick and divide by the full height.
- Units: Both inputs go in feet.
Common mistakes
- Using outside diameter. Walls take up room, so the outside overstates capacity.
- Counting empty headspace. If the overflow sits 6 inches below the rim, the usable height is smaller.
- Applying the percentage to a horizontal tank. Partial levels do not scale linearly there.
- Forgetting the weight. Water adds 8.34 lb per gallon to whatever the tank rests on.
- Mixing US and imperial gallons. This calculator uses US gallons.
Reading a partly full tank
You do not need a percentage to use the tool. Measure the water depth with a stick, divide by the full height, and enter that as the fill level.
A 10 ft wide, 12 ft tall tank holding 5 ft of water is 5 ÷ 12 = 41.7 percent full. That is 7,050 × 0.417 = 2,938 gallons. Per foot of depth, this tank holds 587.5 gallons, so 5 × 587.5 = 2,938 gives the same answer.
Related calculators
For a round pool, use the round pool calculator. Other shapes are covered by the pond volume calculator. To see how much water a connected line holds, use the pipe volume calculator, and for the bare geometry the cylinder volume calculator.
Frequently asked questions
How do I calculate a cylinder tank's volume?
Volume equals pi times the radius squared times the height, which gives cubic feet. Multiply by 7.48 to get US gallons. The calculator does this from the diameter and height you enter, so you do not need to halve the diameter yourself.
Does the fill level work for horizontal tanks?
The fill percentage is linear, which is exact for an upright cylinder where each foot of height holds the same volume. A cylinder lying on its side fills non-linearly, so only the 50 percent point (half the full volume) is right. For other levels use a tank chart.
What is the difference between gallons and cubic feet here?
Both describe the same volume. Cubic feet is the raw geometric result, and gallons is what tanks are usually rated in. One cubic foot holds about 7.48 US gallons, and the calculator shows both.
How accurate is this for a real tank?
It gives the internal geometric capacity. Real tanks lose a little to wall thickness, fittings, and the fact that you rarely fill to the brim. Treat the full number as a maximum and plan for slightly less usable volume. Measure the inside diameter, not the outside.
Can I use this for a round stock tank or cistern?
Yes, any upright round container works: cisterns, rain barrels, stock tanks, and round pools. For a round above-ground pool specifically, the Round Pool Calculator uses the same math with pool-friendly inputs.
How much does the water weigh?
Fresh water weighs about 8.34 pounds per US gallon. A 7,050 gallon tank holds roughly 58,800 pounds of water (7,050 × 8.34), so the base and foundation must be planned for that load, not just the tank's own weight.
Estimates only. Verify quantities with your supplier before purchasing.