Capacity of rectangular and cylindrical water tanks in m³, litres and gallons.
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Tank Volume Calculator: Free Online Tool
The Tank Volume Calculator gives the capacity of a rectangular tank or a vertical cylindrical tank. You enter the length and width, or the diameter, together with the height or water depth, and the calculator returns the volume in cubic metres and cubic feet, plus litres, US gallons and imperial gallons. A filled-level percentage lets you find the water volume at a partial depth.
Plumbers, site engineers, contractors and building owners use this calculation to size overhead tanks, underground tanks, septic chambers and sumps. Knowing the real capacity matters because it decides how many hours of supply a building can hold, how long a pump takes to fill the tank and whether a planned tank is suitable for the number of occupants.
When to use this calculator
Use this calculator when you design or check an overhead tank, an underground water reservoir, a septic tank chamber or a sump. It is also useful when a supplier quotes a tank in gallons and you need to compare it with your requirement in litres. You can enter a lower filled level to find the quantity of water at a given depth, for example to check a half-full tank, or to estimate the volume of water pumped out of a pit or a cistern during a dewatering operation.
How to use the Tank Volume Calculator
- Open the calculator above on your phone, tablet or computer. Nothing needs to be installed and no sign-up is required.
- Choose the unit system first (feet / inches or metres / mm). The input labels change to match your choice.
- Pick the option you need from the drop-down list (Rectangular; Cylindrical (vertical)).
- Enter your values in the input boxes: Tank shape, Length (ft), Width (ft), Diameter (ft), Height / water depth (ft), Filled level (%). The boxes already hold sample values, so overwrite them with your own.
- Press Calculate. The result appears straight away under the button.
- Change any value and press Calculate again to compare options. Press Reset to go back to the sample values.
- Check the result against your drawings, project specification and local code before you order material or approve work.
How the calculation works
For a rectangular tank the volume is length x width x height. For a vertical cylinder it is (pi/4) x diameter² x height. The dimensions are converted to metres, so the volume is first obtained in cubic metres and also shown in cubic feet. The volume is then multiplied by the filled level percentage divided by 100, with a default of 100%. Litres are volume in m³ x 1000. US gallons are litres divided by 3.785411784, and imperial gallons are litres divided by 4.54609. The default rectangular tank is 10 x 8 ft with a 6 ft height. You can edit all dimensions.
Worked example
When you open the page the calculator is filled with sample values. Pressing Calculate with those values gives the result below, which you can use to check that you are reading the output correctly.
| Capacity | |
|---|---|
| Volume | 13.592 m³ (480.00 cft) |
| Litres | 13,592 L |
| US gallons | 3,591 gal |
| Imperial gallons | 2,990 gal |
Tips and common mistakes
- Use internal dimensions of the tank, not the outside measurements, otherwise wall thickness will make the capacity look larger than the real storage volume.
- Choose the correct shape before entering data. The diameter field is for the cylindrical tank, while length and width apply only to the rectangular tank.
- Remember that a tank is never filled to the top. Use the filled level percentage to find the usable water volume below the overflow level.
- Check whether your supplier uses US gallons or imperial gallons. They are different, and the calculator shows both so that you can compare correctly.
- For horizontal cylinders or sloped-bottom tanks, this calculator is not suitable, because it assumes straight vertical sides and a flat base.
Frequently asked questions
How do I find the volume of a cylindrical tank?
Select the vertical cylinder shape and enter the diameter and the height. The calculator uses (pi/4) x diameter² x height, converts it to metres, and shows the result in m³, litres and gallons. Use the internal dimensions of the tank.
How many litres are in one cubic metre?
One cubic metre equals 1000 litres. The calculator multiplies the volume in m³ by 1000, so a 13.592 m³ tank holds about 13,592 litres, as in the default example. The litre value is also useful for pump and consumption calculations.
What is the filled level percentage for?
It scales the full capacity to a partial depth. For example, entering 50% gives the volume of water in a tank that is half full. The default is 100%, meaning a full tank. Use it for tanks that are partly filled.
Why are US and imperial gallons both shown?
The two gallons are different sizes. A US gallon is 3.785 litres and an imperial gallon is 4.546 litres. Showing both avoids confusion when tank sizes are quoted in either unit. Check which one your supplier uses.
Does the calculator include wall thickness?
No. It uses the dimensions you enter. For correct water capacity, use internal length, width or diameter, and the internal water depth, measured inside the tank. Measure inside the walls to avoid overestimating.
Important note
This tool gives estimates for planning, study and quick checks. It does not replace design by a qualified engineer, site-specific testing, or the requirements of your project specification and local building code. Always confirm final quantities, test results and acceptance limits with the responsible engineer before using them for construction or payment.
