How to use it
- Choose the Tank shape that matches your tank.
- Type the inside measurements that the shape asks for: Diameter, Length, Width or Height. The other fields are hidden.
- Type the Liquid depth (cm), measured from the inside bottom of the tank to the surface.
- Read Liquid in the tank, Full capacity and Filled. How it is worked out shows each formula with your numbers.
How the volume is calculated
A vertical cylinder holds its base area times the depth: pi times the radius squared, times the depth. A box holds its length times its width times the depth.
A horizontal cylinder is harder. The liquid's cross-section is a circular segment, the part of the circle below the surface, and its area is not a simple product. The tool uses the exact formula for that segment:
Liquid = [ r² x arccos((r - d) / r) - (r - d) x √(2rd - d²) ] x length, where r is the radius and d the depthUnits and what the tool does not include
- The sizes are typed in centimetres and the result is in litres. One litre is one cubic decimetre, which is 1,000 cubic centimetres (NIST SP 330, Table 8).
- The tool works with the inside measurements you type. It does not allow for wall thickness, domed or rounded ends, fittings or the feet of the tank.
- A tank that is not level holds a different amount at the same reading. Stand the tank level, and measure the depth from the lowest inside point.
- The result is a geometric estimate. A meter, a sight glass or the maker's capacity table gives the figure that counts for sale or for planning a delivery.
Worked example: a horizontal tank 30 cm deep
A horizontal tank has a diameter of 120 cm and a length of 240 cm, and the liquid stands 30 cm deep. The radius is 60 cm, so the depth is half the radius.
The wet cross-section is 2,211.07 cm². Multiplied by the length, the liquid is 530,655.7 cm³, which is 530.7 litres. The full tank holds 2,714.3 litres, so the tank is 19.6 % full.
| Result | Value |
|---|---|
| Liquid in the tank | 530.7 L |
| Full capacity | 2,714.3 L |
| Filled | 19.6 % |
Frequently asked questions
How do I measure the depth of the liquid?
Measure from the inside bottom of the tank up to the surface of the liquid, with a rule or a dipstick, and type that depth in centimetres. Measure inside the walls and do not add the thickness of the sheet metal or plastic.
Why is a horizontal tank harder to calculate than a vertical one?
The liquid's cross-section is a circular segment, so its area does not grow in a straight line with the depth. At exactly half the diameter the tank is half full, but at other depths the share changes along a curve, which is why the tool uses the exact segment formula.
What if I only know the outside measurements of the tank?
Subtract twice the wall thickness from each measurement that goes across the tank, such as the diameter or the width, before you type it. The end walls and the thickness of the bottom are not allowed for, so check the result against the maker's data.
Is one litre the same as one cubic decimetre?
Yes. One litre is one cubic decimetre, which is 1,000 cubic centimetres, and the NIST table of units accepted for use with the SI lists it as 1 L = 1 dm³ = 10⁻³ m³.
Can I use the tool for a tank with domed or rounded ends?
Not exactly. The full capacity is calculated with flat ends, and a domed or dished end adds volume. Use the capacity table of the maker for a tank with such ends.
Why does a tilted tank give a wrong figure?
A tilted tank has a different depth and a different surface than a level one, so the formula no longer describes it. Stand the tank level before you measure the depth.
Sources
- Wolfram MathWorld: Cylinder (volume pi r^2 h, equation 5)
- Wolfram MathWorld: Circular Segment (area in terms of the height, equation 18)
- NIST Special Publication 330, section 4, Table 8: 1 L = 1 dm³ = 10⁻³ m³
Figures and text last checked on Tuesday, November 10, 2026.