Measure tank volume by shape
Use internal dimensions and the water level, not outside tank dimensions.
Back to calculator ↑Choose rectangle or upright cylinder
A rectangular base is length × width. A cylinder base is π × radius², with radius half the diameter.
Measure the inside diameter of an upright cylinder and water depth from the relevant inner bottom. Thick walls or rims make outside dimensions overstate water capacity. Manufacturer capacity data may be better when walls are thick or the shape is unusual.
Measure usable water height
The height field is the water depth being evaluated. Overflow level or inaccessible water at the bottom can make it different from outside height.
A cylinder 1 m in diameter has a 0.5 m radius, so its base area is π × 0.5² ≈ 0.785 m². At 1 m water height it holds about 0.785 m³ or 785 L. This example helps catch the common error of squaring the diameter rather than the radius.
Worked default example
A rectangular tank 2 m long, 1 m wide and 1 m water height holds 2 m³ or 2,000 L.
If the example tank has only 0.8 m of usable water height, it holds 1,600 L. Compare water levels for reserve planning rather than relying on outside tank size alone.
Shapes outside the formula
Domed ends, recessed bottoms and horizontal cylinders need manufacturer capacity data or a different formula. Real shape can differ from the calculated solid.
The formulas assume a constant horizontal cross-section with height. Tapered tanks and horizontal cylinders change cross-section as water level changes, so this tool cannot model them. Compare the estimate with the exact model documentation or a measured fill when appropriate.
If the tank has marked water levels, measure the vertical span of interest rather than always using bottom-to-rim height. For constant-base shapes, usable volume between two levels can be found by subtracting the lower-level volume from the upper. Do not subtract minimum water height twice if a net height was already entered.
Before choosing a pump, check the minimum water level and real outlet position. Not all geometric volume is necessarily pumpable, and the installation may need additional headspace above the water.
Common questions
How many litres are in 1 m³?
1,000 L.
Can I use outside diameter?
No. Wall thickness makes internal water volume smaller.
Can this calculate a horizontal cylinder?
No. This cylinder formula assumes an upright tank.
Sources and calculation guidance
Check these against your chosen product’s label and instructions.