Trade & construction Australia
Tank Volume Calculator
Pick the shape, enter the dimensions, and set a fill level if the tank is not full. A horizontal cylinder is handled properly with the circular-segment maths rather than the linear approximation that makes most dipstick estimates wrong.
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Leave at 0 for full capacity
Capacity
βFull capacity
β
Cubic metres
β
Weight of contents
β
Percent full
β
How do you calculate tank capacity in litres?
Work out the volume in cubic metres, then multiply by 1,000. An upright cylindrical tank is ΟrΒ²h; a rectangular tank is length Γ width Γ height. A tank 1.8 m across and 2.1 m tall holds 5.34 mΒ³, which is 5,340 litres. A horizontal tank filled part way needs the circular-segment formula instead.
Nominal capacity is not geometric volume
A tank sold as 5,000 litres will usually measure slightly more than 5,000 litres geometrically, and hold slightly less in practice.
The geometric figure is the whole internal space, right to the top. Real tanks are never filled to the rim β the outlet sits above the base, the inlet and overflow occupy the top, and the last few centimetres at the bottom contain sediment you do not want drawn into a pump.
So treat a calculated volume as the upper bound. Working capacity on a rainwater tank is typically the geometric volume less whatever sits below the outlet and above the overflow.
This matters most when sizing rather than measuring. A tank chosen because it holds exactly the volume you calculated will be marginal in use.
Weight, and what it sits on
Water weighs one kilogram per litre, so the number in litres is also the number in kilograms.
A 5,000 litre tank is five tonnes when full, sitting on a footprint of about two and a half square metres. That is a load of roughly two tonnes per square metre, which is why rainwater tanks need a properly prepared base β a compacted pad or a slab β rather than levelled ground.
Tank stands for gravity feed concentrate that load further. A full tank on a stand is a structural load, and the stand needs to be designed for it rather than assembled from what was in the shed.
The weight figure in the result is for water. Other liquids differ β diesel is about 0.83 kg per litre, so a diesel tank is lighter than the same volume of water by about a sixth.
How this is calculated
Three shapes, three formulas β and one of them is considerably less obvious than the other two.
upright round V = Ο Γ (d Γ· 2)Β² Γ hrectangular V = l Γ w Γ hhorizontal round, full V = Ο Γ (d Γ· 2)Β² Γ lengthhorizontal round, part filled to depth y, radius r:area = rΒ² Γ arccos((r β y) Γ· r) β (r β y) Γ β(2ry β yΒ²)V = area Γ lengthlitres = V Γ 1,000 Β· water weighs 1 kg per litre
That circular-segment formula is why a horizontal tank cannot be read with a simple ruler and a multiplier. The cross-section of liquid is a segment of a circle, and its area grows fastest through the middle of the tank and slowest at the top and bottom.
In practice that means a horizontal tank half full by depth is exactly half full by volume, but a tank a quarter full by depth holds only about 20 per cent of its capacity β not 25. Reading a horizontal tank as if depth were proportional to volume overstates the contents near the bottom and understates them near the top.
A worked example: a horizontal tank at a third depth
A horizontal cylindrical tank 1.2 metres in diameter and 2.4 metres long, with 400 millimetres of liquid in it.
- Radius0.6 m
- Full capacity β Ο Γ 0.6Β² Γ 2.42.714 mΒ³
- Depth as a fraction of diameter33.3%
- Segment area at y = 0.4 m0.3300 mΒ²
- Volume β 0.3300 Γ 2.40.792 mΒ³
- As a share of capacity29.2%
About 792 litres β 29% of capacity, from a tank that is a third full by depth.
That four-point gap between depth and volume is the error a linear dipstick reading introduces, and it grows toward the ends of the tank: at a quarter of the depth the tank holds under 20 per cent of its capacity.
What this calculator does not do
- Assumes straight sides and flat ends β domed or dished ends hold slightly more
- External measurements include wall thickness and overstate capacity
- Corrugated tank walls are treated as smooth
- Weight figures assume water at 1 kg per litre
Common questions
How do I work out the litres in my water tank?
Calculate the volume in cubic metres and multiply by 1,000. An upright round tank is Ο Γ radiusΒ² Γ height; a rectangular one is length Γ width Γ height. Use internal dimensions where you can.
Why can't I just measure the depth in a horizontal tank?
Because the relationship between depth and volume is a curve, not a line. A horizontal tank a quarter full by depth holds only about 20 per cent of its capacity, not 25.
How much does a full water tank weigh?
One kilogram per litre, so a 5,000 litre tank is five tonnes. Spread over its footprint that is about two tonnes per square metre, which needs a properly prepared base.
Is a 5,000 litre tank actually 5,000 litres?
Not usable. The geometric volume is the full internal space, but the outlet sits above the base and the overflow limits the top, so working capacity is meaningfully less than the number on the label.
Do I measure inside or outside the tank?
Inside, for capacity. An external measurement includes the wall on both sides β negligible on a thin poly tank, but a real difference on a concrete one.
Where these numbers come from
- Wolfram MathWorld β circular segment area The partial-fill formula for horizontal cylinders
Method last reviewed 2026-10-28 This calculator uses fixed rules rather than rates that change, and its method is re-checked against the sources above at least once a year. Out of date? Tell us and we will fix it.
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