Calculate tank capacity in litres
Measure the tank’s internal dimensions in metres. One cubic metre equals 1,000 litres. Choose the shape that matches the main body of your tank, then enter the water depth to calculate the amount currently stored.
Rectangular tank
Capacity (L) = length × width × height × 1,000
A tank 3 m long, 2 m wide and 1.5 m high has a geometric capacity of 9,000 litres. At a water depth of 0.75 m it holds 4,500 litres.
Vertical cylindrical tank
Capacity (L) = π × (diameter ÷ 2)² × height × 1,000
A round upright tank with an internal diameter of 3 m and a height of 2 m has a geometric capacity of approximately 14,137 litres. With 1 m of water, it holds about 7,069 litres. The water percentage matches the depth percentage for an ideal upright cylinder.
Horizontal cylindrical tank
For a cylinder lying on its side, depth and volume are not proportional. This calculator uses the circular-segment area beneath the waterline, multiplied by the cylinder’s length.
Water volume = [r² × arccos((r − h) ÷ r) − (r − h) × √(2rh − h²)] × length × 1,000
Here r is the internal radius, h is water depth measured from the bottom, and the angle is in radians. A 2 m diameter horizontal cylinder, 3 m long, holds about 9,425 litres when full. At 0.5 m depth, it holds about 1,843 litres: roughly 19.6% of capacity, even though the depth is 25% of the diameter.
Common questions
Should I use inside or outside measurements?
Use internal dimensions. External measurements can overstate capacity because they include wall thickness. Check the manufacturer’s rated capacity or calibration chart where available.
Does this work for domed ends or an irregular tank?
No. The cylinder calculation assumes flat ends and a uniform circular cross-section. Rectangular calculations assume straight sides. Rounded, domed, tapered or irregular sections need a manufacturer’s volume chart or a more detailed model.
Can I use all the calculated water?
Not necessarily. Water below the outlet, sediment and any protected reserve may be unavailable for normal use. Use a suitable allowance when you move to the days-of-supply calculator.