Water Tank Capacity Formulas: Litres, m³ and Round Tanks

How to calculate water tank capacity in litres

Measure the internal dimensions of the water-holding space, select its actual shape and calculate volume in cubic metres. Multiply cubic metres by 1,000 to obtain litres. For a rectangular tank, multiply internal length by width by height. For a vertical circular cylinder, multiply π by radius squared by height. The calculator also supports an ideal horizontal cylinder, whose partial-fill volume needs a circular-segment formula. A diameter is twice a radius: entering a radius in the diameter field gives the wrong result.

Internal dimensions and nominal tank capacity

Use internal dimensions rather than external catalogue dimensions. Wall thickness, ribs, domed ends, tapers, fittings and irregular bases can make the real tank different from the ideal geometric shape. A manufacturer’s nominal capacity is a product specification and need not equal the result from a simplified external bounding shape. Check the actual model label, current manufacturer data and any level-to-volume calibration chart. This independent calculator is not an official JoJo sizing tool and does not replace manufacturer capacity information.

Square and rectangular water tank formula

A square tank has equal length and width, so its full volume is side × side × internal height. A rectangular tank uses length × width × height. Dimensions must use the same unit before multiplication. With metre inputs, a 2 m × 1.5 m × 2 m internal rectangular space holds 6 m³, or 6,000 litres, at its geometric brim. If the water depth is 1 m, the same ideal tank contains 3 m³, or 3,000 litres. Freeboard reduces the permitted filling depth.

Round water tank capacity calculation formula

For a vertical cylindrical tank, V = π × (diameter/2)² × water depth. A 2 m internal diameter and 2 m internal height gives approximately 6.283185 m³, or 6,283.185 litres, to the geometric brim. For partial filling, replace the height with the measured water depth. These examples describe a constant circular cross-section, not a domed, corrugated or tapered commercial vessel. Do not use the vertical-cylinder formula for a partly filled cylinder laid on its side.

Horizontal cylinder and partial water depth

For a level horizontal cylinder with flat ends, radius r, straight internal length L and water depth h measured vertically from the bottom, V(h) = L × [r² × acos((r − h)/r) − (r − h) × √(2rh − h²)]. The calculator uses radians internally and handles empty and full limits. At half the internal diameter, the water occupies half the ideal cylindrical volume. At a quarter of the diameter, it occupies less than a quarter of the volume. Curved ends or a sloping tank require another model or calibration.

Water tank capacity in m3, kilolitres and gallons

One cubic metre equals 1,000 litres and one kilolitre. Inputs can use metres, centimetres, millimetres or feet; the tool converts every length to metres before applying the shape formula. It reports US liquid gallons and Imperial gallons separately because they are different units. A 10 × 10 × 10 tank cannot be assigned a capacity without its measurement unit and shape. A rectangular 10 m cube is 1,000 m³; a rectangular 10 cm cube is only 1 litre. Always include units when sharing an estimate.

Frequently asked questions

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