Water Tank Volume Calculator

Water Tank Volume Calculator

Calculate tank capacity, water volume and filling level in litres, cubic metres and gallons.

Tank shape Rectangular tank Vertical cylindrical tank Horizontal cylindrical tank

Dimension unit Metres (m) Centimetres (cm) Millimetres (mm) Feet (ft) Inches (in)

Rectangular tank dimensions

Length (m)

Width (m)

Height (m)

Cylindrical tank dimensions

Internal diameter (m)

Tank length / height (m)

Enter internal dimensions for the best estimate of usable capacity.

Current water depth (m, optional)

Measure the water depth from the tank bottom. For a horizontal cylinder, this means the vertical depth from the lowest internal point.

Total tank capacity

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Capacity in cubic metres: —

US gallons: —

UK gallons: —

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Current water volume

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Enter water depth to estimate the current volume.

Fill level: —

Remaining capacity: —

Water Tank Volume Formulas

Rectangular tank

Volume = Length × Width × Height

Vertical cylindrical tank

Volume = π × Radius² × Height

The radius is half the internal diameter.

Horizontal cylindrical tank

For a completely full tank, the cylinder formula applies. For a partially filled horizontal cylinder, the calculator uses the circular-segment area multiplied by the tank length.

Volume = Length × [r² cos⁻¹((r − h) / r) − (r − h)√(2rh − h²)]

Here, r is the internal radius and h is the vertical water depth, from zero to the tank diameter.

Unit conversions

1 cubic metre = 1,000 litres
1 US gallon ≈ 3.78541 litres
1 UK gallon ≈ 4.54609 litres

Worked Example

Rectangular water tank

A tank is 5 m long, 3 m wide and 2 m high.

Volume = 5 × 3 × 2 = 30 m³.

Capacity = 30 × 1,000 = 30,000 litres.

This is approximately 7,925 US gallons or 6,598 UK gallons.

Frequently Asked Questions

How do I calculate water tank capacity in litres?

Calculate the tank's volume in cubic metres and multiply by 1,000 to obtain litres. Use the formula that matches the tank shape.

How do I calculate the volume of a cylindrical tank?

For a vertical or completely full cylindrical tank, multiply pi by the square of the internal radius and the tank's height or length.

Can I calculate water in a partially filled tank?

Yes. Enter the current water depth. The calculator estimates the water volume and fill percentage. For a horizontal cylindrical tank, it accounts for the curved cross-section.

What is the difference between US and UK gallons?

A US gallon is approximately 3.78541 litres, while a UK imperial gallon is approximately 4.54609 litres. This calculator displays both values.

Should I use internal or external tank dimensions?

Use internal dimensions for the most accurate estimate of water capacity. External dimensions include the thickness of the tank walls and may overestimate usable volume.

Does this calculator account for freeboard?

No. The calculated capacity is the geometric volume based on the dimensions entered. For practical storage or engineering design, allow for freeboard, fittings, unusable volume and the manufacturer's stated capacity.