Method: Rectangular tank volume = length × width × height. Cylindrical tank volume = π × radius² × cylinder length. All dimensions are converted internally to metres before calculating volume. 1 m³ = 1,000 litres. 1 m³ ≈ 264.172 US gallons. 1 m³ ≈ 219.969 Imperial gallons. The result represents the tank’s geometric internal capacity. Actual usable capacity may be lower because of freeboard, internal fittings, rounded corners, inaccessible volume or operating level limits.