Inputs and units
Use internal dimensions in inches. For a cylinder, enter diameter in the first field and cylinder length or height in the third. Width is used only for rectangular tanks. Measure fill depth vertically from the inside bottom. Select the contents to estimate liquid weight; the tank itself is not included.
How the calculation works
Rectangular volume = length × width × liquid depth
Vertical cylinder volume = π × radius² × liquid depth
Horizontal cylinder volume = length × [r² arccos((r − h)/r) − (r − h)√(2rh − h²)]
US gallons = cubic inches / 231The horizontal-cylinder calculation uses a circular segment, so liquid depth and percent full are not generally proportional. Empty, half-depth and full are the familiar reference points. The tool clamps depth to the tank’s physical range. It uses 3.785 litres per US gallon for the displayed conversion and preset liquid densities for approximate weight.
Worked example
A rectangular tank with internal dimensions 36 × 24 × 60 inches holds 51,840 cubic inches, or about 224.4 US gallons. At a measured depth of 30 inches it holds half that amount: 112.2 gallons. A horizontal cylinder needs the segment calculation instead; a quarter of its diameter in liquid depth holds about 19.6% of capacity, not 25%.
Assumptions and limits
Dished ends, rounded corners, internal obstructions, wall thickness and tank tilt are not modelled. Use a manufacturer’s calibration chart when accuracy matters. Liquid density changes with composition and temperature. Contents weight is not a structural assessment: check the actual support footprint, tank weight and load path rather than treating the calculator’s general support notes as a floor rating.
References and calculation details
Page prepared September 18, 2026. Examples describe this calculator’s implementation; reference links do not imply independent certification.