Tank and storage volume calculations that account for actual shape
Vertical, horizontal and rectangular storage, with working freeboard and the resulting usable volume.
The engineering question
A tank is ordered in m³ but its usable volume is less. Which shape are you buying, and what does the freeboard cost you?
Worked example
The figures below were produced by running the Water Tank Storage Calculator with the same inputs .
These figures are worked by the published method rather than executed at build time, because this tool is a self-contained page calculator without a programmatic engine.
- A vertical cylindrical tank is its plan area times the depth.
- A horizontal cylinder needs the segment area, not the bounding box.
- Usable volume is the gross volume less the freeboard actually provided.
Sanity check
A horizontal tank never holds a full rectangle. Treating it as one over-estimates capacity substantially.
Tools for this problem
These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.
- Water Tank Storage Calculator Size cylindrical and rectangular water storage tanks from dimensions or daily demand and residence time.
- Rainwater Harvesting Calculator Annual and monthly rainwater yield from a roof, with a cistern size derived from a daily demand and autonomy period.
- Greywater Reuse Calculator Reusable greywater available per day and the storage volume needed, from residents, laundry flow and the captured fraction.
References & standards
- API 650 — welded tanks for storage
- BS 2594 — tanks for liquids
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