One calculator, two tank shapes
Rectangular tanks are length × width × depth × 7.48 gallons per cubic foot; cylindrical tanks are π × r² × depth × 7.48. The calculator handles both — use the average depth for tapered or partially buried tanks, and remember usable volume is less than brim volume on every real tank.
Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on a regional code, the calculator lets you override it rather than assuming one jurisdiction.
- NIST Office of Weights and Measures — the US customary and SI conversion factors used throughout this tool.
- International Code Council — the International Residential Code and International Building Code.
Figures, formulas and reference tables last verified: 19 September 2026.