Septic systems · tank volume
Septic tank capacity calculator (gallons).
Measure the inside of the tank and the depth of the liquid to get gallons and liters for a rectangular tank, an upright round tank or a horizontal cylinder tank. Runs in your browser; nothing is sent anywhere.
Planning aid: codes and soils vary by location. Confirm septic and well work with your local health department and a licensed professional.
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Worked example
A rectangular tank 8 ft long, 5 ft wide with 4.5 ft of liquid depth: 8 × 5 × 4.5 = 180 ft³. Multiply by 7.4805 gal per ft³ to get 1,346 US gallons. A horizontal cylinder 4 ft in diameter and 10 ft long, filled to 2 ft (half), holds half of π × 2² × 10 = 62.8 ft³, which is 470 gal.
Formulas, sources, and limits
Formulas: Rectangular: length × width × liquid depth. Upright cylinder: π × (diameter/2)² × liquid depth. Horizontal cylinder: length × [r² × arccos((r − h)/r) − (r − h) × √(2rh − h²)], where r is the radius and h the liquid depth. Cubic feet to gallons uses 1 US gal = 3.785412 × 10³ m³ and 1 ft = 0.3048 m exactly, which gives 7.4805 gal per ft³.
Sources (retrieved 2026-10): NIST SP 811 Appendix B.8, conversion factors (gallon, inch, cubic inch).
Limits: Uses interior dimensions with square corners. It ignores baffle walls, curved ends, and manhole volume. Rated capacity on the tank label or permit wins over your own measurement.
Frequently asked questions
How do I find out what size my septic tank is?
Look for the permit or as-built drawing at your county health department, or the label on the tank. If neither exists, measure the inside dimensions and liquid depth and use this calculator.
How many gallons is a rectangular septic tank?
Multiply length, width and liquid depth in feet, then multiply by 7.4805.
What liquid depth should I use?
The depth of the liquid at the outlet level, not the full interior height, since the space above the liquid is air.
Is this the tank’s legal capacity?
No. Regulators set capacity by the liquid volume below the outlet as built; use their number if it is on file.