Measure an existing tank
Septic Tank Volume Calculator
Convert internal tank dimensions to an estimated liquid capacity in US gallons and liters.
Scope: The size and pumping tools use U.S. residential planning values and EPA guidance. They are not permit calculations.
Volume formulas
How tank volume is calculated
Rectangular tanks use internal length × internal width × liquid depth. Cylindrical tanks use π × (inside diameter ÷ 2)² × internal length.
The calculator converts measurements to feet, calculates cubic feet, then converts the result to US gallons and liters. Never enter a septic tank; use records or measurements taken safely by a qualified professional.
FAQ
Septic tank volume FAQ
Should I measure inside or outside?
Use internal liquid dimensions. Exterior dimensions include walls and freeboard and can overstate usable capacity.
Does the result prove the tank's rated size?
No. It is a geometric estimate; compartments, baffles and measurement error affect usable capacity.
Can this tell me what tank size I need?
No. Use the size calculator for early planning and follow your permitting authority's required capacity.