Septic Tank Sizing Calculator

What this calculation does not cover

• This does not replace a percolation test or soil evaluation, and tank capacity is only one line on a permit. Drain field area, depth to the seasonal high water table or bedrock, ground slope, and setbacks from wells, property lines and surface water are all outside this number, and any of them can rule out a conventional system on a site where the tank size is fine.
• Bedroom count is the only thing read. Garbage disposals, hot tubs, whirlpool baths, water softener backwash, a home business, and occupancy well above what the bedroom count implies all add flow or solids, and many jurisdictions require a larger tank for them — none of that reaches the figure shown.
• The bands are a North American health-code convention, and the metric answer is that gallon figure converted rather than a capacity drawn from a local code. Jurisdictions outside North America commonly size tanks from a population-based formula and to their own product standards, so do not hand the litre figure to a supplier as a code minimum.
• Nothing interpolates inside a band and nothing exists outside them. A one-bedroom cabin returns the same minimum as a three-bedroom house, five- and six-bedroom homes return an identical figure, and there is no answer at all above eight bedrooms.
• This is a liquid capacity minimum, not a tank specification. Compartment count, effluent filter, inlet and outlet tee and baffle detail, burial depth and traffic or buoyancy rating, access risers, and the concrete or plastic product standard are separate decisions — as is everything downstream of the tank, including drain field area, distribution box and any pump chamber.