Septic Tank Sizing Calculator

Minimum Tank Size is whichever is larger: the bedroom-based figure or twice your entered Daily Wastewater Flow. For typical households, the bedroom-based figure wins and determines the tank size — but a household with an unusually high daily flow relative to its bedroom count (a home business with heavy water use, for example) can push the flow-based figure higher instead, so it's worth checking both if your actual usage is well above the 75-150 gallons per person/bedroom rule of thumb.

Drain Field Area is your Daily Wastewater Flow divided by a soil application rate that drops sharply as Soil Percolation Rate increases (slower absorption). Fast-draining sandy soil (1-5 min/inch) can accept over an order of magnitude more effluent per square foot than slow clay soil (60+ min/inch), so the same household's drain field can need to be many times larger on clay-heavy ground than on sandy ground.

Pumping Frequency divides about a third of the tank's total capacity by an assumed sludge accumulation rate of 60 gallons per person per year — more people generate solids faster, which shortens the interval between pump-outs for the same tank size. A larger household on an undersized tank will need pumping noticeably more often than the commonly cited 3-5 year rule of thumb assumes.

Most state and local codes size the tank and drain field by bedroom count as a proxy for potential occupancy, regardless of how many people actually live there — that's what Minimum Tank Size and Drain Field Area are based on. Number of Occupants is used separately, only in the Pumping Frequency estimate, since actual sludge accumulation depends on how many people are really using the system day to day.