Septic Tank Size Calculator — What Size Tank Do You Need?

This calculator uses the standard onsite-wastewater sizing basis taught by extension services (University of Nebraska–Lincoln, Penn State): estimated daily flow of 150 gallons per day per bedroom, which comes from assuming two occupants per bedroom at 75 GPD each. That flow drives the standard tank table — 1–2 bedrooms = 750 gallons, 3 bedrooms = 1,000, 4 bedrooms = 1,200, and 5–6 bedrooms = 1,500 — because the tank must hold at least a day's flow long enough for solids to settle before effluent moves to the drainfield.

We then apply the two standard bump-ups: if your household exceeds the two-people-per-bedroom assumption, you go up one size (the table's math no longer covers your real flow), and if you run a garbage disposal you go up one size (ground food solids accumulate faster than tank bacteria digest them — many county codes make this bump mandatory). Water-saving fixtures lower real-world flow and stretch pumping intervals, but we never size down for them: codes size by bedrooms, and an undersized tank fails inspection.

Two honest caveats. First, many states mandate a 1,000-gallon minimum regardless of bedroom count, so if the table says 750, your county may still require 1,000 — we flag this on the result. Second, the tank is only half the sizing problem: the drainfield is sized separately from soil percolation rates, and your county's permit process has the final word on both. Treat this as the design floor and have a licensed local installer confirm against your county code.

Estimates only — independent local providers quote their own pricing. Data last reviewed 2026-07.