No black boxes
Methodology & sources
Every SepticCompass result shows its work. Here is exactly how each calculator computes its answer, the public standards it cites, and where our cost ranges come from.
Tank sizing
Design flow is 150 gallons per bedroom per day (a widely-used onsite-code value derived from the EPA figure of 60 gal/person/day), held for a 48-hour retention. The result is floored at the 1,000-gallon code minimum and rounded up to a standard manufactured tank (1,000 / 1,250 / 1,500 / 2,000 / 2,500+). A garbage disposal adds ~250 gal; high-efficiency fixtures apply ×0.85.
Perc rate & drain field
A percolation rate T (minutes per inch) converts to a soil loading rate with 1.2 ÷ √T gal/ft²/day. Required absorption area = daily design flow ÷ loading rate; trench length = area ÷ trench width. Soil slower than ~60 min/in generally needs an engineered system.
System cost
A national installed base cost per system type is scaled by home size, soil/site difficulty, a regional cost index, and tank material, plus soft costs (perc test, design, permits). Base ranges are cross-checked annually against published 2026 cost data and stored as editable, dated coefficients.
Pump-out frequency
Years between pumping come from the published Penn State / Ohio State Extension tables (after Mancl), interpolated across tank size and household size; a garbage disposal shortens the interval ~25%. The real trigger is sludge depth at an inspection.
State & province differences
Regional cost indices reflect local construction-cost differentials. Tank minimums and perc/permit rules are set by each jurisdiction — we show a baseline and always direct you to your state or county health department, which is authoritative.
Sources
- US EPA — Onsite Wastewater Treatment Systems Manual (EPA/625/R-00/008): 60 gal/person/day design flow
- Soil loading rate from percolation rate (1.2 ÷ √T) — EPA / Wisconsin onsite design model
- Tank-capacity minimums — International Private Sewage Disposal Code (IPSDC) baseline; adopted/amended by each state
- Penn State Extension — septic tank pumping frequency
- Ohio State University Extension (Mancl) — pumping-frequency table
- Installed cost ranges — The Septic Guide (2026)
- Cost data — HomeGuide
- Cost data — Angi
- Your state or county health department — the authoritative source for local code, minimums, setbacks, and permits
All results are estimates for general information only — not engineering, professional, or legal advice — and may not reflect your local code or current prices. Always consult a licensed septic professional and your local health authority before designing, installing, or purchasing a system. See our full disclaimer.