Septic Tank / Leach Field Sizing Calculator
Estimate septic tank capacity and leach field area from bedroom count and soil percolation rate — the same approach county health departments use for a first pass.
How this calculator works
Design flow is estimated at 150 gallons per bedroom per day. Tank size is that daily flow multiplied by 2.1, rounded up to the nearest common tank size (1000, 1250, 1500, or 2000 gallons). Leach field area is design flow divided by your soil's absorption rate (in gallons per day per square foot) — slower-draining soil needs proportionally more field area to handle the same flow.
Where these numbers come from
- Design flow, tank sizing formula, and soil absorption rate table: Inyo County (California) Environmental Health septic system sizing charts, a standard example of the county-level formulas widely used across the U.S.
- Simplified bedroom-to-tank-size baseline cross-checked against Kern County Public Health Services septic guidance
Every jurisdiction has its own code, and many require a licensed installer and a permitted percolation test before any system is approved — treat this as a budgeting and planning tool, not a permit substitute.
Frequently asked questions
Why does bedroom count matter more than household size for tank sizing?
Septic sizing codes are typically based on a home's bedroom count as a standardized proxy for potential occupancy, rather than actual current household size, since bedroom count is a stable, easily verified number that doesn't change as often as who's actually living there.
Why does soil percolation rate matter for the leach field?
Soil that drains slowly needs a larger leach field area to safely absorb the same volume of wastewater compared to fast-draining soil, so the percolation rate directly determines how much field area is actually required.
Is this calculator a substitute for an official percolation test?
No — this gives the same kind of first-pass estimate county health departments use, but an actual on-site percolation test and health department approval are still required before a real system is designed and installed.
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