Septic and wastewater
Septic Tank Size Calculator
Most states size residential septic tanks by the number of bedrooms rather than actual occupants. This calculator applies a widely used rule of thumb, but your state or county health department code always governs.
Formula
Rule of thumb used by many state codes: 1,000 gal minimum for up to 3 bedrooms, plus 250 gal for each additional bedroom. Design flow is commonly 120 to 150 gal/day per bedroom. Retention = tank volume ÷ design flow, and many codes aim for at least 1.5 to 2 days at design flow.
Worked example
A 4 bedroom house: 1,000 + 250 × (4 - 3) = 1,250 gal. Design flow = 4 × 150 = 600 gal/day, giving about 2.1 days of capacity.
Assumptions and limits
- Rule of thumb only; tank minimums, design flows and the use of two-compartment tanks vary by state.
- Garbage disposals, large tubs and home businesses may require a larger tank.
- The drainfield, not the tank, is usually the costlier part to size and depends on a soil or percolation evaluation.
Frequently asked questions
How often should a septic tank be pumped?
EPA's SepticSmart program says household systems should generally be inspected at least every three years and tanks typically pumped every three to five years.
Why size by bedrooms instead of people?
Bedrooms predict maximum potential occupancy over the life of the house, which the system must handle even if current occupants are fewer.
Who designs a septic system?
Depending on the state, a licensed professional engineer, a licensed onsite system designer or a registered sanitarian. A soil evaluation or percolation test is usually required first.