How septic tank size is determined
State onsite-wastewater rules set a minimum liquid capacity by bedroom count, because bedrooms are a stable proxy for how many people a house can hold. This calculator applies the published tables:
| Bedrooms | New York | Maryland | Indiana | Missouri |
|---|---|---|---|---|
| 1–2 | 1,000 | 1,000 | 750 | 1,000 |
| 3 | 1,000 | 1,000 | 1,000 | 1,000 |
| 4 | 1,250 | 1,250 | 1,250 | 1,250 |
| 5 | 1,500 | 1,500 | 1,500 | 1,500 |
| Each beyond | 6 BR = 1,750, then +250 per BR | +250 per BR | +300 per BR over 5 | V = 1.5 × Q + 500 (Q = 120 gpd × BR) |
Gallons of minimum liquid capacity. Sources: 10 NYCRR Appendix 75-A.6, COMAR 26.04.02.05, 410 IAC 6-8.3-60 Table III, 19 CSR 20-3.060 and MU Extension EQ401.
Adjustments the rules spell out
- New York: a garbage grinder adds 250 gallons.
- Indiana: a jetted bathtub over 125 gallons is a "bedroom equivalent" and is counted as an extra bedroom. In a two-compartment tank, the first compartment holds 1/2 to 2/3 of the volume.
- Missouri: MU Extension advises adding one bedroom of capacity for a hot tub or whirlpool, and sizing the tank to hold at least 48 hours of flow.
Worked example
A 4-bedroom house in New York with a garbage disposal: the table gives 1,250 gal for 4 bedrooms, and the grinder adds 250 gal, so the minimum is 1,500 gallons. The same house in Indiana without a disposal needs 1,250 gal. In Missouri, a 6-bedroom house is over the table, so Q = 120 × 6 = 720 gpd and V = 1.5 × 720 + 500 = 1,580 gallons.
Assumptions and limits
- Single-family dwelling with a conventional septic tank. Commercial, multi-family and aerobic treatment units follow different rules.
- The result is a minimum. Counties may adopt stricter rules, and tanks are sold in standard sizes, so round up to the next size available.
- States not listed have their own tables. Use the closest one only as a rough reference and check your state rule.
- Drain-field (absorption area) sizing depends on soil tests and is not calculated here.