Septic tank size by state / West South Central
Oklahoma septic tank size
In Oklahoma the minimum septic tank for a three-bedroom house is 1,000 gallons, and four bedrooms need the same 1,000 gallons. The figure comes from OAC 252:641-7-4(a). That matches the 1,000-gallon median of the 47 states listed here.
What the Oklahoma rule says
The rule is OAC 252:641-7-4(a), administered by Oklahoma Department of Environmental Quality (DEQ). In its own words:
A septic tank used in an individual on-site sewage treatment system for a residential unit with four (4) or fewer bedrooms shall have a liquid capacity of at least one thousand (1,000) gallons. ... An additional two hundred and fifty (250) gallons of capacity must be added for each additional bedroom.
Source: OAC 252:641-7-4(a), read 2026-10-20.
- Above four bedrooms the minimum rises 250 gallons per bedroom.
- Oklahoma DEQ publishes size charts in Appendix H of the rule.
Oklahoma minimum tank size by bedrooms
| Bedrooms | Minimum tank | Versus the median state |
|---|---|---|
| 1 | 1,000 gal | same |
| 2 | 1,000 gal | — |
| 3 | 1,000 gal | same |
| 4 | 1,000 gal | −200 |
| 5 | 1,250 gal | −250 |
| 6 | 1,500 gal | — |
The first step up comes at 5 bedrooms, from 1,000 to 1,250 gallons, and a six-bedroom house needs 1,500 gallons. Beyond the table the rule adds 250 gallons for each extra bedroom. These are minimums: the permit can call for more, and a bigger tank needs pumping less often.
How Oklahoma compares with nearby states
Arkansas requires 1,000 gallons for three bedrooms; Louisiana requires 1,000 gallons for three bedrooms; Texas requires 1,000 gallons for three bedrooms. 37 of the 47 states listed here share Oklahoma's three-bedroom figure.
Before you size a tank in Oklahoma
- Bedrooms, not people, set the size in most rules. Rooms that could be used as bedrooms are usually counted, so check how your permitting office counts a den or a finished basement.
- Garbage disposals, grinder or ejector pumps and large tubs change the answer in many states; the points above list Oklahoma's adjustments.
- The tank is only half the system. The drainfield is sized separately from soil tests, and it is usually the bigger cost.
- A private well on the same lot has its own setbacks from the tank and field; the well pump and pressure tank calculators size the water side.
Figures are the state's minimum as written on 2026-10-20. Local health departments can require more; the permit issued for your property governs. See the methodology for how each state was read.