Septic tank size by number of bedrooms
For a 3-bedroom house the printed minimum is 1,000 gallons in Oklahoma and Missouri and 900 gallons in Tennessee.
Minimum septic tank size by state
Each state sets a minimum liquid capacity by the number of bedrooms. These are the minimums written in each rule.
Oklahoma and Tennessee figures for 5 and 6 bedrooms are calculated from each rule's add-250-gallons-per-bedroom provision. Missouri sizes homes with 6 or more bedrooms like an establishment, using V = 1.5Q + 500, where Q is the design daily flow.
Arkansas: the 2024 rule sets 1,000 gallons as the minimum for smaller homes. Its Appendix C table then lists 1,250 gallons on a row printed as 3 bedrooms, and adds 250 gallons for each additional bedroom. Because that row can be read two ways, ask your Arkansas Department of Health county office which size applies to your house.
| State | 1 BR | 2 BR | 3 BR | 4 BR | 5 BR | 6 BR | Each extra bedroom | Rule section |
|---|---|---|---|---|---|---|---|---|
| Arkansas | 1,000 | 1,000 | See note | See note | See note | See note | +250 gal | Rules Pertaining to Onsite Wastewater Systems (eff. Sep 5, 2024), Section 10.1 and Appendix C |
| Oklahoma | 1,000 | 1,000 | 1,000 | 1,000 | 1,250 (calc.) | 1,500 (calc.) | +250 gal above 4 | OAC 252:641-7-4(a) |
| Missouri | 1,000 | 1,000 | 1,000 | 1,250 | 1,500 | Sized as an establishment: V = 1.5Q + 500 | Not stated; 6+ uses formula | 19 CSR 20-3.060(4)(B)16, Table 4, and (4)(B)17 |
| Tennessee | 750 | 750 | 900 | 1,000 | 1,250 (calc.) | 1,500 (calc.) | +250 gal above 4 | Rule 0400-48-01-.08(1) |
Sources: Arkansas State Board of Health, Rules Pertaining to Onsite Wastewater Systems (effective Sep 5, 2024); Oklahoma DEQ, OAC 252:641 Individual and Small Public On-site Sewage Treatment Systems (amendments effective 9-15-24); Missouri 19 CSR 20-3.060 Minimum Construction Standards for On-Site Sewage Disposal Systems (Code of State Regulations, 9/30/20); Tenn. Comp. R. & Regs. 0400-48-01-.08 Septic Tank Capacity (Cornell LII copy)
What else the rules say about tank size
Missouri: the Table 4 sizes already allow for garbage grinders, washers and other appliances, though garbage grinders are not recommended. No tank may hold less than two days (48 hours) of flow. A tank serving two or more homes must hold at least 1,500 gallons.
Arkansas: the Appendix C sizes allow for standard household appliances, and garbage grinders are not recommended for standard systems.
Oklahoma: in a two-compartment tank, the first compartment must hold one half to two thirds of the required capacity. An aerobic system's trash tank must hold at least 300 gallons or the average daily flow, whichever is greater.
Sources: Missouri 19 CSR 20-3.060 Minimum Construction Standards for On-Site Sewage Disposal Systems (Code of State Regulations, 9/30/20); Arkansas State Board of Health, Rules Pertaining to Onsite Wastewater Systems (effective Sep 5, 2024); Oklahoma DEQ, OAC 252:641 Individual and Small Public On-site Sewage Treatment Systems (amendments effective 9-15-24)
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