Septic Tank Size for a Target Pump-Out Interval
The septic tank working volume that puts a target number of years between pumpings.
Example
You enter
- Target years between pump-outs 5
- Household occupants 4
- Accumulation (gal/person/yr) 30
- Fill fraction before pumping 0.33
You get
- Required tank working volume 1818 gal
- Accumulation at interval 600 gal
Checked against published worked examples from: USEPA Onsite Wastewater Treatment Systems Manual / university extension...
Details, formula, and sources
Scopetank = years x people x accumulation / fill. A 4-person home at 30 gal/person/yr wanting 5 years needs ~1,820 gal. State minimum tank sizes by bedroom count usually govern.
Formula or tableTank working volume = target_years x occupants x accumulation_per_person_per_year / fill_fraction (the inverse of years = tank x fill / (people x accumulation)).
Source and editionUSEPA Onsite Wastewater Treatment Systems Manual (EPA/625/R-00/008); university onsite-wastewater extension pumping-frequency guidance, by name.
Free accessFree at epa.gov/septic. Extension pumping-frequency tables free on land-grant university extension sites.
What governsEstimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.
API field namestarget_years, people, accum_gal_pp_yr, fill_fraction, tank_gal, working_gal
- Default accumulation 30 gal per person per year of net sludge + scum (a garbage disposal roughly doubles it); editableextension pumping-frequency guidance
- Default fill fraction 0.33 of the tank before pumping; editableextension pumping-frequency guidance
- Code minimum governs state minimum tank sizes by bedroom count usually govern the actual tank, overriding this interval-based estimatestate onsite code