Where these numbers come from

State rules

Each of the 50 state pages was built by reading that state's own administrative code or its agency's published design standard. There are99 primary sources cited across the site, each listed on the relevant state page with the date it was checked.

What we take from each source:

• Design flow per bedroom, and any minimum bedroom rating
• Minimum septic tank liquid capacity by bedroom count
• Detention or retention requirement expressed as a multiple of daily flow
• Garbage disposal adjustment, where the code specifies one
• Setback distances to wells, property lines, surface water, and foundations
• Permitting authority, fee range, and licensing requirements
• Reserve area requirement

What we could not verify

Several states publish their sizing tables only as scanned images inside PDFs, which cannot be read reliably. Rather than substitute a plausible-looking number, those fields are flagged directly on the page with a note telling you to confirm locally.

8 states currently carry such a flag: Alabama, Connecticut, Delaware, Illinois, Montana, Nebraska, New Mexico, Texas.

This is deliberate. A site that presents every number with equal confidence is telling you less than one that admits where its confidence runs out.

The engineering model

Design flow

Bedrooms multiplied by the state rate, with the state's minimum bedroom rating applied first. Occupancy is deliberately excluded, because codes size for every future owner rather than the current one.

Tank capacity

The larger of the state's bedroom table and its detention requirement, then rounded up to a size manufacturers actually stock. The calculator shows both inputs so you can see which one is governing.

Soil loading rates

Long-term acceptance rates for septic tank effluent applied to trench bottom area, following EPA 625/R-00/008 Onsite Wastewater Treatment Systems Manual, Table 4-7. Most state codes track these within about 0.1 gpd/ft². We use the conservative column, because the failure mode of an optimistic loading rate is a drainfield that floods in year three.

Drainfield area

Design flow divided by loading rate, then adjusted for the dispersal product: −30% for gravelless chambers, −40% for drip, +30% for beds, and a further −25% where advanced pre-treatment is present. These reductions reflect what most states grant; your own state may differ, and the page tells you to check.

Trench runs are capped at 100 feet, which nearly every code imposes so effluent distributes evenly. Footprint assumes 6-foot centre-to-centre trench spacing.

Pumping interval

Calculated as Y = (T − 49.8 × N) ÷ (76.6 × N), where T is tank capacity in gallons and N is the number of occupants. This is the solids-accumulation model behind the published extension-service pumping-frequency tables, and it reproduces them to within about a tenth of a year across the full range of tank and household sizes. A garbage disposal divides the interval by 1.5.

Cost model

Built from unit costs — dollars per gallon of tank, dollars per square foot of drainfield by system type — multiplied by a state construction cost index relative to the national average. Permit fees come from the researched state figures. Every line item and its assumption is shown in the result, not hidden behind a total.

Costs are planning estimates. They do not include landscaping restoration beyond basic grading, well drilling, tree removal, rock excavation beyond the difficult-site allowance, or long driveway repairs.

Testing

The calculation library is covered by unit tests that check the model against published reference tables — in particular, that the pumping model reproduces the extension-service table, that drainfield area always increases as soil tightens, that cost totals equal the sum of their line items, and that no dispersal adjustment ever points the wrong way.

What we do not do

• We do not sell equipment or installation.
• We do not take contractor referral fees, and we do not operate a lead marketplace.
• We do not gate results behind an email address or a phone number.
• We do not present a national average as if it were your answer.

Corrections

If a figure here does not match what your health department told you, the health department is right and we want to know.Tell us what you found — include the state and the rule section if you have it, and we will check the source and update the page.