Septic tank size calculator.

Answer two questions and get the minimum tank your state’s own rule requires, with the rule cited next to the number and the arithmetic shown. Eight state rules, read from the rule text. Where a rule is ambiguous, this tool quotes it and stops.

What the tool asks, and why

Two questions are always asked and two more are asked only where a rule reads them.

Which state the property is in comes first, because sizing rules are set state by state and they disagree. There is no national answer to size a tank against.

Bedrooms come second. Every rule here sizes on bedrooms, not on how many people live in the house, because bedrooms do not change when the family does.

Floor area is read by one rule in the set, where every row of the design-flow table carries a floor-area ceiling and square footage can move a house up a size on its own. A fixture question is read by two: one names water-saving devices without defining them, the other names a garbage disposal or a sewage ejector and requires half again the capacity when either is present. On the states whose rules read neither, both inputs are switched off in place and say why.

What this number is, and is not

It is a minimum liquid capacity, which is not the number on the side of the tank. Every rule here sets a liquid capacity: the working volume below the liquid line. The space above it is freeboard and does not count toward the figure. It is also not a permit and not a quote. The office that reviews your plan has the last word, and it can require more.

Where the numbers come from

Every figure was read from the rule that sets it, not from a chart, a supplier catalog or another website. The rule and its own publication date sit next to the answer, and the rule text is one click away. Where the answer is our arithmetic on a formula the rule prints, the result says so and shows each step. The full comparison and the reasoning behind it live in the septic tank size chart.

The drain field is not here yet

A drain field is sized on soil, not on bedrooms: the design flow divided by the hydraulic loading rate for the soil the field sits in. Several of these states publish those loading rates as scanned tables we have not read, and a loading rate we have not read is one we will not print. So this tool sizes the tank and stops. Ask your permitting authority for the loading rate assigned to your soil class; it is the one input that decides the field.

What each state actually sizes on

Six of these eight rules publish a bedrooms-to-gallons table you can read straight off. Two publish a daily flow first, then a formula that turns that flow into gallons. One of the six switches to a formula of its own above five bedrooms. That structural split is why a chart copied from one state is wrong in the next, and it is the reason this tool asks which state before it asks anything else. Every rule below was read in the text the state publishes.

Each row carries the agency that adopted the rule and the rule’s own publication date on the page, next to the citation. Those are two different dates and this page keeps them apart.

Two things the tool will not do

Fill a cell the rule leaves open

One provision in this set is genuinely ambiguous about houses of six bedrooms and more. Two readings of the same sentence give two different tanks, and the rule does not say which is meant. The calculator prints the sentence and stops. Guessing would look like an answer and be worth less than none.

Turn a minimum into a recommendation

A legal minimum is a floor, not a recommendation. Designers routinely go up a size for a house with a garbage disposal, a long pump run, or a lot with no room to rebuild a field later. If your installer proposes more than the minimum, ask why, and expect an answer about your site rather than about the rule.