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What size septic tank do I need?

Your state or county code sets the minimum, either by number of bedrooms or by the home's design flow, which itself is based on bedrooms. In Minnesota, a home with 3 or fewer bedrooms needs at least 1,000 gallons and a 4- or 5-bedroom home needs 1,500 (Minn. R. 7080.1930, current rule). Pennsylvania's minimum for any tank is 900 gallons (25 Pa. Code § 73.31, current rule). In some states a garbage disposal raises the requirement.

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Two state codes, side by side

Minnesota figures are read directly from the rule's table (Minn. R. 7080.1930, current rule). Pennsylvania's § 73.31 has no bedroom table. It sets design flow at 400 gallons a day for a single-family home, plus 100 gallons a day for each bedroom over three, and converts flow to tank size with a formula (25 Pa. Code § 73.31, current rule). The Pennsylvania column is our arithmetic from that formula. The two columns aren't on quite the same basis: Pennsylvania's flow figure already allows for a garbage grinder (25 Pa. Code § 73.31, current rule), while Minnesota adds 50% if there is one (Minn. R. 7080.1930, current rule). Confirm with your local permitting office.

New York sizes tanks by bedrooms too. Its code counts an expansion attic as an extra bedroom and adds 250 gallons for each bedroom over six (10 NYCRR 75-A.6, effective February 3, 2010).

The garbage disposal rule

Codes handle a garbage disposal differently:

• Minnesota: the tank must be at least 50% larger, with multiple compartments or tanks (Minn. R. 7080.1930, current rule).
• New York: the rule says a garbage grinder "shall be considered equivalent to an additional bedroom" for tank size, and separately that an additional 250 gallons is required when one can reasonably be expected (10 NYCRR 75-A.6, effective February 3, 2010).
• Pennsylvania: the design flow already includes garbage grinders, so there's no separate increase (25 Pa. Code § 73.31, current rule).

Even where the code doesn't require it, EPA notes a disposal means more frequent pumping (EPA, August 17, 2026).

What a bigger tank costs

HomeAdvisor's tank prices, before installation: $900 to $1,500 for a 1,000-gallon tank and $1,500 to $2,500 for 1,500 gallons (HomeAdvisor, June 20, 2026). Its guide pairs 1,000 gallons with 3–4 bedrooms and 1,500 gallons with 6–7. For a 4-bedroom home, and for 6 or 7 bedrooms, that's below the Minnesota minimums above, so check your code before pricing. HomeAdvisor also says labor is 50% to 70% of a system's installation cost (HomeAdvisor, June 20, 2026).

How our calculator handles size

The calculator assumes a typical tank for each bedroom choice: 750 gallons for 1–2 bedrooms, 1,000 for 3, 1,250 for 4, and 1,500 for 5 or more. It scales the whole installed cost by bedroom count. As the table above shows, some states require more than that (Minnesota requires 1,500 gallons at 4 bedrooms), so treat the calculator as a budget, and your local code as the requirement. Find your issuing office on septic permits by state.

Related questions

Sources

1. Minnesota Rules 7080.1930 (septic tank capacity), current rule.
2. 25 Pa. Code § 73.31 (standards for septic tanks), current rule.
3. New York 10 NYCRR Appendix 75-A.6 (septic tanks), effective February 3, 2010.
4. U.S. EPA, How to Care for Your Septic System, last updated August 17, 2026.
5. HomeAdvisor, Septic System Installation Cost, updated June 20, 2026.