Updated October 2026. Calculations use the Washington and Minnesota septic tank rules as examples; your local health department sets the size for your home.
Septic tanks are sized by the number of bedrooms. Most homes with up to 3 bedrooms need at least a 1,000 gallon tank, and larger homes need 1,250 to 2,000 gallons or more, depending on the state. Pick your bedrooms below to see the minimum under two real state rules. A garbage disposal or a pump to the tank can raise the minimum by half.
Septic tank size calculator
Septic tank size by number of bedrooms
Here are the two rules the calculator uses, side by side. They show how much states differ.
| Bedrooms | Washington minimum | Minnesota minimum | Minnesota with garbage disposal or sewage pump |
|---|---|---|---|
| 1 to 3 | 1,000 gallons | 1,000 gallons | 1,500 gallons |
| 4 | 1,000 gallons | 1,500 gallons | 2,250 gallons |
| 5 | 1,250 gallons | 1,500 gallons | 2,250 gallons |
| 6 | 1,500 gallons | 2,000 gallons | 3,000 gallons |
| 7 | 1,750 gallons | 2,000 gallons | 3,000 gallons |
| 8 | 2,000 gallons | 2,500 gallons | 3,750 gallons |
| 9 | 2,250 gallons | 2,500 gallons | 3,750 gallons |
Washington requires 1,000 gallons for up to 4 bedrooms and 250 gallons more for each additional bedroom (WAC 246-272A-0232). Minnesota requires 1,000 gallons for 3 bedrooms or fewer, 1,500 for 4 or 5, 2,000 for 6 or 7 and 2,500 for 8 or 9, and a tank at least 50 percent larger with a garbage disposal or sewage pumping (Minnesota Rules 7080.1930).
Why tank size is based on bedrooms
Codes use bedrooms because they predict how many people will live in the house over its life, and people determine how much wastewater goes into the tank. Square footage and bathrooms do not matter in most codes. For more detail, see how big of a septic tank you need.
When to choose a bigger tank than the minimum
- More people than bedrooms, or frequent guests
- A garbage disposal, even where the local code does not require extra capacity
- Large tubs, frequent laundry or a home business
- You want more time between pumpings. The EPA says the pumping interval depends on tank size and household size, with most tanks pumped every 3 to 5 years.
Sizing a new or replacement septic tank?
Tank size, permit and soil all have to line up. Talk to a local septic company about the right size and a quote.
Calls are answered by a pay-per-call partner network that connects you with a local septic company. Calling is free; we may be paid for referrals.
Septic tank dimensions by size
Once you know the gallons, see the typical measurements of a 1,000 gallon septic tank and a 1,500 gallon septic tank, or compare concrete and plastic tanks.
Quick answers
What size septic tank do I need for a 3 bedroom house?
Usually 1,000 gallons, which is the minimum in both Washington and Minnesota. Minnesota requires 1,500 gallons if you have a garbage disposal.
What size septic tank do I need for a 4 bedroom house?
1,000 gallons in Washington and 1,500 gallons in Minnesota. A 1,250 or 1,500 gallon tank gives a 4 bedroom house more room between pumpings.
How do I find out what size tank I already have?
Check the septic permit or as-built drawing, an old pumping receipt, or ask the pumper. See how to find your septic tank records.
Sources
- Washington State, WAC 246-272A-0232, septic tank minimum size
- Minnesota Rules, 7080.1930, septic tank capacity
- U.S. EPA, How to care for your septic system