How to Determine the Size of a Septic Tank

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Have your septic system designed by a professional engineer. Plan to locate the tank at least 100 feet from wells or other drinking water sources.


Avoid disposing of grease or hazardous chemicals in a septic tank.

The size of your septic tank will be determined by state and local regulations.

A septic tank is the first stage of the wastewater treatment in a septic system. While in the tank, solids begin to be digested by anaerobic bacteria, and the wastewater is separated into scum, effluent and sludge. In order to be effective, the septic tank needs to be large enough to hold at least one day's worth of wastewater before passing it on to the leach field.

The precise size of the septic tank you'll need will depend on state and local regulations. Local authorities typically use the number of bedrooms to determine the amount of wastewater a residence will generate. If your house has 2 bedrooms, you might be required to have only a 750-gallon tank, whereas if your house has 6 bedrooms, you may need a 1500-gallon tank.

Step 1

Contact your local health department to determine the state and local regulations that govern the size of septic tanks in your city or town.

Step 2

Gather the documents you will need to determine the size of your tank. For a new home, you will need the architectural drawings showing the number of bedrooms that are planned for the house. For a septic system repair for an existing property, the town may want to review the latest assessors report, showing the number of bedrooms that actually exist.

Step 3

Adjust the number of bedrooms if needed. Some towns consider any rooms other than a living room, family room, dining room or kitchen as a bedroom, even if they are labelled for another use. So, if you have a two bedroom house that has a large game room in addition to the "basics," the town may require you to size your septic system for four bedrooms.

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Susan MacDowell

Susan MacDowell is a freelance writer from New England. She is a CPA by training, but has many additional interests, including history, baseball, cooking, and travel. She's a native of New York, who now lives in Massachusetts and Maine.