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How to Size Septic Tanks

Septic tanks are used in areas where a home is not connected to a main sewer system. If you are having issues with your septic tank and need to replace it or find replacement parts, you need to know the size of the septic tank. Measuring the tank is relatively simple, but you should also check with your local government as laws regarding septic tanks may have changed since yours was installed.

Rectangular Tanks

Size Septic Tanks
  1. Get a rough estimate of the size of your tank, in gallons, by counting the number of bedrooms in the house. Typically, two-bedroom houses will have either a 750 gallon or 1000 gallon tank, three-bedroom houses will have a 1000 gallon tank, four-bedroom houses will have a 1200 gallon tank, and five-bedroom houses will have a 1500 gallon tank.

  2. Measure the septic tank's depth. Make all measurements in feet.

  3. Measure the septic tank's width and length.

  4. Calculate the size of the tank with a simple equation: length x width x depth x 7.5.

Circular Tanks

  1. Get a rough estimate of the size of your tank, in gallons, by counting the number of bedrooms in the house. Typically, two-bedroom houses will have either a 750 gallon or 1000 gallon tank, three-bedroom houses will have a 1000 gallon tank, four-bedroom houses will have a 1200 gallon tank, and five-bedroom houses will have a 1500 gallon tank.

  2. Measure the septic tank's depth. Make all measurements in feet.

  3. Measure the radius of the tank.

  4. Calculate cubic capacity with the equation 3.14 x radius squared x depth.

  5. Multiply the cubic capacity by 7.5 to get the gallon size.

About the Author

Antonin Korenek has been writing professionally since 2008. On eHow and Answerbag he specializes in everything geeky and DIY home repair and maintenance. In college he was awarded the Loughead-Eldredge Scholarship in Creative Writing two years in a row. Korenek graduated from Wayne State University with a bachelor's degree in English.