How Do I Determine Tank Size for a Septic System?

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Proper calculations before installation can save you time and money later.
Proper calculations before installation can save you time and money later. (Image: legal pad and mechanical pencil image by alpy7 from Fotolia.com)

When building a new home or installing a new septic tank, it is important to install a tank of the proper size. An error in this area will be difficult, as well as expensive, to correct. Fortunately, calculating the proper tank size can be done with little effort and a high degree of accuracy. The number of bedrooms in the house, regardless of how many will initially be occupied, is the most important factor in determining septic tank size.

Check local building codes to determine whether your area requires a minimum size for the tank to be installed. In many areas, a 1,000-gallon tank is the smallest allowed for a single residence. This is a good size for the standard home and will properly handle the home of the average family. You should also consider that the larger the tank capacity, the less frequently it will need to be emptied.

Count the number of bedrooms in the home. Assume that at some point all bedrooms of the home will be occupied, and the septic tank must be capable of handling the workload associated with this number of residents. For a one- or two-bedroom home, the minimum tank size required is 750 gallons. A three-bedroom home takes a 1,000-gallon tank; a four-bedroom house requires a 1,200-gallon tank. A five- or six-bedroom home needs a minimum of 1,500 gallons.

Calculate the size of septic tank needed based on the water usage of the residence. For this calculation, you need to know how many people will be living in the house. An average person uses 100 gallons of water per day in a home. Multiply this number by the number of people living in the home to determine the average daily water usage in the house.

Add a small fudge factor into the total water usage, especially if the home will be occupied by teenagers or if the residents are retired, are stay-at-home parents or are otherwise in their home more than the average person. Use this total to determine the tank size required. If daily water usage in the house is 500 gallons or less, the minimum tank size is 900 gallons. Daily usage of 501 to 600 gallons of water requires a 1,000-gallon tank, usage of 601 to 700 gallons needs a 1,200-gallon tank and using 701 to 900 gallons calls for a 1,500-gallon tank.

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