How to Figure How Big a Septic Tank Should Be

Getting the proper size septic tank for your home is crucial to the proper and efficient use of the system. Purchasing a tank that is too small for your application will result in a need for more frequent pumping/cleaning and will increase the risk of system failure. Purchasing a unit which is too large increases not only the tank cost but installation cost as well. Fortunately, calculating the proper size tank for your home is not complicated.

Concrete septic tank at construction site
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Step 1

Calculate the size tank based on the average daily water usage in the home. This is by far the most accurate method of sizing a septic tank, but also more difficult. Measuring the daily usage of water will require you to know how much water you will use in the house before the plumbing system is installed. Use your current water bill to determine the number of gallons used in the billing period. Divide that number by the number of days in the billing period to determine your daily usage. If there will be more people in the new home, you will need to increase this amount. If you use 500 gallons or less per day, you need a 900-gallon tank. Up to 700 gallons per day will require a 1,200-gallon tank and 900 gallons per day will take a 1,500-gallon tank.

Step 2

Count the number of bedrooms in the home to determine the size septic tank needed. This method is used with the thought that the number of residents will be limited by the number of bedrooms. If the home has two bedrooms you will need a 750-gallon tank. Three bedrooms will take a 1,000-gallon tank, while a four-bedroom home will take a 1,200-gallon tank. If the home has five bedrooms, the tank will need to be 1,500 gallons. Note: In some locations the minimum size tank which can be installed is 1,000 gallons. Check your local codes and restrictions before purchasing a septic tank.

Step 3

Calculating the size of a tank you already have is a just a matter of measuring and multiplication. If the tank is round, measure the radius. Multiple 3.14 times the radius squared, times the depth of the tank. This will give you the total cubic capacity of the tank. Take the cubic capacity and multiple it by 7.5 to determine the number of gallons the tank will hold. For a rectangular tank, multiply the length times the width times the depth times 7.5 to determine the number of gallons the tank will hold.