Sizing Septic Tanks

- Thursday, October 07, 2021
Morse Engineering and Construction - Septic System

Septic tanks are typically made of fiberglass, plastic or reinforced concrete. Metal tanks also are available, but these can corrode rapidly and aren’t typically recommended. Septic tanks may be constructed in rectangular, round or oval forms, but the shape of a septic tank doesn’t typically affect its function. The most important factor in choosing a septic tank is making sure you have the right size to accommodate your home.

A Range of Tank Sizes

Septic tank sizes are measured in gallons, based on the amount of sewage the tank can hold. Septic tanks must hold at least two days' worth of sewage from your home so that the solids properly separate from the liquids before the liquids move into the absorption field. The tank size is based on the number of bedrooms in your home. Standard tank sizes are typically 1,000, 1,250 and 1,500 gallons, and these suit most homes. Typically, the minimum tank liquid capacity of a one- to three-bedroom home is 1,000 gallons. A four-bedroom home requires a minimum 1,250-gallon tank, and a home with five bedrooms requires a minimum tank liquid capacity of 1,500 gallons. If you have a hot tub or whirlpool, add one-bedroom worth of capacity per fixture.

Estimating Your Needed Tank Size

Tank size is calculated using a simple calculation, although other factors, such as soil type, must be considered as well. For this reason it’s best to have an expert assess the site and your home to determine the ideal size of tank for your needs. However, to get an idea of the minimum size required for your home, multiply 150 gallons per bedroom, and then multiply the total by two to allow for the 48-hour retention time needed. For example, a one-bedroom home would need a minimum tank liquid capacity of 300 gallons. If your home has more than six bedrooms, or you have five bedrooms and a hot tub, you would multiply 150 by 6, and the total, which is 900, is multiplied by 2 to give you a minimum liquid capacity of 1,800 gallons.

For more information on septic tank sizing and construction, contact Morse Engineering and Construction.