Septic tanks must be watertight and built of a corrosion and decay resistant material such as concrete. The capacity of a septic system should match the size of your household. Table 1 shows the recommended septic tank liquid capacities for households of various sizes. The liquid capacity of a septic tank is the capacity below the tank outlet. Recommended septic tank liquid capacities.
Liquid capacity is the tank volume below the outlet.
An additional internal volume equal to 20 percent of the liquid capacity is needed for floating scum storage. If you know the liquid capacity of your septic tank in cubic feet, multiply that figure by 7.5 to find its capacity in gallons. Compare the liquid capacity of your tank with the guidelines given above. If your septic tank is smaller than recommended, you will probably need to pump your septic tank more frequently than otherwise recommended. If you do not know the capacity of your septic tank, a reasonable estimate would be to assume that the capacity is equal to the size recommended for the number of bedrooms that were in the house when the septic tank was constructed.