Sewage System Sizing

Research has shown that additional plumbing fixtures, such as separate showers with whirlpool tubs instead of tub/shower combinations, double lavatories, bidets, etc., increase the amount of wastewater generated by a home. Any two additional fixtures may increase septic tank and/or drainfield sizes. If you may be installing several fixtures, an early consultation with our Sanitarians can identify the proper sizing of the septic tanks and drainfield.

Things to Keep in Mind

  • The listed sizes are for primary drainfield areas. The total drainfield area required is the sum of the primary and reserve areas. The reserve area is 1.5 times that of the primary drainfield.
  • Effluent filters are highly recommended and may be required on certain sites and on alternative on-site sewage systems. Effluent filters remove small solid particles from wastewater leaving the septic tank, preventing the particles from entering the drainfield. Refer to the manufacturer's recommendations for use and maintenance.

A study, den, or similar room which may be converted into a bedroom must be included in the total number of bedrooms.

Sewage System Sizing

Number of
Total Septic
Tank Capacity
Drainfield Size
(Coarse Sand / Gravel)
Drainfield Size
(Clean Medium Sand)
Drainfield Size
(Fine / Loamy Sand)
Drainfield Size
(Fill-Type Drainfield)
2 or less 1500 gallons 800 square feet 1000 square feet 1500 square feet 2500 square feet
3 2000 gallons 1000 square feet 1200 square feet 1800 square feet 2880 square feet
4 2500 gallons 1200 square feet 1600 square feet 2000 square feet
3200 square feet
5 3000 gallons 1600 square feet 1800 square feet
2200 square feet
3500 square feet
6 3500 gallons 1800 square feet 2000 square feet 2500 square feet 3500 square feet