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Septic Tank Size Calculator

Enter your household size and water use to see the septic tank capacity you need.

How it is calculated

Enter the number of people in the household (a whole number, at least 1), the wastewater per person per day in US gallons, and the retention time in days. The tool computes daily flow = people × gallons per person and capacity = daily flow × retention days. If that is below 1,000 US gallons, the tool raises it to 1,000, a commonly used minimum, so small households all show the same result. Litres are shown as gallons × 3.7854. A fractional head count, zero people, or a zero usage or retention value gives no result. The calculation sizes the working volume only: local rules differ on minimum size, whether bedrooms or occupants are counted, and on additional requirements such as multiple compartments, so check the regulations that apply where you live.

Worked example

A household of 8 people, 70 gallons per person per day, with 2 days of retention. Daily flow = 8 × 70 = 560 gallons. Capacity = 560 × 2 = 1,120 gallons, which is above the 1,000-gallon minimum, so no adjustment is applied. In litres that is 1,120 × 3.7854 = 4,240 L. For a household of 4 the same inputs would give a daily flow of 280 and a capacity of 560 gallons, which the tool raises to the 1,000-gallon minimum.

Try this example →

Common mistakes

  • Entering litres per person per day: the field is US gallons, so 150 litres should be entered as about 40.
  • Expecting a small household to show a smaller tank: any result below 1,000 gallons is raised to the 1,000-gallon minimum.
  • Counting only current residents: a tank is sized for the maximum household, including a room that may be added or occupied later.
  • Using a retention time of hours instead of days: 48 hours should be entered as 2.
  • Treating the result as the final design: it ignores local code, soil conditions, garbage disposal use and the drain field size.

About this calculator

Septic tank sizing determines the minimum capacity needed for a household's wastewater system by multiplying the number of occupants by average daily water use per person, then multiplying that daily wastewater volume by the required retention time — typically 24 to 72 hours — that allows solids to settle and partially decompose before the liquid effluent flows out to the drain field. Most residential codes also specify a minimum tank size regardless of household size (commonly around 1,000 US gallons for up to a certain occupant count) to provide adequate settling capacity even for smaller households.

Homeowners, builders and septic installers need this figure when installing a new system, replacing an undersized tank, or adding bedrooms/bathrooms to a home, because a septic tank that's too small doesn't give solids enough time to settle, leading to solids escaping into the drain field and clogging it — a failure that is expensive and disruptive to fix, since it usually means digging up and replacing the entire leach field.

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