About the Septic Tank Size Calculator
This septic tank size calculator estimates the minimum liquid capacity a septic tank must have for a home, using two published state rules as worked examples of how US codes size tanks. Choose a rule, enter the number of bedrooms (or, for the Pennsylvania method, the design daily flow), and it returns the minimum required capacity in gallons and the next common stock tank size.
Codes size residential tanks by bedrooms rather than people because bedrooms set the potential occupancy of the house. Minnesota Rules 7080.1930 sets 1,000 gallons for up to three bedrooms, rising in steps to 2,500 gallons for nine, and requires 50% more capacity when a garbage disposal or a sewage ejector or grinder pump feeds the tank. Pennsylvania (25 Pa. Code § 73.31) assumes 400 gallons per day for a dwelling of up to three bedrooms plus 100 gallons per day for each extra bedroom, converts that flow to capacity with a sliding formula, and sets an absolute minimum of 900 gallons.
Septic sizing is set by your state and often your county, and a licensed designer or the local health department has the final say. Use this as a planning estimate and comparison, not as a permit design; the figures reflect both rules as published and checked in October 2026, and drainfield sizing is not covered here.
With the default inputs, the minimum tank capacity is 1,000 gallons. Change any value above to recalculate instantly.
How to use the septic tank size calculator
- 1Choose the state rule you want to apply or compare.
- 2Enter the number of bedrooms, counting rooms that could be used as bedrooms.
- 3For Minnesota, switch on the toggle if a garbage disposal or ejector pump feeds the tank.
- 4For Pennsylvania, you can instead enter a design daily flow in gallons per day.
- 5Read the minimum capacity and the next stock tank size, then confirm with your local permit office.
Formula and method
Minnesota Rules 7080.1930, Table V, gives the minimum liquid capacity for dwellings by bedroom count. Subparts 2 and 3 require the tank to be at least 50% larger when a garbage disposal is anticipated or installed, or when sewage is pumped to the tank by an ejector or grinder pump; in either case the tank must also have multiple compartments or be multiple tanks. Subpart 4 says the two increases are not added together.
Pennsylvania (25 Pa. Code § 73.31) uses a daily flow of 400 gallons for a single-family dwelling, plus 100 gallons for each bedroom over three. Capacity is then 3.5 × (Q − 400) + 900 for flows up to 500 gpd, 1.5 × (Q − 500) + 1,250 up to 5,000 gpd, 1.45 × (Q − 5,000) + 8,000 up to 7,500 gpd, 1.35 × (Q − 7,500) + 11,625 up to 10,000 gpd and 1.5 × Q above that, never less than 900 gallons. The stock size is the next common tank volume at or above the minimum.
- BR
- Number of bedrooms
- Q
- Design daily sewage flow (gallons per day)
Worked examples
Three-bedroom house, Minnesota rule
Minnesota’s Table V sets 1,000 gallons as the minimum for a dwelling with three or fewer bedrooms, which is also a standard stock tank size.
Four bedrooms with a garbage disposal, Minnesota
Four bedrooms need 1,500 gallons, and the garbage disposal adds 50%: 1,500 × 1.5 = 2,250 gallons. The next common tank size is 2,500 gallons.
Five bedrooms, Pennsylvania rule
The design flow is 400 + 2 × 100 = 600 gpd. Capacity is 1.5 × (600 − 500) + 1,250 = 1,400 gallons, so a 1,500 gallon tank would be used, holding about 2.3 days of flow.
2,000 gpd small facility, Pennsylvania flow method
For 2,000 gallons per day the formula gives 1.5 × (2,000 − 500) + 1,250 = 3,500 gallons, or 1.75 days of flow; a 4,000 gallon tank is the next stock size.
Frequently asked questions
What size septic tank do I need for a 3-bedroom house?+
Many state codes set 1,000 gallons as the minimum for up to three bedrooms; Minnesota is one example. Pennsylvania’s formula allows 900 gallons, but 1,000 gallons is the common stock size installed.
What size septic tank for a 4-bedroom house?+
Under Minnesota rules a 4-bedroom home needs 1,500 gallons, or 2,250 gallons with a garbage disposal or ejector pump. Pennsylvania’s formula gives 1,250 gallons for four bedrooms.
Why are septic tanks sized by bedrooms and not people?+
The number of occupants changes over time, but bedrooms set how many people the house can hold. Sizing by bedrooms makes the system adequate for future owners and is how most state codes define residential design flow.
Does a garbage disposal require a bigger septic tank?+
Often yes. Food waste adds solids that settle and fill the tank faster. Minnesota requires 50% more capacity with a disposal, while Pennsylvania’s 400 gpd design flow already allows for garbage grinders.
How often should a septic tank be pumped?+
The US EPA says the average household system should be inspected at least every three years and the tank is typically pumped every three to five years, depending on tank size, household size and use. Larger tanks go longer between pumpings.
Results are general estimates and not legal advice. Laws vary by jurisdiction — consult a qualified attorney.