Household size
Most state codes size septic by bedroom count (each bedroom = 2 people max). Actual occupants are used to verify reasonableness.
Water usage
Average is the standard assumption. Low fits water-conscious households with low-flow fixtures. High fits homes with long showers, dishwashers, washing machines running daily.
Garbage disposal
Garbage disposals significantly increase solids loading on a septic. Most state codes require 250-500 gal extra tank capacity when one is installed.
Climate / soil
Cold climates slow bacterial breakdown – many state codes require larger tanks to give solids more residence time.
Tank size
Daily wastewater
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gallons per day estimated
Minimum tank size
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gallons (code minimum)
Recommended size
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gallons (with margin)
Pumping interval
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years between cleanings
Septic tank sizing rules vary by state. Most jurisdictions require a 1,000 gallon minimum, sized by bedroom count (not actual occupants), with extras for garbage disposals and large fixtures. This calculator gives a general estimate – always consult your local health department before installing or replacing a septic system. A larger tank is almost always better; the cost difference between sizes is small.