Septic systems · tank size
Septic tank size by bedrooms calculator.
Enter your bedroom count to see the minimum septic tank liquid capacity required by four state rules side by side, or size a larger system from design daily flow with the North Carolina flow table. Runs in your browser; nothing is sent anywhere.
Planning aid: codes and soils vary by location. Confirm septic and well work with your local health department and a licensed professional.
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Worked example
A 4-bedroom house. Minnesota Table V: 4 or 5 bedrooms = 1,500 gal. Washington Table VII: up to 4 bedrooms = 1,000 gal. Maryland: 4 bedrooms = 1,250 gal. North Carolina Table XIV: 4 or fewer = 1,000 gal. The same house needs a different minimum depending on the jurisdiction, which is why you must check your own local rule. Entering a design flow of 900 gpd gives the North Carolina Table XV result 1.17 × 900 + 500 = 1,553 gal.
Formulas, sources, and limits
Formulas: Minnesota: 1,000 gal for 3 or fewer bedrooms, 1,500 for 4 or 5, 2,000 for 6 or 7, 2,500 for 8 or 9, then 2,500 + 250 per bedroom above 9. Washington: 1,000 gal up to 4 bedrooms, plus 250 per additional bedroom. Maryland: 1,000 gal for 3 or fewer bedrooms, 1,250 for 4, plus 250 per additional bedroom (two compartments or two tanks in series required). North Carolina Table XIV: 1,000 gal for 4 or fewer bedrooms, 1,250 for 5; above 5 bedrooms use Table XV with design flow Q in gpd: V = 2Q for Q up to 600; V = 1.17Q + 500 for 600 < Q < 1,500; V = 0.75Q + 1,125 for 1,500 to 4,500; V = Q above 4,500.
Sources (retrieved 2026-10): Minnesota Rules 7080.1930, Table V; Washington WAC 246-272A-0232, Table VII (effective April 1, 2025); Maryland COMAR 26.04.02.05; North Carolina 15A NCAC 18E .0801, Tables XIV and XV.
Limits: These are four examples, not a national standard. Most states and counties publish their own table, and some are stricter or key tank size to design flow instead of bedrooms (Nebraska, for instance, requires at least 1,000 gal under any design). Rules change, so confirm with your local health department before buying a tank.
Frequently asked questions
What size septic tank do I need for a 3 bedroom house?
Three of the four rules shown here give 1,000 gallons for a 3-bedroom house. Your local code governs and may require more.
What size septic tank for a 4 bedroom house?
It varies by rule: 1,000 gal in Washington and North Carolina, 1,250 gal in Maryland, and 1,500 gal in Minnesota.
Is a bigger septic tank better?
A larger tank holds wastewater longer and stores more solids between pump-outs, but the tank is only one part of the system and the drain field must still match the flow. Follow the local design.
Does this replace a septic designer or the health department?
No. The health authority approves the system. This tool only shows published minimums.