What size septic tank do I need?

Quick answer

Tank size by bedroom count

Bedrooms	Minimum tank
1–3 bedrooms	1,000 gal
4 bedrooms	1,250 gal
5 bedrooms	1,500 gal
6 bedrooms	2,000 gal

Find your bedroom count in the table — that's your minimum tank in most jurisdictions. Want it broken down row by row? See the septic tank size chart; for one worked case, the 3-bedroom example.

Where those numbers come from

Codes don't size a tank from how many people live in the house — they size it from design flow, which is a fixed property of the building. The standard assumption is two people per bedroom at roughly 75 gallons per person per day, so 150 gallons per bedroom per day. The tank then has to hold about two days of that flow, because solids need roughly 48 hours of quiet retention to settle out before the effluent moves on to the drain field.

That's why a 3-bedroom house pencils out at 450 gallons a day and 900 gallons of retention — and then lands at 1,000 gallons anyway, because almost every jurisdiction sets 1,000 gallons as the floor no matter how small the house. Under about four bedrooms, the code minimum decides your tank, not the arithmetic.

What pushes you up a size

• A garbage disposal — the single most common bump. Ground food waste adds solids and many codes require the next tank size up, or a larger first compartment.
• A finished basement or bonus room a future owner could sleep in — some inspectors count anything that could become a bedroom, so a 3-bedroom house can be permitted as a 4.
• A whirlpool tub or a high-volume fixture group — large single-draw fixtures spike the flow and can trip a larger tank or a dosing setup.
• Working from home or a home business — some health departments size on actual occupancy when the use is clearly heavier than the bedroom count implies.

Going the other way, high-efficiency fixtures genuinely lower daily flow, but most jurisdictions won't let you size below the bedroom-count minimum on that basis. Efficiency buys you drain-field margin, not a smaller tank.

Don't oversize by much either

Stepping up one size is harmless and often smart if you might add a bedroom. Going far larger is not: a tank that never fills to its working level doesn't scour properly, solids can stratify in the wrong places, and you've paid for concrete and excavation you'll never use. Size to the standard, then confirm the exact figure with your local health department — the minimum is set locally and a few states run higher than the national baseline.

Get your number: Open the septic tank size calculator → — set bedrooms, a disposal, and fixture efficiency for the exact figure and the math behind it.

How it works: How to size a septic tank by bedrooms → — why bedrooms, not occupants, set the size.