What Size Septic Tank Do I Need?
Enter bedrooms and whether you have a garbage disposal — get the code-basis minimum tank size in gallons, the way state codes size it. Your county sets the binding number; this tells you what to expect before you call.
Codes size by bedrooms (as a proxy for occupants), not bathrooms.
How the math works
US state codes overwhelmingly size residential systems off 150 gallons per bedroom per day of design flow (Arizona DEQ, Virginia, and Washington DOH all publish this figure). The tank's liquid capacity runs about twice the daily design flow, rounded up to standard manufactured sizes (750–3,000 gallons), with 1,000 gallons the common floor for new systems. Check it against real code tables and it lands: 3 bedrooms → 1,000 gallons, 4 → 1,250, 5 → 1,500 — exactly what the state tables say.
Honest limits
- Your county's number wins. This is the code-basis estimate; the local health department issues the actual permit and may require more — or size things differently for your soils. Never order a tank off a web calculator alone.
- This sizes the tank, not the drainfield. The field is sized separately from soil texture and percolation test results. A big tank on a small lot with bad soil still needs an engineered field — or doesn't fit at all.
- Garbage disposals are load. Many codes count one as an extra bedroom (New York does explicitly). It also means more frequent pump-outs — run your schedule through our pump schedule calculator.
- Bigger isn't automatically better. An oversized tank doesn't hurt, but you pay for the concrete and the excavation. The expensive mistake is undersizing, which sends solids to the field early.
Frequently asked questions
Can I just install the biggest tank to be safe?
Oversizing the tank is harmless to the system but you're buying concrete and digging you don't need — and the permit still sizes the drainfield off the bedroom count, not the tank. Size it right, spend the savings on the field.
Does a garbage disposal really matter that much?
Yes — food waste is high-strength solids loading, which is exactly what the tank is trying to settle out. Several states treat a disposal as an additional bedroom for sizing. If you're on septic and love your disposal, budget for more frequent pumping.
What if I'm adding a bedroom later?
Size for the future bedroom count now — permits are issued off the bedroom count on the plans, and upsizing a buried tank later costs far more than buying the bigger tank once.
Planning a system? Price it with our septic cost calculator.