Septic tank sizing
Septic tank size calculator
Set your bedroom count and we size the tank from public health-code design flow — then show you the full formula and the standard it cites. No black boxes.
Rough estimate only — local codes vary. Follow your local regulations and consult a licensed septic professional in your area before you build or buy.
The standard
How we size your tank
floored at 1,000 gal, rounded up to the
nearest standard tank size.
Codes size tanks by bedrooms because bedrooms cap a home's design occupancy — a fixed number that won't drift as people come and go. Most onsite-wastewater codes estimate 150 gallons per bedroom per day of design flow; the tank holds that flow for a 48-hour retention so solids can settle. The tank must precede the drain field and keep a minimum vertical separation to the seasonal water table.
For the full size-by-bedroom breakdown with daily design flow, see the septic tank size chart — or jump to the common cases: 3-bedroom, 4-bedroom, or 5-bedroom house.
Full guide: How to size a septic tank by bedrooms → — why bedrooms (not occupants) set the size, and how to verify against your local code.
Questions
Is bedroom count the input — not occupants?
Yes. Codes size tanks by bedrooms because the bedroom count caps a home’s design occupancy and will not change as people come and go. Codes estimate about 150 gallons per bedroom per day of design flow.
Why round up to a standard size?
Tanks are manufactured in fixed capacities (750, 1,000, 1,250, 1,500, 2,000, 2,500 gallons). Sizing up to the next standard never hurts performance and costs little more.
Does a garbage disposal change the size?
Yes — a disposal adds solids, so we apply a 1.5× sludge allowance to the working capacity, which can bump you to the next standard tank size. Toggle it above to see the effect on the readout.
What about high-efficiency fixtures?
Low-flow toilets and fixtures reduce daily design flow, so some codes allow a modest reduction. We apply a conservative 0.85 factor when you toggle it on.
How many bedrooms does a 1,000-gallon tank serve?
A 1,000-gallon tank covers most homes up to three bedrooms (about 450 GPD of design flow, floored at the 1,000-gallon code minimum). Four bedrooms typically steps up to a 1,250-gallon tank.
Is the calculator a substitute for a permit?
No. It gives an independent baseline from national standards. Your local health authority sets the legal minimum and may require a specific size, a perc test, and a permitted design.
Sources & methodology
- US EPA — Onsite Wastewater Treatment Systems Manual (EPA/625/R-00/008): 60 gal/person/day design flow
- Tank-capacity minimums — International Private Sewage Disposal Code (IPSDC) baseline; adopted/amended by each state
- Your state or county health department — the authoritative source for local code, minimums, setbacks, and permits
Estimates compiled from the sources above and standard engineering formulas — not engineering, professional, or legal advice, and may not reflect your local code. See our full methodology and disclaimer.