Septic tank sizing
What size septic tank do I need?
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.
Estimate only. Your local health authority sets the legal minimum — always confirm before buying or installing a tank.
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.
Standard septic tank sizes by bedrooms
| 1–3 bedrooms | 1,000 gal (code minimum) |
| 4 bedrooms | 1,250 gal |
| 5 bedrooms | 1,500 gal |
| 6 bedrooms | 2,000 gal |
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.