Septic Tank Size Calculator — Gallons & Design Flow
Estimate the minimum septic tank size in gallons from your home's bedroom count, plus design flow and retention days — then confirm with local code.
How it works
Residential septic tanks are sized from bedroom count, not from how many people currently live in a house, because bedrooms are a stable proxy for the home's maximum wastewater load. The common code basis estimates about 150 gallons of design flow per bedroom per day, and sets a minimum liquid capacity of 1,000 gallons for a home of up to three bedrooms, adding 250 gallons for each bedroom beyond that.
From those two rules the tool reports three numbers: the minimum tank capacity, the estimated daily design flow, and a nominal retention time — how many days of flow the tank holds, which is what lets solids settle and bacteria do their work. A larger tank buys more retention and more resilience to a busy weekend of guests.
This is a planning estimate only. Percolation rate, seasonal water table, a garbage disposal, water-softener discharge, and local amendments can all push the required size up. Follow your local code and confirm the design with your county health department or a licensed septic installer before you dig or buy — the jurisdiction's table always governs.
design flow = 3 × 150 = 450 gallons per day
minimum tank = base 1,000 gal (up to 3 bedrooms) → 1,000 gal (about 3,785 L)
retention = 1,000 ÷ 450 = 2.2 days