A 3-bedroom house typically requires a minimum 1,000-gallon septic tank. Most codes calculate 150 gallons per bedroom per day. For 3 bedrooms at 150 gpd each, peak daily flow is 450 gpd. A 1,000-gallon tank provides roughly 2+ days of retention time, which is the minimum for proper treatment. Many installers recommend 1,250–1,500 gallons for a 3-bedroom home to provide safety margin.

Septic tank size is based on estimated daily wastewater flow. The standard formula uses 150 gallons per bedroom per day (some codes use 100–200 gpd per bedroom). The tank must hold at least 2–3 days of flow to allow solids to settle and liquids to be treated. Most jurisdictions set minimums by bedroom count: 1,000 gallons for 1–2 bedrooms, 1,200 for 3 bedrooms, 1,500 for 4 bedrooms.

Drain field size depends on your soil's percolation rate (measured in minutes per inch, or MPI). A slow perc rate (clay soil at 60 MPI) requires significantly more drain field area than fast-draining sandy soil (5–10 MPI). A typical 3-bedroom home on average soil requires 600–900 square feet of drain field trench area. Your local health department will specify requirements based on your perc test.

A well-maintained septic tank can last 40+ years. Concrete tanks are most common and very durable. The drain field typically lasts 25–30 years if properly maintained. The most common cause of premature drain field failure is solids overflowing from an overfull tank. Regular pumping every 3–5 years is the single most important maintenance task.

You can estimate the required tank size using bedroom count and daily flow calculations, but final sizing must be approved by your county or local health department. A licensed septic designer or engineer must assess your soil conditions, setback requirements, and local regulations before a permit is issued. Use this calculator for planning and budgeting, not as a substitute for professional evaluation.