Septic Tank Size Calculator
Choose your home's number of bedrooms to get the recommended minimum septic tank size and the estimated daily wastewater flow.
Septic tanks are usually sized by the number of bedrooms rather than the number of people, because bedrooms are a good proxy for the home's maximum wastewater flow. Bigger tanks hold waste longer, giving solids time to settle before liquid moves to the drain field.
Recommended septic tank size by bedrooms
| Bedrooms | Minimum tank size |
|---|---|
| 1–2 | 750–1,000 gallons |
| 3 | 1,000 gallons |
| 4 | 1,250 gallons |
| 5–6 | 1,500 gallons |
| 7+ | +250 gallons per extra bedroom |
How septic tank size is worked out
Sizing is based on estimated daily wastewater flow (commonly about 120 gallons per bedroom per day) and holding it long enough for solids to settle. Local codes then set the minimum tank for each bedroom count.
Estimated daily flow ≈ Bedrooms × 120 gallons
Tank size = code minimum for that bedroom count
Tank size = code minimum for that bedroom count
Frequently asked questions
What size septic tank do I need for a 3-bedroom house?
A 1,000-gallon tank is the common minimum for a 3-bedroom home, though some areas require larger. Check your local code.
Is tank size based on people or bedrooms?
Almost always bedrooms, since that caps the home's potential occupancy and flow. Heavy water use or a garbage disposal can justify going a size up.
Can I install a bigger tank than required?
Yes, and it's often a good idea — a larger tank needs pumping less often and handles occasional heavy use better.
Related calculators
General guidance only. Septic sizing and permitting are governed by local codes and site conditions — consult your health department or a licensed installer.