Septic Tank Calculator
Size septic tank and drain field for your home
Number of bedrooms in home
Daily water use per occupant
Drain Field Area
—
sq ft
Bedrooms Served
—
bedrooms
Tank Size
—
gal
Disclaimer: Results are estimates based on standard formulas. Not a substitute for professional advice from a licensed contractor, engineer, or architect. Verify all calculations before making purchasing or construction decisions.
How to Use This Calculator
Correctly sized septic systems prevent backups and protect groundwater. Our calculator estimates tank and drain field requirements based on household size.
- Enter the number of bedrooms in the home
- Set the daily gallons per person (default 75)
- Review tank size, drain field area, and capacity
- Verify results with local code requirements
Expert Insights
Smart analysis for your Plumbing project
Bedroom-Based Sizing
Code uses bedrooms plus one to estimate occupants. AI applies this convention so your tank matches typical regulatory formulas.
Drain Field Sizing
Drain field area scales with tank size and soil percolation rate. AI estimates 0.6 sq ft per gallon as a planning baseline.
Soil Percolation Impact
Clay soils need larger drain fields; sandy soils need less. AI reminds you to run a percolation test before final design.
Calculation Formula
People = Bedrooms + 1
Tank Size = People x Gallons Per Person x 1.5 (min 1,000 gal)
Drain Field Area = ceil(Tank Size x 0.6)
Bedrooms Served = input value
Septic Tank Size Reference
| Bedrooms | Est. Occupants | Min Tank Size | Drain Field Area | Pump Interval |
|---|---|---|---|---|
| 2 | 3 | 1,000 gal | 600 sq ft | 5 years |
| 3 | 4 | 1,000 gal | 675 sq ft | 4 years |
| 4 | 5 | 1,250 gal | 750 sq ft | 3-4 years |
| 5 | 6 | 1,500 gal | 900 sq ft | 3 years |
Authority References
Our calculations are based on guidelines and standards from these authoritative sources:
- U.S. Environmental Protection Agency (EPA) — Onsite Wastewater Treatment Systems Manual. Learn more
- International Code Council (ICC) — International Private Sewage Disposal Code (IPSDC). Learn more
- National Onsite Wastewater Recycling Association (NOWRA) — Site Evaluation & System Sizing Standards. Learn more
Frequently Asked Questions
How big of a septic tank do I need for a 3-bedroom house?
A 3-bedroom home typically needs a 1,000-gallon septic tank, which is the minimum size allowed by most codes. A 4-bedroom home usually requires 1,250 gallons or more.
How far should a drain field be from the house?
Most codes require at least 10 feet between the septic tank and the house foundation, and a minimum of 5 feet between the drain field and the house. Local setbacks vary, so check county regulations.
How often should a septic tank be pumped?
A residential septic tank should be pumped every 3 to 5 years depending on household size and water usage. Larger households or smaller tanks may need more frequent pumping.