How to Size a Septic System: Complete Guide with Calculator
Learn how to properly size a septic system. Step-by-step guide with septic system sizing calculator, tank size, drain field area, and code requirements.
Step-by-Step Guide
1. Determine Daily Flow
Calculate daily wastewater flow: 120 gallons per bedroom (residential) or actual usage data. For 3-bedroom home: 3 × 120 = 360 gallons per day. Add 20% for safety factor.
2. Size Septic Tank
Tank size = Daily Flow × 2 (minimum 2 days retention). For 360 GPD: 360 × 2 = 720 gallons minimum. Standard sizes: 1000, 1250, 1500 gallons. Always round up to standard size.
3. Determine Soil Type
Perform percolation test or soil analysis. Soil types: Sandy (fast), Sandy Loam (good), Moderately Permeable (moderate), Slowly Permeable (slow), Clay (poor). Affects drain field sizing.
4. Calculate Drain Field Area
Drain field area = Daily Flow ÷ Loading Rate. Loading rates: Sandy = 1.2 gal/sq ft/day, Sandy Loam = 0.8, Moderate = 0.6, Slow = 0.4, Clay = 0.2. Add 100% reserve area.
5. Design Drain Field
Layout drain field with proper spacing: 6-10 feet between trenches, 18-36 inches deep. Include reserve area for future expansion. Our calculator provides exact dimensions.
Frequently Asked Questions
Code Compliance
Septic system sizing must comply with local health department regulations, which typically follow state environmental codes. All systems require permits, soil testing, and professional design.