Drain Field Size Calculator
Estimate the required drain field (leach field) area for a septic system based on household size, daily wastewater flow, and soil percolation rate.
bedrooms
gallons/day
min/inch
feet
feet
Formulas Used
Required Absorption Area:
A (ft²) = Q (gal/day) ÷ R (gal/day/ft²)
Where Q = daily wastewater flow and R = soil loading rate determined from the percolation rate table below.
Daily Flow Estimate:
Q = Bedrooms × 2 persons/bedroom × 75 gal/person/day
Trench Length:
L (ft) = A ÷ Trench Width
Soil Loading Rate Table (EPA):
| Perc Rate (min/in) | Loading Rate (gal/day/ft²) |
|---|---|
| ≤ 5 | 1.20 |
| 6 – 10 | 0.80 |
| 11 – 15 | 0.72 |
| 16 – 20 | 0.64 |
| 21 – 25 | 0.56 |
| 26 – 30 | 0.48 |
| 31 – 45 | 0.40 |
| 46 – 60 | 0.32 |
| > 60 | Unsuitable |
Assumptions & References
- Daily flow estimated at 75 gallons per person per day with 2 persons per bedroom (EPA standard).
- Soil loading rates are taken from the EPA Design Manual: Onsite Wastewater Treatment and Disposal Systems (EPA 625/1-80-012).
- Maximum recommended individual trench length is 100 feet to ensure uniform distribution.
- Trench spacing (center-to-center) must exceed trench width to prevent hydraulic interference between trenches.
- A 100% reserve area of equal size is required by most state and local codes — not included in the calculated area.
- Percolation rates > 60 min/inch indicate unsuitable soil; alternative systems (mound, drip irrigation) should be considered.
- This calculator provides an estimate only. A licensed engineer or soil scientist must conduct a site-specific percolation test and design per local regulations.
- Setback requirements (from wells, property lines, structures) are not included and vary by jurisdiction.