A Drain Field Size Calculator is a tool used to estimate the appropriate size of a drain field (also known as a leach field) for a septic system. The drain field is a crucial component of a septic system, responsible for filtering and dispersing wastewater into the surrounding soil. Determining the correct size ensures efficient wastewater treatment, prevents system failures, and complies with local health regulations.
This calculator helps homeowners, engineers, and septic system designers determine the required drain field area based on daily wastewater flow and soil absorption characteristics.
Formula of Drain Field Size Calculator
The formula for calculating drain field size is:
Drain Field Area (ft²) = Daily Wastewater Flow (gallons) × Soil Application Rate Factor
Where:
- Daily Wastewater Flow: Typically ranges from 120-150 gallons per bedroom per day.
- Soil Application Rate Factor: Depends on the soil’s percolation rate, which affects how quickly water drains.
Soil Percolation Rates and Application Rate Factors
| Soil Type | Percolation Rate (minutes/inch) | Application Rate Factor (ft²/gallon) |
|---|---|---|
| Sand | < 10 | 0.8 – 1.2 |
| Sandy Loam | 10 – 30 | 1.2 – 1.6 |
| Loam | 30 – 45 | 1.6 – 2.0 |
| Clay Loam | 45 – 60 | 2.0 – 2.4 |
| Clay | > 60 | 2.4 – 3.0 |
The slower the percolation rate, the larger the drain field required.
Precomputed Table for Common Values
| Bedrooms | Daily Flow (gallons) | Soil Type | Application Rate | Required Drain Field Area (ft²) |
| 2 | 240 | Sandy Loam | 1.4 | 336 |
| 3 | 360 | Loam | 1.8 | 648 |
| 4 | 480 | Clay Loam | 2.2 | 1056 |
| 5 | 600 | Clay | 2.8 | 1680 |
This table provides quick reference values based on standard assumptions.
Example of Drain Field Size Calculator
Problem:
A house with three bedrooms produces an estimated 360 gallons of wastewater per day. The soil is classified as sandy loam, which has an application rate of 1.4 ft²/gallon. What is the required drain field size?
Solution:
Using the formula:
Drain Field Area = 360 × 1.4
Drain Field Area = 504 ft²
Thus, a drain field of at least 504 square feet is needed to handle the wastewater properly.
Most Common FAQs
Calculating the correct drain field size ensures proper wastewater treatment, prevents groundwater contamination, and reduces the risk of system failure.
If the drain field is too small, wastewater may not be properly absorbed, leading to backups, odors, and environmental contamination.
This calculator is primarily designed for residential use, but it can provide estimates for small commercial properties. However, larger systems require professional engineering analysis.