Drainfield Size Calculator
Calculate the required drainfield (leach field) size based on your soil type, household water usage, and state regulations.
Septic Tank Calculator
Get personalized recommendations based on your state's requirements
What is a Drainfield?
A drainfield (also called a leach field or absorption field) is a crucial component of your septic system. It receives liquid effluent from your septic tank and distributes it into the soil for final treatment. The soil naturally filters and treats the wastewater through biological and physical processes before it reaches groundwater.
Distribution
Perforated pipes distribute effluent evenly across the absorption area
Treatment
Soil bacteria break down remaining contaminants naturally
Absorption
Clean water percolates down to replenish groundwater
Soil Types & Absorption Rates
Drainfield size depends heavily on soil type. A percolation (perc) test determines how quickly water drains through your soil.
| Soil Type | Perc Rate | Absorption Rate | Notes |
|---|---|---|---|
Gravel/Coarse Sand | < 1 min/inch | 1.2 gal/sq ft/day | Fastest draining soil, requires smallest drainfield |
Sandy | 1-5 min/inch | 0.8 gal/sq ft/day | Good drainage, moderate drainfield size |
Sandy Loam | 5-15 min/inch | 0.6 gal/sq ft/day | Average drainage, standard drainfield size |
Loam | 15-30 min/inch | 0.45 gal/sq ft/day | Slower drainage, larger drainfield required |
Clay Loam | 30-60 min/inch | 0.3 gal/sq ft/day | Poor drainage, may require alternative system |
Clay | > 60 min/inch | Not suitable | Often unsuitable for conventional drainfields |
Factors That Increase Drainfield Size
Clay soil: Requires 2-3x larger area than sandy soil
More bedrooms: Each bedroom adds ~150 GPD flow
Garbage disposal: Increases solids, may need 25-50% more area
High water table: May require mounded or raised system
Shallow bedrock: Limits depth, needs wider area
Drainfield Design Requirements
Setbacks: Minimum distance from wells, property lines, buildings
Slope: Maximum 25-30% grade for conventional systems
Reserve area: 100% replacement area required in most states
Trench depth: Typically 18-36 inches below surface
Trench width: Usually 18-36 inches wide
Professional Design Required
Drainfield sizing requires a site evaluation and perc test by a licensed professional. This calculator provides estimates only. Your local health department must approve all drainfield designs before installation.
Need help understanding your state's drainfield requirements?
View State Requirements