Leach Field Calculator
A leach field (also called a drain field or septic absorption field) is where treated wastewater from your septic tank soaks into the soil. Size it too small and the field fails, creating soggy yards and backed-up toilets. This leach field calculator sizes the absorption area and total trench length from daily flow (gal/bedroom/day), percolation rate, and trench width.
Leach Field Size for 3 Bedroom Home
Most US states use 150 gal/bedroom/day as the design flow. A 3-bedroom home is 450 gal/day. Standard leach field size for 3 bedroom home on medium loam soil (30 min/inch perc):
- Application rate: 0.6 gal/sq ft/day
- Required absorption area: 450 / 0.6 = 750 sq ft
- At 3 ft trench width: 250 ft total trench length
Leach field size for 3 bedroom home florida is larger because of the state's high water table. Florida DEP Rule 64E-6 applies a roughly 1.5x factor, so the same 3-bedroom home in FL needs about 1,125 sq ft of absorption area instead of 750.
Leach Field Size for 4 Bedroom
Leach field size for 4 bedroom home at 150 gal/bedroom/day is 600 gal/day design flow. On medium loam: 1,000 sq ft absorption area and 333 ft of 3-ft-wide trench. On heavy clay (60+ min/inch), the same 4-bedroom home needs 2,000-3,000 sq ft.
Leach Field Size for 1 Bedroom
Leach field size for 1 bedroom is minimum-practical: 150 gal/day design flow, 250 sq ft absorption area on loam, 83 ft trench length. Many codes enforce a 300 sq ft minimum absorption area and 500 gallon minimum tank regardless of bedroom count.
Leach Field Sizing Chart
This is the standard leach field size chart for 150 gal/bedroom/day design flow at different soil types. Most states publish a similar leach field size chart pdf; download yours from the state environmental health department.
| Bedrooms | Sand (5 min) | Loam (30 min) | Clay (60 min) |
|---|---|---|---|
| 1 | 125 sq ft | 250 sq ft | 500 sq ft |
| 2 | 250 sq ft | 500 sq ft | 1,000 sq ft |
| 3 | 375 sq ft | 750 sq ft | 1,500 sq ft |
| 4 | 500 sq ft | 1,000 sq ft | 2,000 sq ft |
| 5 | 625 sq ft | 1,250 sq ft | 2,500 sq ft |
| 6 | 750 sq ft | 1,500 sq ft | 3,000 sq ft |
Soil Application Rates
These rates come from EPA Design Manual 625/R-00/008 and are the same values most state codes adopt:
| Percolation Rate | Soil Type | Application Rate |
|---|---|---|
| 0-5 min/inch | Sand, gravel | 1.2 gal/sq ft/day |
| 6-15 min/inch | Sandy loam | 0.8 gal/sq ft/day |
| 16-30 min/inch | Loam | 0.6 gal/sq ft/day |
| 31-45 min/inch | Silty clay loam | 0.45 gal/sq ft/day |
| 46-60 min/inch | Clay | 0.3 gal/sq ft/day |
| 61-120 min/inch | Heavy clay | 0.2 gal/sq ft/day |
| 120+ min/inch | Unsuitable | requires engineered system |
Design Flow by Code
Most states size at 150 gal/bedroom/day. Some exceptions:
- Florida: 100 gal/bedroom/day base, with larger area factor (FL DEP 64E-6)
- California: 150 gal/bedroom/day (Title 22)
- Texas: 75-150 gal/bedroom/day (TCEQ Ch. 285, depends on system type)
- EPA recommendation: 100-150 gal/bedroom/day
How to Get a Perc Test
A percolation test (perc test) measures how fast water drains through your soil. The test is required before permitting any new septic system. Your county health department lists approved soil evaluators. Cost is typically $300-700 per test. Fail the perc test and you may need a mound system or aerobic treatment unit instead of a conventional leach field.