Enter your perc test result and number of bedrooms to calculate the required drain field trench area — based on University of Minnesota Onsite Sewage Treatment Program loading rates.
Required trench bottom area in square feet per gallon per day, by perc rate. Source: University of Minnesota Onsite Sewage Treatment Program.
| Perc Rate | Soil Type | Sq ft / gal/day | 3-BR Home (450 gpd) |
|---|---|---|---|
| Under 5 MPI | Coarse sand | ❌ Conventional drainfield not suitable | |
| 5–15 MPI | Sandy loam | 1.27 | 572 sq ft |
| 16–30 MPI | Loam / silt loam | 1.67 | 752 sq ft |
| 31–45 MPI | Silt loam / clay loam | 2.00 | 900 sq ft |
| 46–60 MPI | Clay loam | 2.20 | 990 sq ft |
| Over 60 MPI | Heavy clay | ❌ Conventional drainfield not suitable | |
The minimum square footage of trench bottom required. Trench width is typically 18–36 inches. Divide total area by trench width to get total trench length. Effluent must be distributed across at least two separate trenches using drop boxes.
Using 12 inches of gravel below the distribution pipe (instead of the standard 6 inches) allows a 20% reduction in required trench bottom area — a meaningful saving on small lots.
Most jurisdictions require a 50–100% reserve drainfield area — unused land set aside in case the primary field fails. This doubles or triples the total land required. Verify your county's reserve area requirement.
Soil application rates from: University of Minnesota Extension, Onsite Sewage Treatment Program; University of Nebraska Extension, "Septic Systems for the Homeowner." Daily flow estimates based on 150 gpd/bedroom standard used by most state health departments. This calculator provides an estimate only — actual drainfield sizing must be determined by a licensed soil scientist or engineer using site-specific perc test results, soil profile data, and local regulatory requirements. Local rules govern.