Calculator
Absorption area is daily flow divided by how fast the soil accepts water. Soil texture matters more here than anything else on the site.
From your perc test or soil evaluation. The number in brackets is minutes per inch.
These are conventional trench loading rates. Your state may publish slightly different figures, and a soil evaluator's reading of the actual profile always wins.
| Soil texture | Perc rate (min/inch) | Application rate (gpd/sq ft) | Area for 450 gpd |
|---|---|---|---|
| Gravel / coarse sand | Under 5 | 1.20 | 375 sq ft |
| Sand | 5 – 15 | 0.80 | 562 sq ft |
| Sandy loam | 16 – 30 | 0.60 | 750 sq ft |
| Loam | 31 – 45 | 0.45 | 1,000 sq ft |
| Silt loam | 46 – 60 | 0.30 | 1,500 sq ft |
| Clay loam | 61 – 90 | 0.20 | 2,250 sq ft |
| Clay | Over 90 | 0.10 | 4,500 sq ft |
Most states require a second area of equal size, kept clear and undisturbed, as the replacement field. It is part of the permit even though nothing is built there. Plan the driveway, the shed and the pool around it, because a lot is only buildable if both areas fit.