Percolation rate to soil-loading-rate bands
A percolation (perc) test measures how fast water drops in a test hole, in minutes per inch. Faster soil carries more effluent per square foot, so it maps to a higher soil loading rate (gpd/sq ft) and a smaller drain field. These are labeled planning bands — your local health department’s own table governs, and very slow soil may not perc at all. Use them in the perc → loading and absorption area tools.
| Perc rate (min/inch) | Soil | Loading rate (gpd/sq ft) |
|---|---|---|
| ≤ 5 | Fast (sandy) | 1.20 |
| 6–30 | Moderate | 0.50 |
| 31–60 | Slow (clay) | 0.20 |
| > 60 | Very slow | may not perc — engineered system |
Labeled planning bands — the local health department’s own perc → loading table governs, and a failed perc test may require an engineered or alternative system. Absorption area = design flow ÷ loading rate.