Design infiltration rates, in inches per hour, for A, B, C, and D soil groups. Corresponding USDA soil classification and Unified soil Classifications are included. Note that A and B soils each have two infiltration rates that are a function of soil texture. Link to this table
Hydrologic soil group Infiltration rate (inches/hour) Soil textures Corresponding Unified Soil Classification
A 1.6a

gravel
sandy gravel

silty gravels

GW - well-graded gravels, sandy gravels
GP - gap-graded or uniform gravels, sandy gravels
GM - silty gravels, silty sandy gravels
SW - well-graded gravelly sands

0.8

sand
loamy sand
sandy loam

SP - gap-graded or uniform sands, gravelly sands

B 0.6 silt loam SM - silty sands, silty gravelly sands
0.3 loam MH - micaceous silts, diatomaceous silts, volcanic ash
C 0.2 Sandy clay loam ML - silts, very fine sands, silty or clayey fine sands
D <0.2

clay loam
silty clay loam
sandy clay
silty clay
clay

GC - clayey gravels, clayey sandy gravels
SC - clayey sands, clayey gravelly sands
CL - low plasticity clays, sandy or silty clays
OL - organic silts and clays of low plasticity
CH - highly plastic clays and sandy clays
OH - organic silts and clays of high plasticity

Source: Thirty guidance manuals and many other stormwater references were reviewed to compile recommended infiltration rates. All of these sources use the following studies as the basis for their recommended infiltration rates: (1) Rawls, Brakensiek and Saxton (1982); (2) Rawls, Gimenez and Grossman (1998); (3) Bouwer and Rice (1984); and (4) Urban Hydrology for Small Watersheds (NRCS). SWWD, 2005, provides field documented data that supports the proposed infiltration rates. (view references

aThis rate is consistent with the infiltration rate provided for the lower end of the Hydrologic Soil Group A soils in the Wisconsin Department of Natural Resources Conservation Practice Standard: Site Evaluation for Stormwater Infiltration.