USDA Forest Service - Trail Difficulty Ratings (Hiking Trails)

Easy:
Grade: 20%
Trail Width: 18-24"
Trail Surface: Spot Gravel

Moderate:
Grade: 30%
Trail Width: 12-18"
Trail Surface: Roots, imbedded rocks, some logs

Difficult:
Grade: 30% or more
Trail Width: 12"
Trail Surface: No graded tread

[fs.usda.gov]