Elevation Gain x 2 x distance (in miles). The product's square root is the numerical rating.
For example, a 10-mile hike that gains 2,200 feet in elevation:
2,200 x 2=4,400
4,400 x 10 = 44,000
Square root of 44,000=209.8
The hike's numerical rating is then tied to one of five descriptors: Easiest, Moderate, Moderately Strenuous, Strenuous, Very Strenuous.
The example hike above would be rated Very Strenuous.
This formula approximates difficulty and is imperfect. Dark Hollow Falls has a numeric rating of 36.3. Under the formula this would be in the "Easiest" range, however the steepness of the trail over such a short distance warrants a "Moderate." When choosing a hike, always consider the specific elevation and distance in addition the the given difficulty rating.
A hike that is generally suitable for anyone who enjoys walking. Mostly level or with a slight incline. Generally less than 3 miles.
A moderate hike is generally suitable for novice hikers who want a bit of a challenge. The terrain will involve a moderate incline and may have some steeper sections. Generally 3 to 5 miles.
Moderately Strenuous hikes will generally be challenging for an unconditioned person. The terrain will involve a steady and often steep incline. Generally 5 to 8 miles.
Strenuous hikes will challenge most hikers. The hike will generally be longer and steeper, but may be deemed "Strenuous" because of the elevation gain. Generally 7 to 10 miles.
Only well-conditioned and well-prepared hikers should attempt very strenuous hikes. The hike will generally be long and steep, and may include rock scrambling, stream crossings, and other challenging terrain. Generally 8 miles and over.
Average Pace is:
1.5 miles per hour for easiest trails.
1.4 mph for moderate trails.
1.3 mph for moderately strenuous trails.
1.2 mph for strenuous and very strenuous trails.
This is hiking time and does not take into consideration fitness, exploration, rest, contemplation, etc.