Mountain biking trails difficulty levels are set by the IMBA guide. Not all trails recognize this system as it is fairly new, but it is the first step towards standardizing mountain biking trails across the United States.
Officially the guide is called the IMBA, TDRS the latter of which stands for trail difficulty rating system. The TDRS rates trails only by their technical difficulty, while the former system rates trails by physical difficulty, which is less objective given the ranges will vary by biker. There are a few other considerations that factor into a trail’s difficulty rating, which include psychological factors, natural obstacles, and tread surface.
There are five actual ratings for trail difficulty in the IMBA TDRS guide:
White circle: A white circle trail is at least 48 inches wide, has a solid surfaced or hardened tread surface, has an average grade of about five percent, and does not have any technical features or natural obstacles.
Green circle: A green circle trail is at least 36 inches wide, has a stable or firm tread surface, has a trail grade of about ten percent, features several small unavoidable obstacles, features several bridges that are at least a width of 24 inches or wider, and has some technical trail features that are two inches high at least.
Blue square: A blue square trail is at least 24 inches wide and is mostly stable with some areas of variability. The grade runs between 10% to 15%. Obstacles are no more than 8 inches high.
Black diamond: A black diamond trail is at least 12 inches wide, has a variable tread surface, has an average trail grade of 15%, features medium obstacles of at least 15 inches, possibly has loose rocks, features bridges that are at least 24 inches wide, and has technical trail features that are four inches high.
Double Black Diamond: A black diamond trail is at least six inches wide, has a variable, unpredictable tread surface, and has an average trail grade of 20% or more, features medium high of at least 15 inches, possibly has loose rocks, features bridges that are 24 inches wide or smaller, and has technical trail features that are four inches high.