This article is from the Running FAQ, by Ozzie Gontang with numerous contributions by others.
Remember your trigonometry. Grade (or incline) = rise/run, opposite/
adjacent, height/length, or whatever you want to call it. For percent
grade, multiply this by 100.
degrees = arctan((percent grade)/100)
percent grade = tan(degrees) * 100
Thus, 1% incline is a mere .57 degrees, 5% incline is 2.9 degrees, 10%
incline is 5.7 degrees, and 15% incline is 8.5 degrees.