A hill's degree, grade or % are all different ways to calculate how steep it is. Degree is a straight forward measurement based on a protractor scale, as in 360 degrees in a circle. % is based on what percent of 90 degrees a slope is. Grade is usually misinterpreted because of the 2 different ways to calculate an elevation change.
So a 15% grade is 15% of 90 degrees or about 13 degrees