Divide vertical rise by horizontal run and multiply by 100 to get percent slope. In other words, it's 100 times the tangent of the angle.
Filbert Street and 22nd Street in San Francisco are two of the steepest navigable streets in the Western Hemisphere at a maximum gradient of 31.5% (approximately 17°). One section of Stanyan St, at Belgrave, hits 33%. The sidewalk-only section of Broderick street (between Broadway and Vallejo streets) is actually steeper, at 38% grade. http://en.wikipedia.org/wiki/Filbert...(San_Francisco)