Slope is measured in degrees for road building grade is measured in percent. A 45 degree slope is a 100 percent grade. The formula is based on:

The slope is the Tangent of the angle.

Slope is rise/run, and the % grade is 100 times the slope. So a slope of 1/1 would be a 100% grade, Let us say 100 feet with 100 feet of gain, which is indeed a 45 degree angle.

So if you know the degree of the angle, just enter it into your scientific calculator then hit Tangent. Or use your Iphone in calculator mode and turn it on its side and hit "TAN"