As a retired highway engineer, much of what I've read here is nit-picking.

First off, highway grades are frequently a combination of grades or in the case of old roads, erratic grades at best. Using topo maps will yield an average grade but won't account for those short stretches of leg searing pitch or transitional vertical curves. Measuring points with a level or other gradient device is only accurate at that point and may change a 100' up the hill.

In other words, significant digits past a whole number, or a tenth at the most is all that one can accurately expect without surveying equipment. Having said that, either formula given in previous posts will yield a percent grade more than accurate enough for real-life purposes.

I had a cyclist in my area ask me the grade of a local road and I went out with a 10' level and an engineering rule to take some quick and dirty measurements. I measured the 1/2 mile grade in three places and got grades of 7.8, 9.2, and 10.5. All that in less than 1/2 of a mile on a relatively new road. So is it a 7.8 or a 10.5% grade? Is it an average of the three? Regardless of what one calls its grade, it's still going to ride the same.