Most of us find that climbing up a 500-foot hill which is steep takes more out of us than a 500-foot hill which is gentle (think Pinehurst vs. Redwood Rd). We can express the steepness of any hill by determining it's grade which is a measurement of the angle of inclination expressed at a percentage.
To calculate the grade of a section of road, simply divide the rise (vertical_climb) by the run (horizontal_distance).
grade % = vertical_climb / horizontal_distance * 100
Both vertical_climb and horizontal_distance should both converted to the same measurement units (usually feet). So if a hill goes up 500 feet in one mile, then we can first convert the mile to 5280 feet -- so the grade is then 9.46% (500 / 5280 * 100).
What does this "grade" number mean?
- 0% grade is exactly flat (and a negative grade, less then zero, is downhill).
- 2-3% grade does not seem very steep, but it's enough to substantially reduce forward speed, and for most riders it will absorb more than half their power output.
- 5-7% grade is enough to cut speed to well under half, and absorb 75-85% of a rider's power output (leaving less than 15-25% to fight air resistance and rolling friction).
- 9-11% grade, and anyone who is not a great shape and frequent rider is off the bike walking&,dash;and anyone who is not a racer is reaching for all the extra power they've got.
- 20+% grade, and you're out of the saddle sucking wind or weaving methodically up the road.
Note: the term grade should not be confused in any way with angle. They are two very different things. For example, a flat surface can be referred to as a 0% grade or a 180-degree angle: there is no change in height as the road continues. (There is no rise over the run.) A 100% grade would be equivalent to a 45-degree angle or, in other words, the rise would be equal to the run.