Found on the San Francisco Exploratorium‘s website, a special series on the Science of Cycling. Check out some of their interesting articles on the history and science of bicycles. In the section about aerodynamics, they’ve come up with a calculation of “Aerodynamic Drag And Propulsive Power Of A Cyclist“. Simply input your velocity, the winds’ velocity (+ for a headwind and – for a tailwind), your weight, and the grade of the road–and the formula will give you the relative velocity, drag, and the power it takes to maintain a constant velocity. Despite a few notes as to why this calculation is not perfect, it is still interesting to see the Watts your 156lb body will expend as you ride at 20mph into a headwind of +12mph up a hill with a 3% grade.
