It is vital to a cyclist's riding strategy to determine hill grades prior to riding. A hill's grade is represented by a ratio between the vertical climb and the horizontal distance. Most common hills traveled by road bikes are less than a ten percent grade. Knowing this information can help a cyclist determine how quickly they can go up a hill and how much they can carry.
Measuring Grade

Rest your level on the ground and hold a ruler perpendicular against the end of the level. Raise or lower the level until it reads level or even.

Determine the vertical climb or rise of the hill. With your level and ruler in position, note the distance it takes for the level to be even on the ruler. For example when held level it is two inches above the roadway.

Measure the horizontal distance or the run. This would be the length of your level. For example your level is 24 inches long.

Divide the vertical climb number by the horizontal distance. Verify that both numbers are in the same unit, inches and inches. For example, two inches divided by 24 inches is 0.083.

Multiple your answer by 100 in order to convert it to a percentage to determine the grade percent of a hill. For example, 0.083 times 100 is 8.3%.
Tip
Plan your route before your ride to determine hill grade in advance.
Use a software or online resource with a database of hill grade in your area.
Warning
Take into account the difficulty of a hill grade and your physical abilities before climbing.