There is not an answer to this that everyone can agree on, because believe it or not, folks can't agree what constitutes a mountain vs. a hill. To flatland Midwesterners, anything more than 100 ft. tall is a small mountain, but the other extreme would be Switzerland, where the definition there is a mountain stops being a hill anytime it rises 600 meters above its surroundings and has some rocky cliffs on it, even if they are covered mostly by trees. Overall, if we take the reasonable-man approach, most observers would generally agree that California is about 45% mountain ranges of one type or another.
