The days and jumps and deadlifts and calf raises rolled on, rep by rep, protein shake by protein shake. Six months became seven, then eight. To protect my right hand, I began wearing a canvas gardening glove with the fingers cut off. It soon became stained with blood—the equivalent of Curt Schilling’s bloody sock, but with one-millionth the significance. The rims where I toiled belonged to me now, such that I barely noticed the toddlers wobbling nearby, the skateboarders swirling around me as day turned to dusk, the elderly couple ambling arm in arm, looking for all the world like my wife helping me to the shower on the morning after a double day.

Cameras of that era were too crude to capture the split second when the rules of both Newton and Naismith were bent, so it was fortuitous that New York Times writer Arthur J. Daley was at the Y that day covering the tournament that would decide which Americans sailed to Berlin for the Olympic debut of the 45-year-old sport. This new “version of a lay-up shot,” Daley wrote, “left observers simply flabbergasted. Joe Fortenberry, 6-foot-8-inch center . . . left the floor, reached up and pitched the ball downward into the hoop, much like a cafeteria customer dunking a roll in coffee.”
I mean, I think you can probably improve your vertical some in a month. I think, though, that for most normal people who aren’t teenagers who are trying out for their basketball team, who don’t have all that time on their hands, I think there’s a much saner way to go about it, where you’re steadily improving your vertical over a period of time. You know, there’s a lot of this kind of slightly crazy, kamikaze, self-improvement type of thing, whether it’s trying to jump higher or do anything else. I’m sure those things work to some extent, but it’s not the way I would have wanted to go about it.
This calculator tells you how much you need to jump to dunk a basketball. It will also give you an estimated force required to jump that high. The more you bent your knees the less force you'll need but you will need a lot of energy to take you from that position to the top. You can increase your vertical by training your legs to be able to deliver that much force.
To perform two-foot dunks, jumpers bend their knees very deeply and spend a lot more time on the ground loading the jump. This increased time during takeoff is useful because it allows the athlete to transfer force into the ground thereby improving height. Using this approach makes it more difficult to transform speed into jump height making a fast approach far less useful than in one-foot jumping.

A second, more efficient and correct method is to use an infrared laser placed at ground level. When an athlete jumps and breaks the plane of the laser with his/her hand, the height at which this occurs is measured. Devices based on United States Patent 5031903, "A vertical jump testing device comprising a plurality of vertically arranged measuring elements each pivotally mounted..." are also common. These devices are used at the highest levels of collegiate and professional performance testing. They are composed of several (roughly 70) 14-inch prongs placed 0.5 inches apart vertically. An athlete will then leap vertically (no running start or step) and make contact with the retractable prongs to mark their leaping ability. This device is used each year at the NFL scouting combine.