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.
Start on a lower hoop and practice on that, just to get the feel of dunking. Jump height is one thing, but you would be surprised at the number of people that find it hard just to slam the ball into the basket, even if they are high enough. Make sure the hoop is high enough for you to only touch the rim. Different jumping styles and distances from the basket can change your vertical drastically and could be the difference between a rim-block and a slam. Keep progressing and eventually you will see results. Good luck!
Vertical jump measurements are used primarily in athletic circles to measure performance. The most common sports in which one's vertical jump is measured are track and field, netball, basketball, football, and volleyball, but many sports measure their players' vertical jumping ability during physical examinations. In addition, single and multiple vertical jumps are occasionally used to assess muscular strength and anaerobic power in athletes.