How tall should you be to dunk a basketball?

A rim is 10 feet. Add 6 inches for the fact that your hand has to be over the rim to dunk. That's 126 inches. Now take your height in inches and multiply that by 1.34 for standing reach. Subtract that from 126 and that's the vertical jump you need to dunk. 

At 5'10 the VJ needed is 32.2 inches. Most pro athletes should be able to do that. There are many pro athletes with VJs beyond 50 inches. For reference, Refrigerator Perry had a 42 inch VJ at well over 300 pounds. 

At 6'2 it's 26.84 inches. Anyone who is reasonably athletic and not overweight shouldn't have a problem reaching 27 inches. 

At 6'5 it starts getting really easy at 22.82. Even the most unathletic person on the planet can jump 24.5 which means Tom Brady can dunk. 

At 6'8 it drops below 20 inches. If you're under 500 pounds and have 2 legs you should be able to jump 20 inches. 

A 7'0 it drops to 13.44 which means standing flat footed you would be about 7 inches below the rim. Someone with exceptionally long arms might be able to touch the rim flat-footed. 

If you can walk right up to a regulation basket and put the ball in without even having to jump, you're hired.

Well unless you're like Spud Webb or Nate Robinson, you have to be at least 6'0 or taller to dunk if you're untalented.