Spud Webb won the 1986 dunk contest at a listed five-foot-seven in shoes. That is why his dunks are still so hard to believe. From the best contest image in the source video, I put his jump at roughly 42 to 44 inches. The often-repeated 42-inch figure may be close, but I have not found the original measurement behind it.
Use the net, not just the rim
The NBA lists Spud at five-foot-six. I allow about an inch for shoes in this film calculation, giving him a working height of five-foot-seven. For his head to reach a ten-foot rim, he would need to jump about 53 inches. That is not what the contest footage shows. His head gets closer to the bottom of the net, which gives us another reference point.
Use an approximately 18-inch net and the bottom hangs around 102 inches above the floor. At a working height of 67 inches, Spud needs about 35 inches just to put the top of his head at the bottom of the net. In the contest photo I use, his head appears another seven to nine inches higher. That puts the estimate at 42 to 44. It is more grounded than judging the dunk by how high his hand reaches.
Why I leave a range
The net is the weakest part of that arithmetic. It can swing, curl or hang at a different length, and the photo is not a calibrated side view. A couple of inches either way would be easy to miss. NC State reportedly listed Spud at 42 inches, but the video does not establish the test protocol; Spud also said he did not know his own exact vertical. I would treat the college figure and the footage estimate as separate pieces of evidence, not as two confirmations of one official result.
The height comparison explains why this contest was so wild. A six-foot-six player can get his head to the bottom of the net on a jump about eleven inches lower than Spud needs. The same frame near the hoop does not mean the same vertical for two players of different heights. See the full NBA table for those comparisons; the Jordan article tackles another famous number that lacks a verified test.