It does depend somewhat on where you are now. Second year of grad school? Fresh out of college? Fresh out of high school? Some people go for that Ph.D. to g

It does depend somewhat on where you are now. Second year of grad school? Fresh out of college? Fresh out of high school? Some people go for that Ph.D. to get a good-paying job in a big institution, typically bumping along a professor's pet project for a few years before they publish a lengthy, painstaking, and never-read-again dissertation. You don't have to go at it that way. Consider doing both at the same time. A tiny minority of Ph.D. students will drive at something that is both their passion and capable of supporting a great business model. If you have a decent professor (the kind that wants to change the world as opposed to the one that wants slaves) it is possible to go that way. Alternatively, you could do both at the same time and leave with a Master's or no degree once your startup requires more of your time. I believe the founders of Google followed this approach. My first startup I did in parallel with graduate school. The two subject matters had zero overlap, and that had its benefits too.