[dmoy]

It's not 4% ROI, it's 4% withdrawal rate. And even that is not super safe if you're retiring for many decades (the RE part).

You can use a variety of online calculators to back test a 4% withdrawal rate - maybe 80% safe, but 20% of the time you'll go broke before dying.

[IvoCass]

Why isn't it safe? Or where can I read more on that?

[dmoy]

Sure, so there's a few things to look at.

The first is going back to the origins of the 4% number in the first place, the trinity study. The parameters for that were a 30 year retirement period, and success was "not completely run out of money after 30 years, 95% of the time". If you extrapolate from that original study, if you retire for more than 30 years (FIRE includes retiring early), success drops from 95%.

There's articles exploring that, e.g.

https://www.fiphysician.com/safe-withdrawal-rate-early-retir...

https://www.madfientist.com/safe-withdrawal-rate/#:~:text=Th...

And then there are a variety of online calculators where you can play with the numbers yourself.

The other elephant in the room is pre-Medicare healthcare costs.