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« on: September 22, 2018, 04:22:11 PM »
It's based on the performance history of the US stock market.
Certain calculators such as FIREcalc take your planned spending amount and your starting amount and create multiple historical projections on how it would have fared during different 30 year periods in the market and then calculates in how many of those you would have made it to the end of those 30 years without running out. the 4% rule simply comes from the fact that at about 4% you have about 95% chance of still have some amount of money left after 30 years. It's a little arbitrary in the sense that it's simply that enough people agreed with 95% chance of success being a good level of risk and so the rule spread.
As such there's also a 3.5% rule which is that if you have a SWR of 3.5% you would never have run out of money after 30 years in any 30 year period of the US stock market's history.
This would of course assume a 100% stock portfolio and one matching the stock market as closely as possible. Often something like a S&P 500 index funds etc.
There is also a camp of people that say the 4% rule is way to conservative for early retirees as you are still capable of working and you could always take on a side job if there is a market downturn and reduce your withdrawal rate during down times to avoid some of the losses that lead to you running out of money.
In the end it comes down to how much risk you are personally comfortable with, as you can see from my own plan I'm pretty risk averse in that I'm shooting for 4%, but then planning on coasting to 3.5% before fully considering myself FIRE. I'm comfortable enough with quitting my current job and moving to less lucrative things once I do reach 4% though. As for my actual allocation between stocks and bonds I'm far more comfortable with risk as I have about 95% in stocks, I feel comfortable that once I reach 3.5% SWR even with an allocation of 100% stocks there isn't really any risk of running out.