The 4% Rule is a retirement spending guideline suggesting that you can withdraw 4% of your portfolio in your first year of retirement and adjust that amount for inflation each year, with a high probability of your money lasting at least 30 years. It was developed from historical market data by financial planner William Bengen in 1994.

Think of it as a starting point for figuring out how much you can spend without outliving your savings. While widely used, the 4% Rule has its critics, and factors like market conditions, portfolio mix, and a longer-than-average retirement can all affect whether it holds up for your specific situation.