The 4% rule is a retirement rule of thumb with a real study behind it. It says: in your first year of retirement, withdraw 4% of your invested savings. Every year after that, withdraw the same amount adjusted for inflation. If history repeats, your money has a very good chance of lasting at least 30 years.
Say you retire with $1,000,000 invested. Year one, you take out $40,000. If inflation runs at 3%, year two you take $41,200, and by year ten you are taking about $52,200. The dollar amount climbs, but it always buys what $40,000 bought on day one. Meanwhile the portfolio stays invested, and in most historical periods it grows faster than you drain it.
Financial planner William Bengen crunched US market data back to 1926 in a 1994 paper and found that a first-year withdrawal around 4% would have survived every 30-year stretch he tested, including the ones that started right before crashes. A few years later, three professors at Trinity University ran a wider version of the test. In what became known as the Trinity study (published in the AAII Journal in 1998), a portfolio split 50/50 between stocks and bonds, tapped at 4% a year with inflation adjustments, survived 95% of all 30-year periods between 1926 and 1995.
Flip the rule around and it tells you how much you need. If 4% is a safe slice, then you need 25 times your annual spending saved up. Spend $30,000 a year and the target is $750,000. Spend $60,000 and it is $1,500,000. That is the math behind most FIRE numbers you will see online, including our own FIRE number calculator.
It is a rule of thumb, not a law, and the fine print matters. The studies used US market history, which has been unusually kind. They assumed a 30-year retirement; if you retire at 40, you might need the money to last 50 years or more, and many planners suggest starting closer to 3.5% for horizons that long (about $35,000 a year per million, or 28.6 times your spending). The original tests also ignored fund fees and taxes, both of which come out of your pocket in real life. And the biggest hidden risk is bad luck early on: a crash in your first few years of withdrawals hurts far more than the same crash later.
Treat 4% as a planning anchor, not an autopilot. People who do well with it tend to stay flexible: they skip the inflation raise after a bad market year, keep a cash cushion so they are never forced to sell in a crash, and recheck the math every year or two instead of setting it and forgetting it.
Sources: AAII Journal: what the withdrawal rate studies show; Trinity study overview.
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