Ask how much you need to retire and someone will eventually say "use the 4% rule." It is a genuinely useful rule of thumb, but it gets repeated far more often than it gets explained. Knowing where it comes from, and where it bends, makes it much more useful.
What the rule says
The 4% rule says that if you withdraw 4% of your portfolio's value in your first year of retirement, then adjust that dollar amount for inflation each year afterward, your savings have historically been very likely to last around 30 years.
Note the structure: 4% sets only the first year's withdrawal. After that you are not taking 4% of whatever the portfolio happens to be worth. You take last year's dollar amount plus an inflation bump, whether markets rose or fell.
Where it came from
The rule grew out of research in the 1990s, most famously the Trinity study, in which professors at Trinity University tested how various withdrawal rates would have survived across historical periods of US stock and bond returns. Their finding was that an initial withdrawal rate of about 4%, with inflation adjustments, survived the overwhelming majority of historical 30-year retirements, including ones that began just before terrible markets.
That framing matters. The rule is a summary of what worked in the past. It is not a promise about the future, and it was never designed to be.
The 25x shortcut
Flip 4% around and you get the version most people actually use for planning: multiply your desired annual spending by 25. That works because 4% is one twenty-fifth, so a portfolio of 25 times your spending yields your spending at a 4% withdrawal rate.
A concrete example: if you want your portfolio to cover $50,000 per year of spending, the 25x shortcut points to a target of $1,250,000. Check it in the other direction and it holds: 4% of $1,250,000 is $50,000. That single multiplication turns a vague ambition to "save enough" into a specific number you can plan toward with our retirement calculator.
What the rule assumes
The historical success rate rests on a few conditions:
- A diversified portfolio. The research assumed a substantial allocation to stocks alongside bonds. A portfolio sitting in cash does not support a 4% inflation-adjusted withdrawal.
- A roughly 30-year horizon. The rule was tested against traditional retirement lengths, not 50-year early retirements.
- Disciplined, inflation-adjusted withdrawals. The math assumes you follow the schedule, not that you double your spending in good years.
Fair criticisms
The rule has real weaknesses. The biggest is sequence-of-returns risk: two retirees can earn the same average return over 30 years, but the one who hits a deep market slump in the first few years, while withdrawing, can run out of money even though the averages looked fine. Early losses plus withdrawals compound against you.
Longer retirements strain it too. Someone retiring at 45 may need the money for far more than 30 years, which is why many early retirees, including people pursuing strategies like the one in our Coast FIRE calculator, plan around more conservative assumptions. And some researchers argue that with today's market valuations, a lower starting rate such as 3.5% is a safer default, which raises the savings target accordingly.
How to actually use it
Treat the 4% rule as a planning compass, not an autopilot. It is excellent for answering "roughly how big should my number be?" decades in advance. It is weaker as a rigid spending policy once you are retired, when a flexible approach, spending a little less after bad market years, tends to be far more robust. Start with 25x your spending, stress-test it with a calculator, and revisit the plan as your real life and real markets unfold.