The 4% Rule Explained
The 4% rule is the single most-cited number in retirement planning, and it's the default our retirement calculator uses. But most people who repeat it don't know where it comes from, what it actually assumes, or when it stops being a good idea. Here's the short version.
Where the number comes from
In 1994, financial planner William Bengen studied historical U.S. market returns going back to 1926 and asked a simple question: if a retiree withdrew a fixed percentage of their portfolio in year one, then adjusted that dollar amount for inflation every year after, what was the highest percentage that would have survived every 30-year period in history without running out of money? The answer he landed on was close to 4%.
The idea was popularized further by the "Trinity Study" in 1998, which tested different withdrawal rates and portfolio mixes of stocks and bonds, and found that a 4% rate had a very high historical success rate over 30-year periods.
How to use it
The math runs in reverse of how it sounds. Instead of asking "what can I withdraw," flip it around to size your goal: divide your annual expenses by your chosen withdrawal rate.
Example: $40,000/year ÷ 4% = $1,000,000 needed.
This is exactly the calculation our calculator runs, using your monthly expenses instead of annual ones.
Why some people use a lower rate
Bengen's original research was based on a 30-year retirement, starting around age 65. If you're planning to retire early — say, at 40 or 45 — your money needs to last 50 years or more, not 30. A lower withdrawal rate, such as 3% to 3.5%, gives you a larger margin of safety for a longer time horizon and for the possibility that future market returns are lower than the historical average used in the original study.
A higher withdrawal rate, such as 5%, might make sense if you have flexibility to cut spending in bad market years, other income sources like a pension, or a shorter expected retirement.
What the rule doesn't account for
- Large one-time expenses, like a home repair or medical event
- Changes in spending as you age (often higher early, lower in the middle years, higher again for healthcare later)
- Taxes on withdrawals, which vary a lot depending on account type and country
- Sequence-of-returns risk — a market crash in your first few retirement years can hurt more than the same crash happening later
Use the 4% rule as a starting point, not a guarantee. Our calculator lets you adjust the withdrawal rate directly, so you can see how a more conservative number changes your target and your timeline.
A worked example across withdrawal rates
The difference between a 3% and a 5% withdrawal rate is bigger than the two percentage points suggest, because you're dividing the same annual expense by a smaller number. Here's the same $36,000/year in expenses ($3,000/month) calculated at four common withdrawal rates:
| Withdrawal rate | Nest egg needed | Vs. the 4% baseline |
|---|---|---|
| 3.0% | $1,200,000 | +$300,000 |
| 3.5% | $1,028,571 | +$128,571 |
| 4.0% | $900,000 | baseline |
| 5.0% | $720,000 | −$180,000 |
Moving from 4% to 3% doesn't just add "a little more margin" — it adds a third more to your target. That's the real cost of planning for a longer or less certain retirement, and it's why the rate you choose in the calculator matters more than almost any other input.
Frequently asked questions
No. Bengen's research modeled pre-tax portfolio withdrawals. If your retirement accounts are taxed on withdrawal, you'll need to withdraw more than your target spending amount to cover both expenses and the tax bill — effectively aiming for a larger nest egg or a lower effective withdrawal rate.
The original research used U.S. market data. Later studies applying the same method to other countries' market history found lower safe withdrawal rates in several cases, since not every country's history was as favorable as the U.S. one used in the original study — one more reason a lower rate is often recommended outside the U.S.
Both views exist among researchers. Some argue lower expected future returns and longer lifespans make 4% too risky and favor 3-3.5%. Others argue the original study already used worst-case historical periods, and that modest spending flexibility in bad years makes 4% (or higher) perfectly workable. There's no consensus — it depends on your flexibility and risk tolerance.