The 4% rule: what it is, and what it is not

The 4% rule is the most cited guideline in retirement planning. It says that if you withdraw 4% of your portfolio in the first year of retirement, and then increase that amount each year to keep up with inflation, your money has historically had a good chance of lasting around 30 years. The idea is attractive because it is simple. This article explains where it comes from, how to use it, where it can fail, and ways to make it more resilient.

Where the rule comes from

The guideline is associated with research from the 1990s by financial planner William Bengen, and with a later academic paper often called the Trinity Study. Both looked at historical US returns for portfolios of shares and bonds, and asked what starting withdrawal rate would have survived every 30-year retirement period in the data. The answer for a diversified portfolio was in the region of 4%.

That is an important detail. The rule describes what would have worked in a particular set of historical periods. It does not predict what will work in the future.

How it works in practice

Start with your portfolio at the beginning of retirement. In the first year, withdraw 4%. In each later year, take the same amount and increase it by the previous year's inflation, regardless of how the portfolio performed.

A worked example. With a portfolio of 1,000,000, the first-year withdrawal is 40,000. If inflation is 3% over the next year, the second-year withdrawal is 41,200. If it is 3% again, the third year is about 42,436. The target of 25 times annual spending is the same rule turned around: spending of 40,000 divided by 4% is 1,000,000.

Safe withdrawal rates for different portfolio sizes and lengths

The same arithmetic shows how sensitive the target is to the rate you choose. If the amount your portfolio must cover is 30,000 a year:

• At 4%, you need about 750,000.
• At 3.5%, about 857,000.
• At 3%, about 1,000,000.

A lower rate is more cautious and needs more capital. A higher rate needs less capital but carries more risk of running short.

Why it is only a guideline

• It is based on history. It reflects US data over particular decades. Other countries and future periods may differ, and starting valuations matter.
• It assumes about 30 years. People retiring in their 40s may need their money to last 40 to 50 years or more. Some analysts suggest a lower starting rate, often in the 3% to 3.5% range, for very long retirements.
• It assumes a fixed pattern of spending. Real spending is rarely exactly inflation-adjusted every year.
• Fees and taxes reduce the amount you can safely spend. A 1% annual fund fee, for example, takes a meaningful share of a 4% withdrawal.
• Sequence risk. Poor returns in the first few years can do outsized damage. We cover that in a separate article.
• It treats a plan that ends with money left over as a success. Many historical cases ended with far more than the starting amount, which means a rigid rule may have been too cautious.

Making it more resilient

Many planners recommend a flexible approach instead of a rigid one.

• Spending guardrails. Set an upper and lower limit. If your withdrawal rate drifts above a threshold after a market fall, cut spending by a set percentage. If it falls well below, allow a raise.
• Skip the inflation adjustment in a year after a poor market, then catch up later.
• Keep a cash or short-term reserve equal to one or two years of spending, so that you need not sell shares in a downturn.
• Use other income. A pension, part-time work or rental income reduces how much you must draw.
• Diversify. A mix of shares, bonds and cash can reduce the severity of falls.
• Reassess regularly. Look at your plan annually and adjust.

Common mistakes

• Treating 4% as a promise.
• Applying it to a 50-year retirement without any margin.
• Forgetting fees, taxes and the effect of inflation on medical and other costs.
• Being unwilling to adjust spending in a bad year.
• Using the rule on a portfolio that is not diversified.

A short checklist

• I understand that 4% is a historical guideline, not a guarantee.
• I have tested a lower rate, such as 3.5%, for my situation.
• I know what spending I could cut in a poor year.
• I hold a reserve so I need not sell during a downturn.
• I will review the plan every year.

The 4% rule is useful as a starting point because it gives you a number to aim for and shows how spending, capital and withdrawal rate fit together. Treat it as the beginning of your planning, not the end.