«Safe» withdrawal rate (SWR) and the 4% rule

You have invested capital and need to live off it for the next thirty years. How much can you withdraw each year without the risk of running out of money by the end of the period — or of your life? It's the central question of any retirement plan, and also the hardest one to answer with a single number. The «safe» withdrawal rate, or SWR, is the most common attempt at an answer. The quotes around «safe» mark a factual point: the rate describes what held in the historical cases analysed, not a promise about the future.

What the «safe» withdrawal rate actually is

SWR is the percentage of your starting capital you withdraw in the first year of retirement, then adjust for inflation every year after that. A $500,000 portfolio, a 4% SWR: you take out $20,000 in year one. If inflation runs at 3%, year two you withdraw $20,600, not 4% of a revalued balance, just the same purchasing power as year one.

The capital stays invested the whole time, typically a mix of stocks and bonds. It's not a checking account draining in a straight line. It keeps generating returns, and in plenty of historical scenarios it ends up larger than it started, even after thirty years of withdrawals.

The 4% rule, and where it came from

In 1994 financial adviser William Bengen published an analysis that reshaped how people think about retirement. He tested different withdrawal rates against every possible 30-year period in US market history going back to 1926. Four percent was the highest rate that held up in nearly every scenario, including the worst one on record: someone retiring in 1966, walking straight into a decade of high inflation and flat markets.

Four years later, three professors at Trinity University published what's now known as the Trinity Study: same idea, a wider dataset, portfolios with different stock-bond splits. They confirmed 4% worked in over 95% of historical 30-year scenarios for a balanced portfolio. From there the number turned into a standard, almost a piece of personal-finance folklore.

Why 4% isn't a magic number

It's a reasonable starting point, not a guarantee. Three things complicate it.

The first is sequence risk: once withdrawals begin, the order returns arrive in weighs at least as much as their average — and more in the early years. Two retirees with identical average returns over thirty years can end up in completely different places if one starts with a crash and the other doesn't, because withdrawals during the crash lock in losses at low prices, permanently shrinking the base the portfolio needs to recover from.

The second is that 4% is a historical result, not a law of physics. It holds for the sample it was calculated on, US stock and bond markets across the twentieth century. Whether it holds elsewhere, or in a world where expected returns for the next thirty years look lower than the last thirty, is an open question.

The third is that Bengen himself, in a book published in 2025, revised his own estimate upward to 4.7%, adding more asset classes to the original portfolio (small caps, micro caps, international equities). Pulling the other way, a 2013 study by Finke, Pfau, and Blanchett recalculated historical returns using the much lower valuations and rates of that period, landing at around 3%. The 4% figure survives as a useful reference precisely because neither camp is entirely right. Where you land depends on how much risk you're willing to take of having to cut spending if things go badly.

There's a technical distinction the phrase "4% rule" tends to hide. A 95% success rate in the Trinity Study doesn't mean every single historical year held up at exactly 4%. The so-called safemax, the highest withdrawal rate that survived even the worst historical 30-year scenario (again, the 1966 retiree, on a 50/50 stock-bond portfolio), sits at around 4.15%. Four percent is the threshold chosen because it works for the large majority of cases and is easy to remember, not because it's the most critical point ever observed: the historical worst case, in fact, held up just above 4%.

Long horizons: the FIRE case

Thirty years covers someone retiring at 65 just fine. Someone chasing FIRE and quitting work at 40 is looking at a potential horizon of fifty years or more, and the math shifts.

A withdrawal rate that's sustainable over thirty years isn't automatically sustainable over fifty or sixty. The longer the money has to last, the lower the starting withdrawal needs to be to absorb an unlucky sequence. The gap between the two horizons isn't cosmetic: the Early Retirement Now blog, already cited in BacktestFolio's sequence-risk methodology, calculates that a 4% withdrawal on a 50/50 stock-bond portfolio holds up in about 95% of historical scenarios over thirty years, but only 65% over sixty. Hence its entire series of posts dedicated to exactly this gap between the classic 4% and realistic rates for much longer retirements.

SWR, PWR and LTWR: three different thresholds

SWR answers one specific question: what's the highest rate that, in the worst historical scenario, doesn't exhaust the capital before the chosen horizon ends? It accepts that by the end of the period, the balance might be close to zero.

The PWR, or Perpetual Withdrawal Rate, is more conservative. It's the rate that preserves the full starting capital in real terms, even in the worst-case scenario. Useful if you want to leave an inheritance, or if you never want to see your balance drop below where it started.

Sitting between the two is LTWR, the arithmetic average of SWR and PWR: neither bare survival nor full preservation, a middle ground.

A numbers example

Take two people retiring with the same capital, $600,000, the same 4% withdrawal rate, the same 60/40 allocation. One retires in 1982, the other in 2000.

The first rides one of the longest bull markets in US history. After thirty years of withdrawals, the portfolio is worth more than double what it started with, in real terms.

The second walks straight into the dot-com bust, then 2008. The same starting 4%, applied to that scenario, would have drained the capital much faster, coming close to zero by the end of the thirty years. Same withdrawal rate, same expected long-run average return, completely different outcomes. That's sequence risk playing out, not a theoretical worry.

In BacktestFolio

The Withdrawal Rate Analysis calculates SWR, PWR and LTWR on your own portfolio using two methods side by side. The first takes every possible starting month in your historical series, roughly 360 scenarios with thirty years of data each, and checks how many of them the capital would have survived. It's the same logic as the Trinity Study, applied to the portfolio you actually built rather than a generic index. The second generates thousands of forward-looking scenarios with Monte Carlo simulation, using Stationary Bootstrap by default, to explore market conditions that history hasn't produced yet.

You can stretch the horizon out to 80 years, the FIRE case described above, apply a tax rate on withdrawals to see net spending power, and turn on a glidepath that gradually raises the equity share in the early retirement years specifically to cushion sequence risk. None of it is locked behind a fixed, preset number.

You can try it free right after any backtest, on your own real portfolio: there you get both the historical path and Monte Carlo. For a guided path built around decumulation, with the 25x rule as a quick estimate and then a Monte Carlo stress test in the Planner, there's the FIRE calculator. On the Free plan, backtest history is capped at 10 years; Plus and Pro unlock the full available history.

Frequently asked questions

Does the 4% rule hold outside the United States?

The number comes from US market data. Countries with different market histories, ones hit by wars or currency crises that never show up in the American sample, have often produced lower historical SWRs. It's one reason it pays to test the rate against your own actual portfolio rather than apply the number from memory.