“How long will my money last” isn’t really one question, it’s three stacked on top of each other: how much do you have, how much are you taking out each year, and what does the market do while you’re taking it out. Change any one of those and the answer shifts, sometimes by decades.

That’s why two retirees with identical account balances can have wildly different outcomes. One withdraws conservatively and retires during a strong stretch of markets. The other withdraws aggressively and retires right before a downturn. Same starting balance, completely different 25-year story.

Your withdrawal rate is the percentage of your portfolio you take out in the first year of retirement, with that dollar amount then adjusted upward each year to keep pace with inflation. It’s the single biggest lever you control, and it’s also the one most people get wrong by picking a number that feels reasonable rather than one that’s actually been tested against historical market data.

Our 4% Rule guide walks through where the commonly cited 4% figure came from and the specific 30-year market history it was built on. The short version: a 4% starting withdrawal rate, adjusted for inflation every year after, historically survived every rolling 30-year period in U.S. market history, including retirements that started right before major crashes. Push that rate up to 5% or 6%, and the failure rate climbs meaningfully, particularly for anyone retiring earlier than 65.

Here’s the counterintuitive part. Two retirees can have the exact same average annual return over 30 years and end up in completely different places, purely based on when the good and bad years happened. Someone who hits a market crash in year one or two of retirement, while still withdrawing money, does far worse than someone who hits that same crash in year 25, because early losses combined with ongoing withdrawals shrink the portfolio right when it has the least time to recover.

This is called sequence of returns risk, and it’s the reason retiring right before a downturn is genuinely more dangerous than retiring right before a strong bull run, even if the average return across your full retirement ends up identical either way. It’s also why some retirees build in flexibility, cutting discretionary spending during down years, rather than withdrawing a fixed amount no matter what the market is doing.

The 4% rule was modeled around a 30-year retirement, roughly what someone retiring at 65 and living into their 90s would need. Retire at 55 instead, and your money may need to stretch 40 years or more, which generally calls for a lower starting withdrawal rate, often closer to 3.3% to 3.5%, to keep the same margin of safety.

On the other end, someone retiring at 72 with a shorter expected time horizon can often sustain a somewhat higher withdrawal rate without meaningfully increasing their risk of running out. Time horizon isn’t just about your age, either. Family longevity, health, and how much you value leaving money behind for heirs all factor into how conservative you should be.

A portfolio sitting entirely in cash is safe from market crashes but nearly guaranteed to lose purchasing power to inflation over a multi-decade retirement. A portfolio sitting entirely in stocks has historically produced the highest long-term growth but comes with the sharpest downswings, which matters more in retirement than during your working years because of sequence of returns risk.

Most retirement research settles on some blend of stocks and bonds, often shifting to be somewhat more conservative over time, as the approach that’s historically balanced growth against the risk of a bad early sequence wiping out the portfolio too fast. The right mix for you depends on your own risk tolerance, other income sources, and how large a cushion you built in the first place.

The 4% rule assumes you withdraw a fixed dollar amount every year, adjusted only for inflation, regardless of what the market is doing. That’s useful for modeling and easy to communicate, but it isn’t how most retirees actually behave once they’re living through a real downturn. In practice, many retirees cut back on discretionary spending, travel, big purchases, dining out, during years when their portfolio is down, and loosen up again once it recovers.

This kind of flexible approach, sometimes called a “guardrails” strategy, generally extends how long a portfolio lasts compared to a rigid fixed withdrawal, because it reduces the damage done during the exact years when sequence of returns risk is doing the most harm. The tradeoff is less predictable annual income, which suits some retirees and genuinely stresses out others. Neither approach is wrong, but it’s worth deciding in advance which kind of retiree you are, rather than discovering it for the first time during an actual market crash.

Standard withdrawal rate modeling generally assumes your spending grows at a steady pace with inflation. Healthcare costs, particularly for retirees, have historically outpaced general inflation for extended stretches, and they tend to spike unpredictably rather than rise smoothly, a major diagnosis or a long-term care need can reshape a spending plan overnight in a way the underlying models don’t anticipate.

This is one reason many retirees build in a separate buffer specifically for healthcare, on top of their general withdrawal rate calculation, rather than assuming their day-to-day spending model already has enough slack absorbed into it. It’s also part of why retiring before Medicare eligibility at 65 tends to carry more financial risk than retiring after, since private health coverage in the interim is both expensive and hard to predict several years out.

Social Security, a pension, rental income, or part-time work in early retirement all reduce how much your portfolio needs to cover on its own, which directly extends how long it lasts. If your total spending is $50,000 a year but $20,000 of that comes from Social Security, your portfolio is really only supporting $30,000 a year of withdrawals, a meaningfully lower burn rate than the headline spending number suggests.

This is a common point people miss when they picture their retirement math: running a withdrawal-rate calculation against total spending, rather than spending minus other guaranteed income, makes the situation look more fragile than it actually is.

Say you retire with $900,000 saved, and your total spending plan is $54,000 a year, with $18,000 of that covered by Social Security. Your portfolio needs to cover $36,000 a year, which works out to a 4% starting withdrawal rate on your $900,000 balance, right at the historically tested threshold for a 30-year retirement.

Now imagine you didn’t account for Social Security and ran the math against the full $54,000. That would put your starting withdrawal rate at 6%, a rate with a meaningfully higher historical failure rate over a 30-year period. Same portfolio, same spending plan, but a very different risk picture depending on whether you did the subtraction correctly.

It’s tempting to plug a few assumptions into a calculator, get a reassuring result, and consider the question settled. The honest version of this exercise involves testing more than one scenario: a below-average market return, a longer-than-expected lifespan, an unplanned medical expense in your 70s. If your plan survives the pessimistic version, not just the middle-of-the-road one, that’s a genuinely stronger signal than a single optimistic projection.

It’s also worth revisiting the math whenever something material changes, a new health diagnosis, a spouse passing away, an unplanned early retirement, rather than only checking in once a decade. The inputs shift more than people expect, and a plan built on assumptions from ten years ago is really a different plan than the one you’re actually living.

Before you can estimate how long your savings will last, you need a clear target for what you’re working toward in the first place. Our guide on how much money you actually need to retire walks through the calculations for setting that target, and our Retirement Calculator projects your balance forward based on your current savings, contributions, and expected growth, so you can see whether your trajectory lines up with the withdrawal rate you’re planning to rely on.

None of these numbers are set-and-forget. Market performance, health, spending habits, and life circumstances all shift over time, which means the math is worth revisiting every few years rather than calculated once at 60 and never touched again.