What Monte Carlo Success Rate Should You Actually Target for Early Retirement?

Run any retirement calculator and it hands you a single number: 82%, 94%, 100%. Most people treat it like a test score. Below 90 feels like you failed something. This is the wrong way to read a Monte Carlo result, and for anyone planning a 40 or 50-year early retirement instead of a standard 30-year one, it's an expensive mistake. It pushes people to work extra years chasing a few extra percentage points of certainty they never actually needed.

Let's get into the actual math, then translate it into a number you can use.

What "success rate" is actually measuring

A Monte Carlo retirement simulator doesn't predict the future. It takes historical market data and reshuffles it into hundreds or thousands of alternate versions of history your retirement could have landed in, then checks how many of those versions leave you with money at the end.

Our own engine, the one behind the free calculator at the bottom of this article, does this by running 1,000 independent trials sampled from 98 years of real U.S. market returns (1928 to 2025), using block bootstrap sampling in 5-year chunks so it preserves real crashes, real recoveries, and the real relationship between stock and bond returns, rather than treating every year as an independent coin flip. Everything is calculated in inflation-adjusted terms, so a "success" means your spending power held up, not just the nominal dollar figure.

The "success rate" is simply: of those 1,000 alternate histories, what percentage never ran your portfolio to zero. That's it. It's not a probability assigned by a statistician who has looked at your specific life. It's a description of how your specific withdrawal rate, asset allocation, and time horizon would have fared across a wide sample of the market conditions we've actually lived through.

Which means the number is only as meaningful as the assumptions feeding it, and one of those assumptions, time horizon, matters enormously more than most people realize.

Why 95% was calibrated for a 30-year retirement

The success rate targets most people have absorbed (90%, 95%, "why not just aim for 100%") trace back to research built around a 30-year retirement: retire at 65, plan to age 95. William Bengen's original 1994 paper and the Trinity Study that followed both used that window, and a 4% withdrawal rate landed around 95% success in that context (Pfau, Retirement Researcher).

A FIRE retirement is a different shape entirely. Retire at 35 or 40 and you're underwriting a 50 to 60-year spend-down, not 30. More years means more exposure to the handful of historically bad sequences that wreck a portfolio, and more years of inflation compounding against a fixed real withdrawal. We ran this directly through our simulation engine, holding everything constant except the number of years, at a 4.0% withdrawal rate on a $1.5M portfolio, 70/30 stock-to-bond allocation:

Same withdrawal rate, same portfolio, same allocation. The only thing that changed is how many years the money has to last, and that alone knocked 13.6 points off the success rate. If you retire early and plug your numbers into a calculator expecting a 30-year retirement's worth of comfort, the honest FIRE number is going to look scarier than you were braced for. That's not the tool being pessimistic. It's the math of a longer time horizon.

The real lever: withdrawal rate over a 50-year horizon

Here's where it gets more useful, because withdrawal rate is the variable you actually control. Holding the 50-year horizon fixed and moving only the withdrawal rate:

Notice the shape of that curve. Dropping from 4.0% to 3.5%, a 12.5% cut in annual spending, buys you 8 points of success rate. Dropping from 3.25% to 2.75%, a comparable-sized cut, buys you only 4.6 points. Each additional slice of safety costs more than the last one and returns less. This is the mathematical version of the argument Dr. Jim Dahle makes at White Coat Investor: chasing a 99% or 100% success rate usually means saving well past the point of diminishing returns, working extra years to buy certainty against outcomes that were already unlikely (White Coat Investor).

It's also worth noticing what doesn't happen as horizon stretches from extreme to more extreme. Push the same 70/30 portfolio out to 60 years and a 3.25% withdrawal rate still clears 90.9%, only about two points below its 50-year result. Sequence of returns risk is heavily front-loaded: the years that determine whether your portfolio survives are mostly the first 10 to 15 years of retirement. A portfolio that makes it through that window intact tends to keep compounding well past it, which is why the marginal danger of retiring at 35 instead of 45 is smaller than most people assume.

A "failure" almost never means you actually run out of money

The other correction worth making is about what a sub-100% result actually implies. Michael Kitces and Derek Tharp have made the case that "probability of success" is the wrong label entirely, and that it should really be read as a "probability of adjustment." Their research modeled a retiree couple and found that if you're willing to revisit your plan periodically and trim spending when markets underperform, the gap between targeting 95% success and targeting 50% success barely shows up in your median outcome. The main thing that changes is how much you're allowed to spend on day one (Kitces.com).

That reframing matters most if you're the kind of person who checks in on your plan every year or two, which describes most FIRE households already. A 15% chance of "failure" in a static model usually just means: at some point, in some simulated world, you'd have needed to spend a bit less than planned. It rarely means destitution, especially if your floor expenses (housing, food, insurance) are smaller than your total withdrawal and could absorb a cut without changing your life.

Guardrails: targeting a lower number on purpose

If adjusting spending in response to markets is the real safety valve, some retirees build that adjustment into the plan up front instead of leaving it implicit. This is the idea behind the Guyton-Klinger guardrails approach: start at a lower initial success rate target (WCI's example uses 80%), but pair it with a rule that cuts spending if your ongoing probability of success falls to 25%, and a rule that raises spending if it climbs back to 100%. Because the plan is designed to flex both directions, you can safely start from a lower, less conservative number than a static plan would ever tolerate.

Morningstar's research team (Blanchett, Finke, and Pfau) found that this kind of dynamic, guardrails-based withdrawal strategy supported a 5.2% starting withdrawal rate in their modeling, well above the roughly 3.9% starting rate their research recommends for a fixed, never-adjusted withdrawal plan (Morningstar). WCI walks through a real client example built on this framework: a couple able to move from $172,000 to $220,000 in after-tax annual spending by adopting guardrails instead of a fixed real-dollar withdrawal, while their probability of success actually improved, from 69% under the old approach to 99% under the new one, because the plan could correct itself instead of running on autopilot for three decades (White Coat Investor).

So what number should you actually target?

There isn't a single correct answer, but here's a defensible framework:

If you're building a plan you intend to set and mostly forget, without regular check-ins or the willingness to trim spending, lean toward 90% or higher, and build in a real cash or bond buffer for the early years specifically, since that's where sequence risk is concentrated.

If you plan to revisit your numbers annually and can flex discretionary spending when markets are unkind (which is most people who got into FIRE by tracking their finances closely in the first place), an 80 to 85% initial target is defensible, provided your non-negotiable expenses sit comfortably below your planned withdrawal.

Weight time horizon more than the success rate itself. A 50-year retirement at 3.25% and an 85% success rate is a materially different, and generally better-built, plan than a 30-year retirement at 4% and a 94% success rate, even though the second number looks more comforting on its face.

None of this accounts for taxes or investment fees, and none of it is personalized financial advice. It's a description of how the math behaves so you can read your own results with more context than a single percentage gives you.

Run your own numbers

The fastest way to see how your specific horizon, withdrawal rate, and allocation interact is to run them, not estimate them. Our free Monte Carlo retirement simulator runs the same 1,000-trial, block-bootstrap engine used for the examples above against your actual numbers, with support for Social Security, custom glide paths, and side-by-side what-if comparisons. No account required, nothing saved unless you choose to share the link.

Try the free Monte Carlo retirement calculator →

Sources

• White Coat Investor, "Risk-Based Guardrail Retirement Withdrawal Strategy"
• White Coat Investor, "Reasons Your Retirement Plan Is Too Conservative"
• Kitces.com, Derek Tharp, "A Monte Carlo 50% Retirement Success Probability Can Work"
• Morningstar, "The Best Strategies for Boosting Starting Withdrawal Rates in Retirement"
• Wade Pfau, Retirement Researcher, "Safe Withdrawal Rates for Retirement and the Trinity Study"