A Monte Carlo retirement simulation runs a financial plan through many modeled investment and inflation paths. It helps reveal how outcomes vary around a smooth forecast. The success percentage is conditional on the model and your inputs, so read it alongside shortfalls, spending cuts and the time horizon.
Why a smooth return can hide a difficult retirement
A forecast with a constant annual investment return is easy to compare and inspect. Real markets move unevenly. A sharp fall while you are withdrawing can force sales from a smaller portfolio, leaving less invested for a recovery. This is sequence-of-returns risk.
Consider two simplified portfolios starting at €100,000. With a €5,000 withdrawal at the start of each year, returns of −20% then +25% leave €88,750 after two years. Reversing those returns leaves €91,000. The return factors are the same; withdrawals make their order matter. This arithmetic example excludes taxes and fees.
What Zenarios varies
Zenarios generates investment-return and inflation paths around the assumptions in your plan. Its market model allows more extreme returns than a simple normal distribution. It runs at least 300 futures and can continue to 1,000, stopping adaptively as the estimated funding share becomes more stable.
Scenarios of the same plan meet the same generated market futures. That makes comparing “keep working” with “retire sooner” more useful: differences are not merely caused by drawing a new random set of markets. A changed assumption still changes the question being tested.
The methodology page links to the maker’s technical explanation. No simulator captures every possible market, household event or future tax change.
How to read a simulated success rate
If 900 of 1,000 modeled futures meet a funding test, that is a 90% simulated success rate under those inputs. It is not a 90% guarantee about your life. Different assumptions, horizons and definitions of success can produce different percentages, even for similar starting balances.
Ask what the test requires. Does money last only to the planned retirement date, or through the years after it? What spending is funded? Is a long life included? Compare plans over the period you actually need to fund.
When flexible spending is enabled, a plan may avoid running short by cutting expenses. Inspect how often and how deeply it cuts. “Bills stay covered” and “the original lifestyle stays funded” are different outcomes.
Historical replay asks a different question
A historical replay applies an observed market sequence instead of generating one. Zenarios includes US market returns and inflation from 1872–2025, with preset starts in 1929, 1966, 1973, 2000 and 2008. The app’s underlying datasets are described in the methodology, including Robert Shiller’s market data.
A replay is useful for asking how your plan behaves in a particular bad period. US history does not represent every country’s experience, and surviving past crises does not establish a floor for future outcomes.
Use a named shock to find a specific weakness
Zenarios also has 13 shock tests, including job loss, a market crash, inflation, higher loan rates and longevity. Their verdicts—Holds, Gets tight and Runs short—help locate a problem you can inspect. A job-loss test can be useful even when market simulations look comfortable.
A practical review is to inspect the baseline, test a relevant shock, run market futures and repeat with a lower return or higher cost assumption. Change something actionable, such as the retirement date or spending, then compare again. Read the guardrails guide before treating a higher success rate as an improvement.
Common questions
Is a 90% simulated success rate a 90% guarantee?
No. It describes the share of modeled futures that pass the simulation's funding test for the chosen horizon and assumptions. It is not a measured probability of your real-world future, and changes in inputs or model design can change it.
How many futures does Zenarios simulate?
Zenarios runs up to 1,000 model-generated futures, with adaptive stopping after at least 300. It also offers historical US market replays and 13 named shock tests. These are different ways to explore uncertainty.
Explore Zenarios
See the Zenarios financial planning features, explore life scenarios and financial decisions, or read about how Zenarios calculates your forecast and how your financial data stays private.
Try Zenarios in your browserWritten by the maker of Zenarios, a software engineer. These guides explain planning concepts and the product; they are not a personalized financial recommendation. Illustrative calculations are not results from your plan.