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What are the odds your money lasts: Monte Carlo simulation

Markets do not return the same every year. This calculator simulates thousands of possible futures, with good and bad years, and shows the probability that your money lasts through retirement, the range of savings you may have and the monthly withdrawal that is safe with a 90% chance.

Educational simulation. Not investment advice. Results are estimates based on the data you enter and do not replace advice from a qualified professional.

Reviewed on Oct 31, 2026

Everything in today’s money, after inflation. Example values.

Saving phase
Withdrawal phase
Investments

Rough guide: bonds have 1% to 5% volatility a year; a balanced portfolio 8% to 12%; all stocks 15% to 25%. A 3% to 5% real return is a prudent assumption for diversified portfolios.

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—Chance the money lasts to the end
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The range of possible futures

Savings year by year. The light band holds 80% of the simulations; the dark band 50%. The line is the middle path (median).

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Chance of success by monthly withdrawal

Probability that the money lasts through all withdrawal years, for each monthly withdrawal.

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What you may have when withdrawals start

Distribution of savings at the start of withdrawals across all simulations.

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Savings by year (percentiles)

Next step

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The summary lists your inputs, results, formulas, assumptions and sources. The full report also includes the charts and interpretations, in color. In the print dialog, choose Save as PDF.

Method, assumptions and sources

Formulas

Monthly return = exp(μ + σ × Z), with μ = ln(1 + real return) ÷ 12, σ = volatility ÷ √12 and Z standard normal (Box-Muller, fixed seed)
Saving: P = P × return + contribution; withdrawing: P = (P − withdrawal) × return; if P ≤ 0 the simulation fails
Chance of success = simulations in which the money lasts ÷ total; safe withdrawal = largest withdrawal with chance ≥ 90% (binary search)

Assumptions and limits

  • Probabilities are model results under your assumptions, not guarantees. This is not investment advice.
  • Contributions at the end of each month while saving; withdrawals at the start of each month; constant in today’s money.
  • It ignores taxes, fees, pensions and other income, and changes in behavior during crises.

Official sources and references

Frequently asked questions

What is a Monte Carlo simulation?

A method that draws thousands of possible sequences of yearly returns, good and bad, and counts in how many of them your plan works. It shows risk that a fixed-rate calculator hides, especially the danger of bad years right after you start withdrawing.

How do I read the default example?

With $100,000 invested, $1,500 a month for 20 years, a 5% real return and 12% volatility, a $4,000 monthly withdrawal lasts 30 years in 62 out of 100 futures. On the middle path you reach retirement with $889,184, but in 10% of cases with less than $533,394. The safe withdrawal, with a 90% chance, is $2,398 a month.

What chance of success is good enough?

Many planners aim for 85% to 90%. Lower than that, the plan relies on luck; much higher, you may be saving more or spending less than you need. Being willing to cut withdrawals in bad years makes a lower figure acceptable.

What return and volatility should I use?

Use the portfolio you actually plan to hold. A balanced stock and bond mix is often modeled with a 3% to 5% real return and 8% to 12% volatility; all stocks with higher return and 15% to 25% volatility. Keep the real return conservative.

Why do results not change when I reload?

The simulation uses a fixed random seed, so the same inputs always give the same answer. That makes it easy to compare scenarios: only your changes move the result.

What does the model leave out?

It assumes constant contributions and withdrawals in today’s money, ignores taxes, fees and other income such as pensions, and draws returns that are independent from month to month. Real crises can be longer and deeper than the random draws.

When should I talk to a professional?

When your chance of success is below 85%, you are a few years from retiring or you need to decide on pensions, annuities and withdrawal order. A fee-only financial planner can build the portfolio, set flexible withdrawal rules and stress-test the plan.

Related calculators

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Monte Carlo simulation for retirement

Simple calculators assume the portfolio earns the same rate every year. Real markets have good and bad years, and the order in which they come matters: a crash in the first years of withdrawals hurts far more than the same crash near the end, because you are selling assets at low prices to pay the bills. A Monte Carlo simulation draws thousands of possible sequences of returns and counts in how many of them the money lasts.

What you need to enter

  • Expected real return: the average return above inflation. For diversified portfolios, 3% to 5% a year is a prudent assumption.
  • Volatility: how much returns swing from year to year. Short-term bonds are close to zero; a balanced stock and bond mix runs about 8% to 12%; an all-stock portfolio, 15% to 25%.
  • Contributions and withdrawals: in today’s money, so you can compare them with your current budget.

How to read the result

The chance of success is the share of simulations in which the money lasts all the withdrawal years. Many planners aim for 85% to 90%. Below that, the plan depends heavily on luck. The safe withdrawal is the largest monthly amount that keeps a 90% chance.

The median is the middle path: half of the simulations do better, half do worse. The lower edge of the band (worst 10%) shows what happens in a bad but plausible future.

How to improve the odds

  • Save more or work a few more years: each extra year adds contributions and cuts one year of withdrawals.
  • Withdraw less in bad years: flexible spending is one of the most effective protections.
  • Combine with guaranteed income, such as a public or workplace pension, which reduces how much the portfolio must provide.

The model uses lognormal monthly returns with a fixed seed, so the results do not change with every click. It is a planning tool, not a forecast.