Monte Carlo Retirement Calculator

Start by entering your timeline: Current Age, planned Retirement Age, and Life Expectancy. A longer retirement period requires a higher success rate to feel confident.

Enter your Current Savings across all retirement accounts and your Monthly Contribution until retirement. Contributions grow with inflation each year — a $1,500/month contribution at 2.5% inflation becomes ~$2,400/month by retirement 20 years later.

Choose an Asset Allocation preset: Aggressive (90/10), Growth (70/30), Balanced (60/40), or Conservative (40/60). Each sets a return and volatility appropriate for that mix. Or select Custom to enter your own values. Set your Annual Fee (expense ratios, advisory fees) — this is subtracted from returns each year.

Enter your planned Annual Spending in today's dollars, your expected Social Security benefit, and the age you'll start collecting. Use the Social Security calculator to estimate your benefit.

Open advanced settings to adjust inflation, simulation count, or the random seed. Click "New seed" to rerun with different random sequences.

Current Age, Retirement Age, and Life Expectancy

These three inputs establish the simulation timeline. The period before retirement is the accumulation phase (contributions added, growing with inflation). The period after retirement is the withdrawal phase (spending subtracted). Planning for a longer life expectancy (90–95) stress-tests your plan against longevity risk.

Current Savings

The starting portfolio balance for every simulation. Include the total across all accounts: 401(k)s, IRAs, 403(b)s, and taxable brokerage accounts intended for retirement.

Monthly Contribution

The amount you save each month until retirement. Unlike a flat nominal contribution, this calculator grows your contribution with each simulation path's inflation rate — modeling the realistic pattern of increasing savings as your salary grows. By retirement, your nominal contribution will be significantly higher than today's amount.

Asset Allocation

Determines the expected return and volatility used in simulations. The presets are based on historical stock/bond blended returns:

The relationship between allocation and volatility is what makes Monte Carlo valuable — a higher return comes with wider outcome dispersion.

Annual Fee

Investment fees (expense ratios, advisory fees, fund costs) subtracted from the gross return each year. A 0.25% fee on a 7% return produces a 6.75% net return. Over decades, even small fees compound into a large drag on portfolio growth. Low-cost index funds typically charge 0.03–0.10%; active management or advisory services may charge 0.5–1.5%.

Annual Spending in Retirement

Your target annual cost of living in today's dollars. The calculator adjusts this for inflation each year using each simulation's random inflation path. A common target is 70–85% of pre-retirement income. For a detailed estimate, use the retirement expense calculator.

Social Security (Monthly) and Start Age

Your expected monthly benefit in today's dollars. This income is inflated with each simulation's path and reduces portfolio withdrawals every year it's active. Delaying Social Security from 62 to 70 significantly increases your monthly benefit. See when to take Social Security.

Inflation Rate & Volatility

The mean inflation rate drives contribution growth during saving years, spending growth during retirement, and Social Security COLA. Inflation volatility adds realism — the calculator generates a different random inflation rate for each year of each simulation, so some paths experience high-inflation decades and others don't.

Number of Simulations

How many times the scenario runs with different random sequences. 1,000 simulations is sufficient for stable success rates. More simulations (5,000+) give smoother percentile bands but take longer to compute.

Seed & Rerun

The calculator uses a seeded pseudorandom number generator (mulberry32). The same seed always produces the same results, making your analysis reproducible. Click "New seed" to generate a fresh set of random sequences. The seed is displayed so you can share or compare specific runs.

Simulation Loop

For each of 1,000 simulations, the calculator projects your finances year by year from your current age through life expectancy:

1. Generate random return and inflation for the year using Box-Muller normal sampling with the seeded PRNG
2. During accumulation (before retirement): portfolio grows by the random net return, then receives the inflation-adjusted annual contribution
3. During withdrawal (retirement): inflation-adjusted spending minus Social Security is withdrawn; remaining balance grows or shrinks by the random return
4. Track success: a simulation succeeds if every year's withdrawal is fully funded — a portfolio that pays every bill and ends at exactly $0 is a success
5. Guardrail scenario: a parallel simulation cuts spending by 10% after any year with negative returns

Contribution Growth

Contributions grow with cumulative inflation each year:

year_contribution = monthly_contribution × 12 × cumulative_inflation_multiplier

This models the realistic pattern of increasing savings as salaries grow. A $1,500/month contribution at 2.5% inflation reaches ~$2,400/month after 20 years — more than doubling in nominal terms.

Net Return (After Fees)

net_return = mean_return - annual_fee
random_return = max(-100%, normal(net_return, volatility))

Fees are subtracted from the mean before sampling. This means fees reduce the average outcome while the volatility (risk) stays the same — exactly how real fees work.

Success Definition

A simulation is a success if every year's full spending need was met from the portfolio plus income. A path that pays every bill and ends at $0 counts as a success — you don't need leftover money. A path where the portfolio runs out before covering a year's spending is a failure.

Guardrail Scenario

The calculator runs a second parallel simulation for each path that cuts spending by 10% in any year following a negative portfolio return. This models a common real-world strategy: tightening the belt during bad markets. The guardrail success rate is typically 5–15 percentage points higher than the base rate, showing the power of flexible spending.

Random Variable Generation

random_return = max(-1, normal(net_mean_return, volatility) / 100)
random_inflation = max(-0.5, normal(inflation_rate, inflation_volatility) / 100)

Pre-Retirement Year

annual_contribution = monthly_contribution × 12 × cumulative_inflation
portfolio = max(0, portfolio × (1 + random_return)) + annual_contribution
cumulative_inflation *= (1 + random_inflation)

Post-Retirement Year

cumulative_inflation *= (1 + random_inflation)
adjusted_spending = annual_spending × cumulative_inflation
ss_income = IF(age >= ss_start_age, monthly_ss × 12 × cumulative_inflation, 0)
net_withdrawal = max(0, adjusted_spending - ss_income)
portfolio = portfolio × (1 + random_return) - net_withdrawal

Success Rate

success_rate = (paths_that_never_ran_short / total_simulations) × 100

There is no universal "correct" number — it depends on your flexibility and risk tolerance.

• 90% or higher: A strong plan that withstands most bad-luck sequences. Most conservative planners target this level.
• 75% to 89%: Reasonably strong, but with a notable chance of failure. Small adjustments often push this above 90%.
• 50% to 74%: A borderline plan with significant risk. Meaningful changes to savings, retirement age, or spending are likely needed.
• Below 50%: Most simulated paths run out of money. Significant changes are needed.

The guardrail rate matters too: if your base rate is 80% but your guardrail rate is 95%, you have a strong plan as long as you're willing to cut spending in bad years.

Sequence of returns risk is one of the biggest threats to a successful retirement and the primary risk Monte Carlo analysis quantifies.

The order in which you experience returns matters enormously, especially in the years just before and after retirement:

• Bad sequence: Poor returns early in retirement force you to sell more shares at low prices to fund living expenses. This depletes principal faster, leaving less to recover when markets rebound.
• Good sequence: Strong early returns grow your portfolio, allowing you to fund withdrawals from gains while leaving principal intact.

A simple calculator using a flat 7% annual return misses this risk entirely. By generating thousands of different return sequences, Monte Carlo simulation shows how vulnerable your plan is to an unlucky start.

Success Rate: The percentage of simulated scenarios where your portfolio fully funded every year's spending through life expectancy. A path ending at exactly $0 after paying the final year's bills counts as a success.

With Guardrails: The success rate if you cut spending by 10% after any year with negative returns. The difference between base and guardrail rates shows how much spending flexibility improves your plan.

Summary Cards:

• Median at Retirement: The 50th percentile projected portfolio value at your retirement date.
• Median End Balance: The median amount remaining at life expectancy.
• With Guardrails: Success rate with the 10% spending-cut rule.

Portfolio Balance Projections Chart: The "cone of outcomes" over time.

• Median Line (50th): The average-luck path.
• Inner Band (25th–75th): A 50% probability range.
• Outer Band (10th–90th): The broader range including very good and very bad luck.

End Balance Distribution: The spread of final portfolio values from worst to best outcomes.

If your success rate is lower than you'd like, test these changes:

1. Save more: Increasing your monthly contribution is the most powerful lever. Contributions grow with inflation, so increases compound over time.
2. Retire later: Each additional year gives investments more time to grow and reduces withdrawal years.
3. Reduce spending: A 10–15% reduction can dramatically increase portfolio longevity. The guardrail rate shows the impact of just a 10% cut in bad years.
4. Delay Social Security: Claiming at 70 instead of 62 provides a much larger, inflation-protected income stream for life.
5. Reduce fees: Switching from a 1% advisory fee to a 0.25% index fund saves hundreds of thousands over a 30-year retirement.
6. Adjust allocation: Very high volatility increases the chance of catastrophic early losses. A slightly more conservative allocation may narrow your range of outcomes while only modestly reducing the median.

1. Overly optimistic assumptions: A 12% return with low volatility produces a great score but doesn't reflect reality. Use the preset allocations as a starting point.
2. Ignoring fees: Even 0.5% annually compounds into a massive drag over decades. Always include your actual investment costs.
3. Flat contributions: Many calculators keep contributions flat in nominal dollars, but real savings grow with income. This calculator grows contributions with inflation for realism.
4. Focusing only on the median: The real value is in the 10th and 25th percentile outcomes — the scenarios where things go wrong.
5. Treating the result as a guarantee: A 95% success rate still means 5% of modeled paths fail. Unmodeled events (health crises, legislative changes) can always occur.
6. Running it once: Re-run your simulation annually or after major life changes. Use different seeds to verify your results are stable.

Use the calculator above to go beyond simple averages and explore how your plan performs under a range of market conditions. Compare your base success rate to the guardrail rate to understand the value of spending flexibility. Test different asset allocations and fee levels to find the right balance.

For more tools, explore our full suite of retirement calculators. Create a spending plan with the retirement budget guide, estimate healthcare costs, or optimize your Social Security claiming strategy.