What is a Monte Carlo retirement simulation?

Monte Carlo simulation runs thousands of hypothetical futures using randomly sampled historical market returns. Instead of assuming average returns every year, it models the real variability of markets — including the devastating "sequence-of-returns risk" where bad years early in retirement can derail an otherwise sound plan. The result is a probability of success, not a single optimistic number.

What does "success rate" mean?

Success rate is the percentage of the 5,000 simulated futures where your portfolio lasts to your target life expectancy (default 95) without running out of money. A success rate of 87% means that in 87% of the simulated histories, you never ran out of money before age 95.

The 4% rule (Bengen rule, or Trinity Study guideline) says that withdrawing 4% of your starting portfolio in year one of retirement — then adjusting that dollar amount for inflation each year — has historically lasted 30 years with a high success rate. This calculator implements the 4% rule exactly: a fixed real withdrawal that doesn't change with market performance.

What is VPW (Variable Percentage Withdrawal)?

VPW is an alternative withdrawal strategy where you withdraw a fixed percentage of your current portfolio value each year, based on your age. The rate increases as you age. Unlike the 4% rule, a VPW portfolio mathematically cannot run out of money — but your annual income will fluctuate with market performance. Many FIRE practitioners prefer VPW for its flexibility.

What is a FIRE number?

Your FIRE number is the portfolio size needed to retire. The classic formula is 25× your annual spending — derived from the inverse of the 4% rule. If you plan to spend $60,000/year in retirement, your FIRE number is $1,500,000. This calculator shows your FIRE number alongside your success probability.

Coast FIRE is the point at which your current portfolio, left to grow with no additional contributions, will reach your FIRE number by traditional retirement age. Once you've "coasted," you only need to earn enough to cover current living expenses — not save for the future. The calculator shows your Coast FIRE number: the amount you need invested today to stop contributing.

Is my financial data safe?

All calculation happens in your browser via a Web Worker. No financial data is ever sent to any server. There is no login, no account, and no tracking of your inputs. You can verify this by checking your browser's Network tab — no POST requests are made when you run the simulation.

What historical data does this tool use?

The simulation uses annual real (inflation-adjusted) returns for US stocks (S&P 500) and bonds (10-year Treasury) from 1928 to 2024, derived from Damodaran's dataset and the Shiller CAPE database. The block bootstrap method samples 10-year blocks of contiguous history to preserve autocorrelation in returns.

How accurate is the tax modeling?

The tool uses 2026 US federal income tax brackets, the standard deduction, and simplified LTCG rates. It models pre-tax (ordinary income), Roth (tax-free), and taxable brokerage (LTCG) withdrawals separately. RMDs from pre-tax accounts starting at age 73 are included. State taxes are not modeled in v1. This is an estimate for planning purposes, not a substitute for a tax professional.

Retirement & FIRE Calculator

Run 5,000 Monte Carlo simulations to find your probability of a successful retirement. Covers 4% Rule, VPW, Social Security, RMDs, and all three account types — free, browser-only, no signup.

Last updated: April 2026

What it does

5,000-path Monte Carlo

Block bootstrap from 97 years of US market history (1928–2024), run entirely in your browser via a Web Worker. Results in under 2 seconds.

Three account types

Models pre-tax (401k / Traditional IRA), Roth (Roth 401k / Roth IRA), and taxable brokerage — each with correct tax treatment on withdrawals.

4% Rule and VPW

Compare fixed real withdrawals (4% Rule) against Variable Percentage Withdrawal — the age-based strategy that cannot deplete a portfolio.

Social Security & RMDs

Input your estimated SS benefit and claiming age. Required Minimum Distributions from pre-tax accounts are modeled starting at age 73 per SECURE 2.0.

Interactive fan chart

Pure SVG fan chart shows P10/P25/P50/P75/P90 percentile bands across your full retirement horizon. Hover for year-by-year values.

Shareable plan links

Your entire plan is compressed into a URL — share with a partner, advisor, or your future self. No account required.

How to use Retirement & FIRE Calculator

1
Enter your current situation
Input your age, retirement target age, current portfolio, monthly contribution, and expected spending in retirement. Use today's dollars — the calculator adjusts for inflation automatically.
2
Set your account split
Drag the sliders to reflect how your savings are split between pre-tax (401k/IRA), Roth, and taxable brokerage accounts. This affects your tax modeling during withdrawals.
3
Review your probability
The hero number shows the percentage of simulated futures where your portfolio lasts to your chosen life expectancy. 85%+ is generally considered a strong plan.
4
Explore the fan chart
The fan chart shows P10 (worst 10%), P50 (median), and P90 (best 10%) outcomes year by year. Hover over any year to see the exact values. The vertical dashed line marks your retirement age.
5
Refine with Tier 2 settings
Expand Income & Savings, Investment Settings, Retirement Phase, and Withdrawal Strategy to model Social Security, employer match, healthcare costs, and compare 4% Rule vs VPW.

When to use this

Am I on track for retirement at 65?

Enter your current age, portfolio, and monthly contribution. See your success rate and fan chart instantly.

Can I retire early (FIRE)?

Switch to FIRE mode, set a lower retirement age, and see how much longer your portfolio needs to last.

When should I claim Social Security?

Toggle between SS claiming at 62 vs 67 vs 70 and compare success rates to find the optimal age.

How much does spending affect my plan?

Adjust monthly retirement spending by $500 increments and watch the success rate and fan chart update in real time.

What is my Coast FIRE number?

The Coast FIRE number appears in the results panel — it's the amount needed today so you never have to contribute again.

Technical details

Simulation method	Block bootstrap (block size 10 yrs) from Shiller/Damodaran US annual real returns 1928–2024 (97 years). 5,000 paths per run.
Return data	S&P 500 and 10-yr US Treasury real total returns, derived from Damodaran's annual returns dataset.
Tax modeling	US federal 2026 brackets, standard deduction, LTCG rates, Social Security ordinary income inclusion, RMD per IRS Uniform Lifetime Table (age 73+).
Withdrawal strategies	4% Rule (fixed real) and Variable Percentage Withdrawal (VPW) using Bogleheads age-based rate table.
Computation	All simulation runs in a Web Worker — no server, no data transfer. Supports TypedArray transfer for zero-copy percentile results.
Privacy	No account. No server. localStorage only. Shareable URLs encode inputs via LZ-string — no server storage.

Frequently Asked Questions

What is a Monte Carlo retirement simulation?: Monte Carlo simulation runs thousands of hypothetical futures using randomly sampled historical market returns. Instead of assuming average returns every year, it models the real variability of markets — including the devastating "sequence-of-returns risk" where bad years early in retirement can derail an otherwise sound plan. The result is a probability of success, not a single optimistic number.
What does "success rate" mean?: Success rate is the percentage of the 5,000 simulated futures where your portfolio lasts to your target life expectancy (default 95) without running out of money. A success rate of 87% means that in 87% of the simulated histories, you never ran out of money before age 95.
What is the 4% rule?: The 4% rule (Bengen rule, or Trinity Study guideline) says that withdrawing 4% of your starting portfolio in year one of retirement — then adjusting that dollar amount for inflation each year — has historically lasted 30 years with a high success rate. This calculator implements the 4% rule exactly: a fixed real withdrawal that doesn't change with market performance.
What is VPW (Variable Percentage Withdrawal)?: VPW is an alternative withdrawal strategy where you withdraw a fixed percentage of your current portfolio value each year, based on your age. The rate increases as you age. Unlike the 4% rule, a VPW portfolio mathematically cannot run out of money — but your annual income will fluctuate with market performance. Many FIRE practitioners prefer VPW for its flexibility.
What is a FIRE number?: Your FIRE number is the portfolio size needed to retire. The classic formula is 25× your annual spending — derived from the inverse of the 4% rule. If you plan to spend $60,000/year in retirement, your FIRE number is $1,500,000. This calculator shows your FIRE number alongside your success probability.
What is Coast FIRE?: Coast FIRE is the point at which your current portfolio, left to grow with no additional contributions, will reach your FIRE number by traditional retirement age. Once you've "coasted," you only need to earn enough to cover current living expenses — not save for the future. The calculator shows your Coast FIRE number: the amount you need invested today to stop contributing.
Is my financial data safe?: All calculation happens in your browser via a Web Worker. No financial data is ever sent to any server. There is no login, no account, and no tracking of your inputs. You can verify this by checking your browser's Network tab — no POST requests are made when you run the simulation.
What historical data does this tool use?: The simulation uses annual real (inflation-adjusted) returns for US stocks (S&P 500) and bonds (10-year Treasury) from 1928 to 2024, derived from Damodaran's dataset and the Shiller CAPE database. The block bootstrap method samples 10-year blocks of contiguous history to preserve autocorrelation in returns.
How accurate is the tax modeling?: The tool uses 2026 US federal income tax brackets, the standard deduction, and simplified LTCG rates. It models pre-tax (ordinary income), Roth (tax-free), and taxable brokerage (LTCG) withdrawals separately. RMDs from pre-tax accounts starting at age 73 are included. State taxes are not modeled in v1. This is an estimate for planning purposes, not a substitute for a tax professional.

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