Monte Carlo Retirement Simulator
Probabilistic retirement planning, without the spreadsheet theatre.
Most retirement calculators assume steady returns and a life that goes to plan. ButterflyForge runs your plan against thousands of market histories - booms, crashes, and the inflation surprises in between - and shows you where it bends.
Free. No signup. Tweak any input, watch the percentile bands move.
Try it now
See what your financial future could look like
How it works
Three steps. Real math underneath.
Describe your situation
Age, savings, contributions, when you want to retire. Start simple.
Run the simulation
1,000 independent paths - each with its own randomised sequence of returns and inflation.
See where it breaks
Find out how your plan holds up - and what to adjust to improve the odds.
Why most retirement simulators get it wrong
They assume 7% annual returns, steady inflation, and a life that goes exactly according to plan. Real markets have crashes, booms, and the occasional decade that makes no sense.
ButterflyForge shows you the spread of outcomes - thousands of paths, percentile by percentile - so you plan against the range, not a single average.
Is this you?
Fresh out of college
Alex is 28 with €20k saved, adding €500/month into a World ETF. With rising life expectancy, the old assumption of a 30-year retirement may not hold for his generation. At a 4% withdrawal rate, does the portfolio survive 30+ years?
One engine, many ways in
Under the hood is a fast simulation engine with a flexible rule language. You don't have to touch it - but it's there when you need it.
Modular builder
Compose holdings, withdrawals, life events, and triggers to mirror your actual situation.
Shared scenarios
Browse what others have built, fork a scenario, tweak the assumptions, make it yours.
AI assistant
Describe your situation in plain language. The AI builds the scenario, you refine it.
Not all simulations are created equal
How you model the market matters as much as what you model. The free simulator uses a simple normal distribution. The Personal Edition adds methods calibrated on decades of observed behaviour.
Normal distribution
Returns drawn from a bell curve with fixed mean and volatility. Simple and fast, but assumes crashes and booms are equally unlikely and treats inflation as independent of market movements - real markets have fatter tails and cluster their bad days together.
Historical block bootstrap
Draws sequences of returns from observed market history, preserving the autocorrelation and volatility clustering that actually happened. No distributional assumptions - if it happened before, it can happen in your simulation.
Regime-switching model
Markets alternate between distinct regimes - bull runs, bear markets, sideways grinds - each with different return characteristics. Inflation is modeled in lockstep with market movements, not as an independent random variable. This model captures those transitions using parameters estimated from long-run market data, generating realistic sequences of correlated returns and inflation.
Is this you?
Riding it out to retirement
Margaret is 57 with €600k in government bonds. Conservative, cautious, and six years from retirement. With Guyton-Klinger guardrails at 3% - does the adaptive strategy keep her safe for 30 years?
Beyond the 4% rule: test it, don't trust it
The 4% rule never adapts. You take the full hit in bad years and leave gains on the table in good ones - and the original study only had to survive one historical sequence.
Compare it head to head with Guyton-Klinger guardrails: spending cuts 10% when the portfolio falls past a threshold, lifts 10% when it grows past another. Switch strategies in the free simulator, hold everything else fixed, watch the failure rate move.
The Personal Edition adds custom guardrail thresholds, smoothed withdrawals, floor and ceiling amounts, and rebalancing rules.
What real plans actually look like
Real plans have one-time events, multiple income sources, and inflection points. The Personal Edition models all of them.
Inheritance at 52
Model a €150k lump sum hitting your portfolio mid-career and see how it shifts your retirement date.
Buying a house abroad
A €300k withdrawal at 60 for a place in southern France - what does that do to a 35-year retirement?
State pension kicks in at 67
Model reduced withdrawals once your pension starts, with a different withdrawal rate before and after.
Career break at 40
Two years of zero contributions and partial withdrawals. How much does it cost you at 65?
Free vs Personal Edition
The free simulator tests the basics. The Personal Edition models life as it actually is.
| Feature | Free | Personal Edition |
|---|---|---|
| People | 1 | Unlimited |
| Portfolio Holdings | 3 presets | Custom |
| Return Distribution | Normal | Normal, Log-Normal, Historical, Correlated |
| Inflation | Fixed rate | Normal + Historical |
| Withdrawals | Simple % | Fixed, Dynamic, Smoothed + Guardrails |
| Contributions | Basic monthly | Custom frequency, growth, age ranges |
| Rebalancing | - | Target allocations + drift tolerance |
| One-Time Events | - | Lump sums, windfalls, expenses |
| Threshold Triggers | - | Conditional rules on portfolio value |
| Control Interface | - | Sliders, toggles & inputs |
| Save & Share | Share link | Save, revisit, share read-only links |
| Simulation Runs | 1,000 | 1,000 or 10,000 |
| Analytics | Portfolio, failure rate, distributions | Full charts, stats, yearly breakdowns |
| ButterflyForge is currency-agnostic and does not simulate taxes, transaction costs, or jurisdiction-specific rules. | ||
Start simple. Go as deep as you want.
Run the free simulator. When you need the full picture - custom portfolios, life events, shared scenarios, AI-assisted planning - the Personal Edition is a one-time purchase, yours forever.
One email when we launch, then we delete your address. Nothing else.
This tool provides statistical simulations based on historical data and modeled assumptions. Results are not predictions or guarantees of future performance. This is not financial advice. The creators accept no liability for decisions made based on these simulations. Please seek professional financial advice before making investment decisions.