Free Monte Carlo retirement simulator
A Monte Carlo retirement simulator runs your plan through many different market sequences instead of assuming one steady average return. That's the honest way to see retirement risk, because the order of returns — not just the average — decides whether a portfolio lasts. Coastline's simulator replays real historical market cycles and random draws, with full tax math on every path.
The same plan, across many market histories
Ending balance in today's dollars across 67 market histories (1928–2024) for a $1,000,000 portfolio at 65 (70% stocks), with $17,333/yr of Social Security, at four spending levels:
| Spend / yr | Bad luck (10th %ile) | Median | Good luck (90th %ile) |
|---|---|---|---|
| $40,000 | $1,533,625 | $2,781,756 | $5,338,023 |
| $50,000 | $741,822 | $2,003,167 | $4,395,534 |
| $60,000 | $0 | $1,476,860 | $3,549,687 |
| $70,000 | $0 | $839,808 | $2,715,981 |
Notice how spending more doesn't just lower the average — it widens the gap and pushes the bad-luck case toward zero. That spread is exactly what a Monte Carlo simulation exists to reveal and a single "average return" projection hides.
If you came here looking for Vanguard's Retirement Nest Egg Calculator, which Vanguard withdrew in 2024: the Vanguard Nest Egg Calculator replacement re-runs its inputs — balance, yearly spend, years, stock/bond mix — through this simulator and shows the odds and the range.
Why sequence of returns is the real risk
Two retirees can earn the identical average return over 30 years and get completely different results. The one who hits a crash in their first few retirement years — selling assets while they're down, with no paycheck to offset it — can run out, while the one who gets the same crash a decade later sails through. This is sequence-of-returns risk, and it's invisible to any calculator that uses a flat return. A Monte Carlo (or historical-cycle) simulator is the only way to size it.
Why Coastline's version is different
Most free calculators hand you a single headline number with the assumptions hidden. Coastline is built the opposite way:
- Real tax math, not a flat rate. It applies federal and state income tax, long-term capital-gains rules, and account-by-account treatment (taxable, traditional, Roth) — in both your working years and retirement.
- The whole journey. It models accumulation (saving and investing) and drawdown (spending it down), so you see how the plan connects end to end, not just one half.
- The math is shown. Every figure has a click-through breakdown of how it was computed — a level of transparency even paid tools rarely expose.
- All 50 states and DC, itemized deductions, IRMAA, ACA credits, 72(t) and student-loan forgiveness tax — modeled with the published rules. Each is the actual rule on inflation-indexed thresholds, applied in the year it lands, so a Roth conversion at 63 shows up as a Medicare surcharge at 65 and a forgiven loan balance shows up as that year’s tax bill.
- Free, no signup. No account needed, no account-linking.
Assumptions: single filer, no-income-tax state, 60% taxable / 30% traditional / 10% Roth, 70% stock allocation, historical-cycle simulation, figures deflated to today's dollars. Your accounts, allocation, and other income change the picture — run yours in the calculator's Simulation Tools.
Common questions
What is a Monte Carlo retirement simulator?
It’s a tool that runs your retirement plan through many different market sequences (random or drawn from history) instead of one fixed average return. The result is a range of outcomes and a success rate, which captures the sequence-of-returns risk that a straight-line projection misses.
Is Coastline’s Monte Carlo simulator free?
Yes — it’s completely free with no signup. Coastline’s Simulation Tools replay real historical market cycles and Monte Carlo draws, with full federal and state tax math applied on every simulated path.
How many simulations should I run?
Historical mode runs every full window of real returns once — 67 for a 65-to-95 plan (1928–2024) — so the count is fixed. Random draws need a few hundred or more. What matters more than the count is that the model captures taxes, your account mix, and the order of returns — not just the average.
Monte Carlo or historical simulation — which is better?
Historical-cycle simulation replays actual past market sequences (preserving how crashes, recoveries, and inflation really clustered); Monte Carlo draws random returns from an assumed distribution. Coastline offers both, so you can stress-test a plan two ways.