How this works — and where the numbers come from
Coastfarer simulates 100,000 versions of a financial life and reports what tends to happen. "On track" means one thing throughout: the money lasts through the plan horizon (age 90 by default). Everything is in today's dollars — inflation is already taken out.
The market model
Each simulated life draws its investment returns by replaying blocks of real market history — inflation-adjusted US large-cap annual returns, 1928–2025 (Damodaran's S&P 500 total-return series deflated by BLS CPI), resampled in multi-year blocks so crashes stay streaky and recoveries stay slow, the way they actually were. History's average is shifted to a conservative 5% real forward assumption. This preserves the fat tails and bad decades that a smooth "average return" hides — the entire reason outcomes spread the way they do.
Taxes
In gross mode, each year of each life is taxed with 2025 law: federal single-filer brackets, FICA (Social Security to the $176,100 wage base + Medicare incl. the 0.9% additional), and a representative top-marginal state rate. No deductions or credits are modeled — skipping the standard deduction makes the tax bite slightly worse than reality, which is the conservative direction.
Social Security
Estimated with the SSA's actual formula: earnings capped at the wage base, averaged over a 35-year career, run through the 2025 bend points, adjusted for claim age (62–70). The default then pays 75% of the scheduled benefit — because the SSA's own Trustees project the trust fund depletes around 2033, after which ongoing payroll taxes still cover roughly 75–79% of benefits. "It won't exist" is too pessimistic; ~three-quarters is the honest do-nothing case. You can set full, reduced, or none.
Life events — calibrated to real data
The events that happen to you carry base rates and magnitudes from primary US sources — not ballpark guesses. Job loss also couples to the market: it's three times likelier in a crash year, so simulated people lose jobs exactly when their portfolios are down, the way real recessions work. The events you choose (marriage, kids, home, sabbatical…) use your own answers from the "Life plans" panel.
| Event | Rate & effect | Primary source |
|---|---|---|
| Job loss | 2.5%/yr, ×3 in a crash; a year at $0 then a ~5% wage scar | BLS Displaced Workers, JOLTS |
| Severe pay cut | 0.4%/yr, ×2 in a crash; −15% for 3 years | NBER WP 25470 |
| Medical shock | 3→8%/yr rising with age; −$12k | ACA OOP max, AHRQ MEPS |
| Disability | 0.3→1.3%/yr rising with age; −50% income | SSA actuarial |
| Inheritance | 2%/yr in the receiving window; +$120k (conservative) | Penn Wharton, Fed FEDS note |
| Divorce | 1.8%/married-yr, front-loaded; −$30k + household reversion | CDC/NCHS, IFS |
| Eldercare | 4.5%/yr mid-life; −$8.5k/yr for 5 years | AARP |
| Lawsuit | 0.2%/yr; −$40k — deliberately unsourced | no credible base rate exists; kept small |
Sourcing honesty: some of these are direct statistics, others are reasoned conversions (a lifetime share turned into an annual rate), and the lawsuit rate has no defensible source at all — the widely-quoted "10% a year" traces to insurance marketing, so we keep it small and say so. The full working notes, with confidence levels per number, live in the project's calibration document.
Known limits — stated plainly
- Single-filer taxes only (married-filing-jointly is a known gap; single is the conservative default).
- Withdrawals aren't taxed separately (no LTCG vs. ordinary distinction yet).
- Divorce models legal costs, not the asset split.
- Event magnitudes are point estimates, not distributions.
- Age ramps are linear where real incidence curves accelerate.
- One market model (US large-cap history); no asset-allocation choices yet.
Reproducibility
The simulation is deterministic: the same inputs and the same seed produce the same 100,000 lives, to the dollar. That's why the numbers on the page don't wobble between visits — and why results can be reproduced exactly.
Privacy
Every simulation runs in your browser. Your numbers are never sent anywhere — the site's Content-Security-Policy (connect-src 'self') forbids the page from talking to any other server, so this is enforced by your browser, not just promised. No analytics, no ads, no accounts.
This is not financial advice. Coastfarer is an educational simulation. It models one simplified version of the world with the assumptions above, and reality will differ. Nothing here accounts for your full situation — for decisions, consult a licensed professional.