How we calculate things

FIRE number = annual spending ÷ safe withdrawal rate (the classic 25× at a 4% SWR). The horizon projects your investable net worth — your portfolios plus your cash/bank accounts — forward with monthly contributions at your blended expected real (after-inflation) return: the value-weighted average of each account's own rate (so cash, at its low rate, adds to your number while easing the growth). It compounds with a geometric monthly rate and finds the month the projection crosses your target. Everything is in today's dollars.

Lean and Fat are the same formula at 60% / 150% of your spending (or your custom amounts). Coast FI is the amount that grows to your FIRE number by your target retirement age with zero further contributions. Barista FIRE funds only the gap between spending and part-time income.

How money leaves the portfolio in retirement is your choice, and the Horizon line, the Monte Carlo fan, and Readiness all apply the same rule. Fixed dollars (default) withdraws your annual spending every year — the classic 4%-rule paycheck, steady but a long bad market can deplete it. Percent of balance withdraws your SWR of the live balance — it can never hit zero, but income rises and falls with markets. Guardrails is percent of balance recomputed once per retirement year and clamped between a floor you can count on and an optional cap that banks good years — a binding floor CAN deplete the balance, which is exactly what the success rate then measures.

Smoothing (percent and guardrails) clamps the year-over-year change in your withdrawal to +5% / −2.5% (Vanguard-style dynamic spending), so income never whipsaws — at the cost that the limited descent can lag a fast crash. The FIRE number is spending ÷ SWR in every mode; the strategy changes only the drawdown.

We run up to 10,000 independent paths (1,000 on the free plan) of your plan. Each month, the portfolio earns a random return drawn from a lognormal model: monthly log-returns are normal with drift ln(1 + μ)/12 and volatility σ/√12, where μ is your blended expected real return and σ = 0.15 (roughly equity-heavy portfolio volatility). Contributions are added monthly until your projected FIRE month, then withdrawals follow your chosen strategy until age 95.

A path fails if it runs out of money before the end — once depleted, it stays depleted. The success rate is the share of paths still solvent at 95. The fan on the Horizon chart shows the 10th–90th (light) and 25th–75th (darker) percentile bands, sampled quarterly.

Limits worth knowing: returns are independent month to month (no momentum, no mean reversion), volatility is constant, and withdrawals follow your strategy. Social Security and pension inputs offset withdrawals after their start ages; taxes are not modeled. Treat the success rate as a stress test, not a prophecy.

TTM income = the per-share dividends with ex-dates in the trailing 12 months × the shares you hold today. It is a forward-income estimate, not received cash — shares bought last week count a full year of payouts. Yield on cost divides that by your cost basis; coverage divides it by your target retirement spending.

Your transaction ledger is replayed against end-of-day market closes at month-end samples. We use unadjusted closes because dividends already appear in your ledger as cash — adjusted prices would count them twice.

Scenarios re-run the deterministic horizon math with your overrides — same formulas, different inputs, computed instantly in your browser. Your real settings never change until you change them.