All projections are in today's dollars
Every chart and number on this site is expressed in real (inflation-adjusted) terms. The real return is computed by division, never subtraction:
real return = (1 + nominal) ÷ (1 + inflation) − 1
With the default 7% nominal and 3% inflation, that's 3.883% — not the 4% the shortcut gives. Over 30 years the shortcut inflates a portfolio projection by roughly 3%, which is exactly the kind of quiet optimism a retirement calculator shouldn't smuggle in. Monthly compounding uses the exact twelfth root, (1 + r)1/12 − 1, so twelve months compound to precisely the annual rate.
The FIRE number, taxes included
Withdrawals from pre-tax accounts are taxable income, so we gross up spending before dividing by the withdrawal rate: gross spend = annual spending ÷ (1 − effective tax rate), and FIRE number = gross spend ÷ SWR. The default safe withdrawal rate is 4% (the Trinity study convention), adjustable from 2.5% to 6% — because reasonable people disagree, and the slider shows what each opinion costs.
Pensions and Social Security: two phases, not a hand-wave
A pension that starts at 67 doesn't reduce the portfolio you need at 55 by its face value. We price the bridge years and the pension years separately: the years before the pension starts are an annuity of full gross spending (discounted at the real rate), and the years after need only (spending − pension) ÷ SWR, discounted back from the pension start date. If the pension is already flowing at retirement, it simply reduces spending.
Coast numbers and the coast age
The coast number at any age is the FIRE number discounted back at the real return: coast(age) = FIRE number ÷ (1 + r)retire age − age. Your coast age is the first age at which your projected portfolio — current assets plus contributions, compounding monthly — meets that curve. From there, compounding alone finishes the job.
Debt payoff: the snowball that actually rolls
The debt calculator simulates month by month at each card's APR ÷ 12 — the same convention as your statement. The total monthly budget never shrinks: when a debt dies, its minimum payment rolls into the next target (smallest balance for snowball, highest APR for avalanche), and any leftover cascades within the same month. Calculators that quietly drop freed minimums overstate your payoff date; ours is tested against hand-derived schedules.
What we deliberately simplify
- Constant returns. Real markets deliver sequences, not averages. The crash-test toggle (assets × 0.7) exists precisely because a smooth 7% line is a fiction — use it.
- One effective tax rate. Real withdrawal taxation is bracket-by-bracket and account-by-account. A single effective rate keeps the input honest without pretending to be tax software.
- No fees input. If you pay meaningful fees, subtract them from your nominal return — that's exactly what they do to you.
Every formula above is enforced by unit tests against hand-derived golden numbers, and all of it runs in your browser — your inputs never reach a server. Found an error? That's a bug, not a rounding choice — tell us and we'll fix it.
FireVerdict is educational, not financial advice. Start with the Coast FIRE calculator or browse all twelve tools.