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How the FIRE Number and Timeline Are Calculated

The tool works in two steps. First it sets your target: FIRE number = annual spending ÷ safe withdrawal rate, which is mathematically identical to annual spending × 25 at the classic 4% rate (since 1 ÷ 0.04 = 25). Second, it projects a path to that number: starting from your current savings, it adds your monthly contribution and compounds everything at your expected real (after-inflation) rate of return, month by month, until the balance crosses the target.

Worked example: you spend $40,000 a year, so your FIRE number at the 4% rule is $40,000 × 25 = $1,000,000. Say you currently hold $100,000 in invested savings, add $2,000 a month, and assume a 5% real annual return (≈0.407% monthly). Compounding that contribution schedule forward, the balance crosses $1,000,000 in roughly 17-18 years. Raise the monthly contribution to $3,000 with everything else unchanged, and the same target is reached in closer to 13-14 years — the nonlinear payoff of a higher savings rate compounding sooner.

The 4% figure itself comes from the Trinity Study, which back-tested US historical stock/bond returns over rolling 30-year retirement periods and found a 4% initial withdrawal, adjusted for inflation each year after, rarely exhausted the portfolio within 30 years. It is a historically-grounded rule of thumb, not a mathematical guarantee.

What the Assumptions Are Sensitive To

Two inputs move the outcome more than anything else you can adjust: the withdrawal rate and the assumed return. Retiring earlier than a standard 30-year retirement, or simply wanting a wider safety margin, argues for a more conservative 3-3.5% withdrawal rate — which raises your target multiple to roughly 28-33× annual spending instead of 25×. The Trinity Study's 4% also reflects specifically US historical market returns; research on international and more recent data often supports the lower end of that range instead.

Savings rate matters more than almost any other lever, because it works twice: spending less means saving more each month and needing a smaller total portfolio to cover that reduced spending. Under typical assumptions, saving roughly 10% of income implies a path to independence measured in decades (~45-50 years); 50% savings compresses that to roughly 15-17 years; 70% savings can get there in under 10.

Only count assets that could actually replace income: investment accounts, and rental property income capitalized at a reasonable rate. A paid-off home does not belong in the portfolio total — instead, its effect shows up as lower annual spending, since you are not paying rent or a mortgage. Expected state or workplace pensions can be modeled by subtracting their monthly amount from spending starting the age they begin. None of this is a guarantee: market returns, inflation, and sequence-of-returns risk in the early retirement years can all push the real outcome away from the projection. This calculator is for informational and educational purposes only, not personalized financial advice.