The FIRE number formula
FIRE number = annual expenses ÷ withdrawal rate — at 4% that's simply expenses × 25. Spending $50,000 a year means a target of $1,250,000. The logic comes from the Trinity study: a portfolio of stocks and bonds has historically survived 30-year retirements at a 4% initial withdrawal rate with high probability. Want more margin, or a retirement longer than 30 years? Use 3.5% (× 28.6) or 3.25% — the calculator lets you set any rate.
Years to FIRE — the worked example
Starting from $100,000, saving $1,500/month at 7% returns, the $1.25M target arrives in about 20 years and 8 months. Two levers move that date far more than investment skill: your savings rate and your expenses. Cutting $500/month of permanent spending does double duty — it adds $500 to savings AND lowers the target itself by $150,000 (at 4%), which is why the FIRE community obsesses over expenses rather than stock picks.
Rules of thumb worth knowing
Savings rate maps to working years almost mechanically: at a 10% savings rate, retirement is ~50 working years away; at 25%, ~32 years; at 50%, ~17; at 65%, ~10 (assuming ~5% real returns). Also budget honestly for the expenses your job currently hides — health insurance before Medicare is the big one for early retirees — and remember the target is in today's dollars, so keep contributions rising with inflation.
How to use this calculator
Start with your expected annual spending in retirement — in today's dollars, and honest about the costs your job currently hides, like health insurance before Medicare. Set a withdrawal rate (4% for a standard 30-year retirement, 3.25-3.5% for a longer early-retirement horizon), and the calculator returns your FIRE number. Add your current invested savings, monthly contribution, and an expected real return of around 5% to see how many years away the target is. Then experiment: cutting annual spending does double duty, lowering the target and raising your savings rate at once, which moves the date more than any plausible change in investment returns. That's why the FIRE community focuses on expenses and savings rate rather than stock picking.