Your FIRE number is 25 times your estimated annual expenses in retirement, derived directly from the 4% safe withdrawal rate (SWR). The 4% rule comes from the Trinity Study (1998, updated 2011), which analyzed historical stock and bond returns and found that a 4% annual withdrawal from a diversified portfolio has historically lasted 30+ years in virtually all market scenarios. If you plan to spend $50,000/year in retirement, your FIRE number is $1,250,000 — meaning once your portfolio hits that mark, you can withdraw $50,000 annually (inflation-adjusted) without depleting it. For extra-early retirements (age 35-45), consider targeting 3-3.5% withdrawal rate to account for longer retirement periods.

Compound interest is the core mechanism of FIRE — your portfolio earns returns on both your original contributions and all previously accumulated gains. At 7% annual return, $100,000 doubles to about $196,715 in 10 years with no additional contributions. Add $2,000/month and that same account reaches over $500,000. The critical insight is that growth accelerates over time: the first $100k is the hardest and slowest to accumulate; each subsequent $100k happens faster because your earnings base is larger. This is why starting early matters enormously — every 7 years at 10% return, your money roughly doubles.

The S&P 500 has returned approximately 10% annually (nominal) over the past century, or about 7% after accounting for 3% average inflation. For FIRE planning, most practitioners use 6-7% as their real (inflation-adjusted) return assumption for a diversified equity-heavy portfolio. If you're conservative, use 5-6%. If your portfolio includes bonds or more conservative assets, use 4-5%. Keep in mind that sequence-of-returns risk (bad years early in retirement) matters more than average returns, which is why the 4% SWR was designed to survive the worst historical scenarios, including the Great Depression and 1970s stagflation.

Yes, and it happens more commonly than you might think — the FIRE movement includes thousands of people who retired in their 30s and 40s. The key levers are savings rate and time: saving 50% of a $100k income and investing in low-cost index funds creates a powerful compounding engine. A 30-year-old with $100k saved and contributing $3,000/month at 7% returns would reach a $1.25M FIRE number in approximately 14-15 years, retiring around age 44-45. The math is real. The critical moves: maximize tax-advantaged accounts (401k, IRA, HSA), keep housing costs below 25% of income, and avoid lifestyle inflation as income grows.