The Real Math Behind FIRE: How to Calculate Your Retirement Number
Most FIRE (Financial Independence, Retire Early) content repeats the same headline advice — save aggressively, follow the 4% rule — without walking through the actual math behind your specific number. Here's how that number is actually calculated, and where the popular shortcuts break down.
The 4% rule, in plain terms
The 4% rule comes from research on historical U.S. market returns: if you withdraw 4% of your portfolio in year one of retirement, then adjust that dollar amount for inflation each year after, your money has historically lasted 30+ years in most market conditions. Flip that around, and it gives you a target: your FIRE number is roughly 25 times your annual expenses (since 1 ÷ 0.04 = 25).
Why 25x isn't the whole answer
The 4% rule was built around a traditional ~30-year retirement, starting around age 65. If you're retiring at 35 or 40, your money potentially needs to last 50-60 years, not 30 — and over a longer horizon, market downturns early in retirement (sequence-of-returns risk) can do much more damage. Most early-retirement researchers now suggest a more conservative 3-3.5% withdrawal rate for very early retirees, pushing your real number closer to 28-33x annual expenses rather than a flat 25x.
The actual calculation
Start with your real annual expenses — not your income, your actual spending. Then divide by your chosen safe withdrawal rate:
FIRE Number = Annual Expenses ÷ Safe Withdrawal Rate
Example: if you spend $40,000/year and use a 3.5% withdrawal rate (conservative, appropriate for an early retirement), your FIRE number is $40,000 ÷ 0.035 = $1,142,857. Using the more standard 4% rate instead gives $1,000,000 — a meaningful difference, which is exactly why picking the right rate for your specific timeline matters.
Traditional retirement vs. FIRE — different math, same formula
The formula doesn't change between a traditional 65-year-old retiree and a 35-year-old FIRE devotee — what changes is the safe withdrawal rate you plug in, based on how long your money needs to last. Someone retiring at 65 with a ~25-30 year horizon can reasonably use closer to 4%. Someone retiring at 35 with a 55+ year horizon should lean more conservative, in the 3-3.5% range.
Don't forget Social Security or pensions
If you'll eventually receive Social Security or a pension, you don't need your portfolio to cover 100% of your expenses forever — only the gap between your expenses and that guaranteed income. Subtract expected guaranteed income from your annual expenses before applying the formula, and your real number often comes in meaningfully lower than the headline calculation suggests.
Run your own numbers
Rules of thumb are a starting point, not a personalized answer — your real number depends on your actual expenses, timeline, and risk tolerance. The free FIRE Calculator lets you plug in your real numbers and compares Lean, Fat, Coast, and Barista FIRE scenarios side by side. If you're closer to a traditional retirement timeline instead, the Retirement Calculator walks through the same 25x/4% math with a more conventional withdrawal horizon.