igmetricYour numbers
How the math works
- FIRE number = annual spending ÷ withdrawal rate. At 4%, every $1,000 of yearly spending needs $25,000 invested.
- Years to FI is solved in closed form: with monthly rate i, current savings C, and monthly savings M, months m = ln((FIRE + M/i) / (C + M/i)) ÷ ln(1+i). Monthly savings are assumed invested at month end.
- The 4% rule comes from the Trinity study — a 50/50 portfolio survived 30-year retirements in nearly all historical US periods. It assumes inflation-adjusted withdrawals, 30 years, and US market history repeating. Longer retirements or retiring into a crash argue for a lower rate.
- Savings rate dominates. Doubling your savings rate cuts the time to FI by roughly 40% — it grows the pile faster and shrinks the spending that pile must support.
Frequently asked questions
What is a FIRE number?
Your FIRE number is the invested portfolio balance associated with your chosen withdrawal rate — annual spending divided by the rate. At the classic 4%, $60,000 of yearly spending needs $1.5 million.
Is the 4% rule safe?
The 4% rule comes from the Trinity study: a 50/50 stock/bond portfolio survived 30-year retirements in nearly all historical US periods at a 4% inflation-adjusted withdrawal. It is a rule of thumb, not a guarantee — longer retirements, high fees, or retiring into a crash all argue for a lower rate or more margin.
What counts toward my FIRE number?
Invested assets you can actually draw from. Most people exclude home equity (you still need somewhere to live) and count retirement accounts with a plan for accessing them before 59½.
How does savings rate affect years to FI?
Enormously — more than returns do. Going from saving 20% to 40% of income cuts the time to FI by roughly 40% at typical returns, because it both grows the pile faster and shrinks the spending the pile must support.
Does this include inflation?
The projection uses a nominal return. For a cleaner read, enter a real (after-inflation) return — e.g., 5% instead of 7% — and think of the FIRE number in today's dollars.