- Your FIRE number is annual spending ÷ withdrawal rate — 25 times spending at 4%, and 33 times at 3%.
- Cutting spending works twice over: it lowers the target and raises the amount you can invest towards it.
- The 4% rule was measured across 30-year US historical windows, so a 45-year early retirement is a different question — which is why 3–3.5% is common among young retirees.
- Coast, Barista, Lean and Fat FIRE are the same equation with a different spending level or a different stopping rule.
How the FIRE calculator works
FIRE — Financial Independence, Retire Early — is one piece of arithmetic surrounded by a great deal of argument about which number is safe. The arithmetic: once a portfolio is large enough that a sustainable withdrawal covers a year of spending, paid work becomes optional. The calculator takes your annual spending and a withdrawal rate, returns that portfolio size, then projects how long your current balance plus your contributions take to reach it.
Two inputs dominate everything else. Spending sets the target: at a 4% withdrawal rate, every $1,000 a year you do not spend removes $25,000 from the number — and frees up $1,000 a year to invest. The withdrawal rate sets the multiple: 4% is 25 times spending, 3.5% is about 28.6 times, and 3% is 33.3 times.
The variants you will see named — Coast, Barista, Lean and Fat FIRE — are the same equation with a different spending level or a different stopping rule, not different maths. Each one is defined in the key terms below.
The math
The target is annual spending ÷ withdrawal rate. The timeline solves the same annuity relationship for the number of years, with your existing balance included: n = ln((target × r + C) ÷ (B × r + C)) ÷ ln(1 + r), where B is what you have invested today, C is the annual contribution and r is the expected real return. Contributions are treated as a single deposit at the end of each year and the return as constant.
The 4% rule deserves more caution than it usually gets. It comes from William Bengen’s original safe-withdrawal-rate paper and the Trinity study that followed, which asked what fixed, inflation-adjusted withdrawal a US stock-and-bond portfolio would have survived across historical 30-year windows. Roughly 4% did. But the horizon was 30 years, not the 45 to 55 a retirement at 40 implies; the data was US returns, among the best in the world over the twentieth century; and the model carried no fees and no tax. Later work on global data and on lower forward-looking returns tends to land nearer 3% to 3.5% for very long horizons, which is why those rates are offered here.
A withdrawal rate is also a starting rate, not a vow. Sequence-of-returns risk — a bad first decade — is what actually ends plans, and in practice it is met by trimming spending or earning a little for a while. That flexibility is worth more than any decimal place on the rate you pick today.
Worked example
At the defaults — $60,000 a year of spending drawn at 4% — the FIRE number is $1,500,000. Starting from $120,000 already invested, adding $30,000 a year and earning 5% real, the calculator puts that about 21.9 years away.
Change only the withdrawal rate and the price of caution becomes visible: at 3.5% the target is $1,714,286 and the horizon 23.9 years; at 3% it is $2,000,000 and 26.3 years. Four and a half extra years of work is what that margin costs. Spending is the stronger lever — dropping to $50,000 a year cuts the 4% target to $1,250,000 and the timeline to about 19.3 years on unchanged contributions, and to roughly 16.4 years if the $10,000 you stopped spending is invested instead.
Key terms
- FIRE number
- The invested total that covers a year of spending at your withdrawal rate, indefinitely — annual spending ÷ the rate, so 25 times spending at 4% and 33 times at 3%.
- Safe withdrawal rate
- The share of the starting portfolio you draw in year one, then adjust for inflation each year after. 4% is the historical benchmark for a 30-year retirement; 3–3.5% is the usual adjustment for a 45-year one.
- Coast FIRE
- The point where what you have already invested will grow to your FIRE number by a normal retirement age with no further contributions. You still need income to cover this year’s spending, but you can stop saving.
- Barista, Lean and Fat FIRE
- Barista FIRE covers part of your spending from the portfolio and the rest from light or part-time work. Lean FIRE is full FIRE at a deliberately low spending level; Fat FIRE is the same at a high one. All three use the same spending ÷ withdrawal-rate division — only the spending figure changes.