Your FIRE number is your annual retirement spending divided by your safe withdrawal rate: at $48,000 a year of spending and a 4% withdrawal rate, it is $48,000 ÷ 0.04 = $1,200,000. That is the portfolio where investment income, not a salary, covers your life — and work becomes optional. With $150,000 already invested, $1,500 a month of contributions and 7% annual returns, a month-by-month simulation reaches $1,200,000 in 224 months, about 18.7 years.
The tool recommended here — Asad Webs' free FIRE Number Calculator — works out the number from your expenses and withdrawal rate, then shows the years to FIRE, the projected date, and your coast FIRE number: the amount that could grow the rest of the way on its own. It runs in your browser, free, with no sign-up.
Try it now: FIRE Number Calculator — Your FIRE Number, Years to FIRE and Coast FIRE. Enter your annual expenses, pick a withdrawal rate, add your savings and monthly contributions, and see the date your portfolio could carry you.
The formula: 25 times your annual spending
The classic 4% safe withdrawal rate comes from the Trinity study: a portfolio that can historically support spending 4% of its starting value each year, adjusted for inflation. Dividing by 0.04 is the same as multiplying by 25, so the rule of thumb is simple — your FIRE number is 25 times what a year of your life costs in retirement. The spending figure is what matters, not your salary: a household that lives on $48,000 needs $1,200,000, whether it earns $70,000 or $170,000.
A lower withdrawal rate is safer but demands a bigger portfolio, because the rate is the divisor. At 3.5% you need about 28.6 times annual spending; at 3%, about 33.3 times. The table shows what that means for the same $48,000 of spending.
| Withdrawal rate | Multiple of spending | FIRE number on $48,000 |
|---|---|---|
| 4% (classic) | 25× | $1,200,000 |
| 3.5% | 28.6× | $1,371,429 |
| 3% | 33.3× | $1,600,000 |
FIRE number on $48,000 annual spending, by withdrawal rate
The worked example: $48,000 a year becomes $1.2 million in 18.7 years
Start with the spending: $48,000 a year in retirement, and a 4% withdrawal rate, gives a FIRE number of $1,200,000 exactly. Then the path: $150,000 already invested, $1,500 added each month, 7% annual returns. The calculator simulates month by month — the balance grows at the monthly equivalent of 7% a year (about 0.5654% a month) and the $1,500 lands at each month end — and the balance first passes $1,200,000 at month 224, about 18.7 years, ending that month at roughly $1,203,130. At month 223 it is still about $1,194,831, short of the line.
Two things decide how fast that date arrives: how much you already have, and how much you add each month. A bigger starting balance and a bigger monthly contribution both pull the date earlier, and the contribution matters more than most people expect early on — in the first years the monthly additions do more work than the growth does.
Estimate retirement spending from your life, not your payslip. Housing, food, travel and everything else you will actually pay for — that figure, not your salary, sets the target. If the mortgage will be gone by retirement, leave it out; if travel will rise, put it in.
Lean FIRE and Fat FIRE: the same formula, a different life
The FIRE number is not one figure. Lean FIRE scales spending down for a stripped-back retirement; Fat FIRE scales it up for a generous one. The calculator uses ×0.75 for Lean and ×1.25 for Fat, applied to the same $48,000 of spending, and the formula does the rest.
| Variant | Annual spending | FIRE number at 4% |
|---|---|---|
| Lean FIRE (×0.75) | $36,000 | $900,000 |
| Full FIRE (×1.00) | $48,000 | $1,200,000 |
| Fat FIRE (×1.25) | $60,000 | $1,500,000 |
Lean FIRE arrives years sooner, but it only works if the stripped-back spending is a life you would actually enjoy for decades — a target you hate living on is a target you will abandon. Fat FIRE is the opposite trade: a longer working life for a retirement with real slack in it.
Coast FIRE: the number that lets you stop contributing
Coast FIRE asks a different question: how much would you need invested today so that growth alone — no more contributions — reaches your FIRE number by your retirement age? The formula is the FIRE number divided by (1 + return) raised to the years left: $1,200,000 ÷ 1.0735 ≈ $112,396 for someone aged 30 retiring at 65. With $150,000 already invested, that person is already past coast FIRE: even if contributions stopped, growth alone is projected to reach the target by 65.
Crossing coast FIRE changes the options, not the destination. Contributions become optional if you are willing to wait until the retirement age — everything you add after only pulls the date earlier. The calculator also tracks half FIRE and coast FIRE as milestones, so you can see when each point arrives on the same path.
Do not read the projected date as a promise. The calculator models returns as a smooth average; real markets arrive in lumps, and a bad sequence of returns early in retirement can strain even a 4% plan. Taxes, fees and inflation are not modelled either — many planners use a return figure net of inflation so the target stays in today's money.
What the number does not cover
The FIRE number covers spending at the withdrawal rate you chose — nothing else. It does not include the cost of a house you still owe on, a business you want to start, or a long period of high healthcare costs. It also assumes the spending figure stays honest: if retirement spending creeps up by $6,000 a year, the FIRE number at 4% rises by $150,000. That is why the spending estimate deserves more care than the rate debate — a $6,000 error in spending moves the target five times further than a 0.5-point error in the rate.
Related guides
Build the safety net first with how much emergency fund you really need, set the habit with a monthly budget that sticks, and set the next goal with a savings goal you will actually hit.
Frequently Asked Questions
How do I calculate my FIRE number?
Divide your annual retirement spending by your safe withdrawal rate. At $48,000 a year and 4%, that is $48,000 ÷ 0.04 = $1,200,000 — or 25 times your annual spending.
What is the 4% rule?
The classic safe withdrawal rate from the Trinity study: a portfolio can historically support spending 4% of its starting value each year, adjusted for inflation. A lower rate (3.5% or 3%) needs a bigger portfolio but survives more bad sequences of returns.
What is coast FIRE?
The amount you need invested today so compound growth alone — with no further contributions — reaches your full FIRE number by your retirement age. On a $1,200,000 target at 7% returns, age 30 retiring at 65, coast FIRE is about $112,396.
What is the difference between Lean and Fat FIRE?
Lifestyle variants of the same target. Lean FIRE scales spending down (×0.75 here) for a stripped-back retirement and a much smaller, earlier number; Fat FIRE scales it up (×1.25) for a generous one.
How long until I reach FIRE?
It depends on your starting balance, monthly contributions and returns. The worked example — $150,000 invested, $1,500 a month, 7% returns — reaches $1,200,000 in 224 months, about 18.7 years, by month-by-month simulation.
Is the FIRE Number Calculator really free?
Yes. The Asad Webs FIRE Number Calculator runs in your browser with no sign-up and no payment; it shows your FIRE number, years to FIRE, projected date, coast FIRE and milestones.
Run your own numbers with the free FIRE Number Calculator, then talk to Asad Webs about tools and websites that help your business plan the same way.
Sources: Asad Webs FIRE Number Calculator tool page (formula, worked example, coast FIRE, Lean/Fat multipliers, Trinity study 4% reference — verified live 2026-11-06). All figures are the article's own worked example on $48,000 annual spending: 25× = $1,200,000 at 4%; $1,371,429 at 3.5%; $1,600,000 at 3%; Lean $900,000; Fat $1,500,000. Returns are modelled as a smooth average; taxes, fees and inflation are not modelled.
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