Your FIRE number is the portfolio that covers your spending without you working. The usual shorthand is 25 times your annual spend, which is the 4 percent rule turned upside down. Spend $48,000 a year and the number is $1.2 million.
Coast FI is the quieter and more useful figure. It is the amount that, left alone, grows into your FIRE number by the age you want it. Once you pass it you can stop investing entirely and still retire on time, which usually arrives years before anyone expects.
It came from the Trinity study using US historical returns over 30 year retirements. For a retirement starting at 40 or 45 the horizon is closer to 50 years, and most researchers drop the figure to somewhere between 3.25 and 3.5 percent for that. At 3.5 percent your target is 28.6 times spending rather than 25.
FIRE is the finish line, the amount that funds your life forever. Coast FI is the point where you can stop adding money and compounding does the rest. Coast FI always arrives first, and hitting it is what turns a stressful job into an optional one.
Use what actually leaves your account in a year, including tax you will still owe in retirement. Retirement income from a traditional 401k or IRA is taxed as ordinary income, so budgeting your gross spend rather than your take home avoids a shortfall of exactly the size of your tax bill.
In the US it changes it a lot if you retire before 65. Marketplace premiums count as spending, and the 400 percent federal poverty level subsidy cliff came back in 2026, which for a couple lands at $84,600 of income. Cross it by a dollar and the premium credit goes to zero.
FIRE number calculator
Three numbers, one page: what you need invested to quit, how long it takes to get there, and the amount that gets there on its own if you never invest another dollar.
Step 1 · Your FIRE number
4% is the classic rule. 3% is the more conservative choice if you plan to retire early and need the money to last far longer than 30 years.
Your FIRE number
$900,000
$36,000 a year of spending, divided by a 4% withdrawal rate.
The shortcut version: monthly bills × 300 = $900,000. Same answer — because at a 4% rate, 12 months ÷ 0.04 is exactly 300.
Step 2 · Time to get there
Time to your number
23 years, 6 months
Starting from $0, adding $800 a month, growing at 10% a year.
That puts you at your FIRE number around age 53 — $900,000 invested, covering $3,000/mo.
Step 3 · Coast FI
Hit this amount and you can stop contributing entirely. Left alone, it compounds into your full FIRE number by your retirement age. You still have to cover your bills until then — but the investing part is done.
Your Coast FI number at 30
$32,026
Invested today at 10% a year, that grows into $900,000 in 35 years — with nothing added.
You are $32,026 away from being able to stop contributing.
What it takes at every age (retiring at 65)
| Age | Years of growth | Invest once |
|---|---|---|
| 20 | 45 | $12,347 |
| 30 | 35 | $32,026 |
| 40 | 25 | $83,066 |
| 50 | 15 | $215,453 |
| 60 | 5 | $558,829 |
Same destination, wildly different price. Every decade you wait multiplies what a one-time investment has to be.
The 4% rule. It comes from studies of historical US stock and bond returns which found that a retiree withdrawing 4% of their starting portfolio in year one, then adjusting that amount for inflation, would usually not run out of money over a roughly 30-year retirement. Dividing your annual spending by 4% is just that rule run backwards. Because a year is 12 months and 12 ÷ 0.04 = 300, it is identical to the “monthly bills × 300” shortcut.
What it assumes, and ignores. A roughly 30-year horizon — retire at 40 and you may need the money to last 50+ years, which is why many people use 3% to 3.5% instead. Historical US market returns, which are not a promise about the future. It ignores taxes, healthcare costs, and sequence-of-returns risk — a bad crash in your first few retirement years does far more damage than the same crash later, even if the long-run average is unchanged.
The timeline. Your existing balance and your monthly contributions are compounded at your expected return, monthly, until they reach your FIRE number. The return is applied as a flat, smooth rate — real markets are not smooth, so treat the answer as a rough middle, not a date. Returns here are nominal; if you want the answer in today's money, enter a return net of inflation (e.g. 7% instead of 10%) and keep spending at today's level.
Coast FI. Same compounding, run in reverse: the FIRE number discounted back to today at your expected return over the years until retirement. It assumes zero further contributions and that you leave it invested and untouched the whole time.
This is an estimate for planning and curiosity, not financial advice. Everything you type stays in your browser — nothing is stored, sent, or tracked. Not sure what you actually spend each month? That is the number Deco works out for you automatically.