FIRE stands for Financial Independence, Retire Early. It is a shorthand for a single question: how large does an investment portfolio have to be before the income it can safely produce covers what you actually spend? Once a portfolio crosses that line, paid work becomes optional rather than necessary — you can keep doing it, change to something that pays less but suits you better, or stop altogether. Everything on this page and the calculators beneath it is arithmetic built around that one idea, applied from a dozen different angles.
The term dates back to the 1992 book Your Money or Your Life by Vicki Robin and Joe Dominguez, which reframed spending as hours of life traded for money rather than pounds handed over. It picked up its current name and a large online following in the 2010s, driven by bloggers such as Mr Money Mustache, who popularised the "shockingly simple maths" behind it: your time to independence is determined almost entirely by your savings rate, not by how much you earn.
The 4% rule and where it comes from
The usual starting point is the 4% rule, which came out of the Trinity Study — a 1998 analysis by three finance professors at Trinity University who tested historical US stock and bond returns against thirty-year retirement periods. They found that a portfolio invested in a mix of stocks and bonds, with an initial withdrawal of 4% of its value in year one and that amount then rising each year with inflation, survived the great majority of thirty-year periods in American market history without running out of money. Inverted, that 4% figure means a portfolio of roughly 25 times annual spending is considered sufficient — the "25x rule" you will see repeated across every calculator on this page.
It is a rule of thumb, not a guarantee, and it comes with real caveats worth stating plainly. It was derived from one country's market history over one period, and past returns are not a promise of future ones. It assumes a specific asset mix, roughly 50–75% equities. And thirty years was the horizon tested — someone retiring at 35 might need their money to last fifty years or more, which is why many independent analyses of early retirement suggest a more conservative 3.25–3.5% is the safer figure once the horizon stretches well beyond three decades. The calculators here default to 4% but let you move it, which is the point: the rule is a starting assumption to stress-test, not a fixed law.
The three numbers that actually matter
Strip away the acronyms and FIRE math comes down to three inputs: what you spend in a year, what you save in a year, and what return you assume markets will deliver after inflation. Of the three, savings rate does almost all of the work. Someone saving 10% of take-home pay is on a multi-decade timeline under almost any realistic return assumption; someone saving 50% can plausibly reach independence in well under twenty years. The reason is not just that a higher savings rate builds the pot faster — it is that it shrinks the target at the same time, because living on half your income means you only need to replace half your income later. That double effect is what makes the savings rate the single most powerful lever in the whole calculation, more powerful than the return you earn on your investments.
The variants: Lean, Fat, Coast and Barista
FIRE is not one plan but a family of them, and the calculators below cover the common variants. Lean FIRE targets a deliberately minimal budget, reaching independence sooner at the cost of very little slack for the unexpected. Fat FIRE goes the other way, targeting a retirement at or above your working-life standard of living, which takes longer to reach but leaves more room to breathe. Coast FIRE is the point at which your existing investments, left alone with no further contributions, will compound into a full FIRE number by a normal retirement age — useful for anyone who wants to stop saving aggressively without giving up the eventual goal. Barista FIRE sits between full independence and full-time work: a portfolio large enough to cover most of your spending, topped up by modest part-time income, which the calculator shows can remove a surprising amount of required capital for a comparatively small amount of continued work.
What these calculators are not
None of this is financial advice, and none of it accounts for your specific tax position, health, family circumstances or risk tolerance. The calculators use standard, published formulas — the same maths behind the Trinity Study and the annuity mathematics used throughout personal finance — applied to the numbers you enter. Treat the output as a planning estimate to revisit every time your income, spending or the market moves meaningfully, not as a figure carved in stone the day you first calculate it.
Where to start
If none of the terms above are familiar yet, the FIRE number calculator is the natural starting point: it takes a single spending figure and a withdrawal rate and returns the portfolio size the rest of this section works from. From there, the FIRE age and years-to-financial-independence calculators turn that target into an actual date given what is saved today and what continues to be saved each month. Coast FIRE and Barista FIRE are worth a look once a full target feels a long way off, since both describe a smaller, nearer milestone that still moves the eventual date meaningfully closer. Everyone else's numbers will look different from the examples used throughout this section — that is expected, and the point of a calculator rather than a table of fixed figures is that your own numbers are exactly one edit away.