FIRE Number Calculator
Calculate your FIRE number from annual expenses and a safe withdrawal rate, and how many years of saving at your current rate gets you there.
How to use this calculator
- 1Enter your expected annual expenses once financially independent — not your current income, but what you would actually need to spend.
- 2Choose a withdrawal rate — 4% is the traditional starting point, though a longer retirement horizon generally calls for a more conservative figure.
- 3Enter your current savings and monthly contribution to see how many years, in today's money, it takes to get there.
How the calculation works
FIRE number = Annual expenses ÷ Safe withdrawal rate- Annual expenses
- What you expect to spend per year once no longer working
- Safe withdrawal rate
- The share of the portfolio withdrawn in the first year, later adjusted for inflation — 4% is the traditional figure
A 4% withdrawal rate is mathematically the same as needing 25 times annual expenses (1 ÷ 0.04 = 25) — the "25x rule" and the "4% rule" are the same statement in two forms.
Years to reach the number are projected using the real (inflation-adjusted) rate of return, so the result is expressed in today's purchasing power rather than requiring the expense target itself to be inflated forward year by year.
Worked example
$48,000 annual expenses at a 4% withdrawal rate
- 1.FIRE number: $48,000 ÷ 4% = $1,200,000 — equivalently, 25 × $48,000.
- 2.Real annual return: (1.07 ÷ 1.025) − 1 ≈ 4.39%.
- 3.Starting from $60,000 and adding $2,000 a month at that real return, the balance crosses $1,200,000 in about 24.4 years.
Result: FIRE number of $1,200,000, reached in about 24.4 years
Where the 4% rule actually comes from
The "4% rule" traces back to a 1994 study by financial adviser William Bengen, who tested how a portfolio would have survived historical US market returns across every rolling 30-year retirement period back to 1926. He found that withdrawing 4% of the starting balance in year one, then adjusting that dollar amount for inflation every year after, survived every historical 30-year period he tested — even retirements that began right before major market crashes. The 1998 "Trinity Study," by three professors at Trinity University, extended the same approach across different withdrawal rates and stock/bond mixes and became the more widely cited academic source for the same conclusion.
Why 25x and 4% are the same idea
If a portfolio can sustainably support a 4% withdrawal, then by definition the portfolio itself needs to be 25 times the annual amount withdrawn — 1 divided by 0.04 is exactly 25. This is why the FIRE community uses "25 times annual expenses" and "the 4% rule" completely interchangeably; they are algebraically the same statement, just phrased as a multiple in one case and a rate in the other.
The rate itself remains genuinely debated
Bengen's original study and the Trinity Study both used historical US-only market data over a specific stretch of the 20th century, which several later researchers have argued was an unusually favorable period for US stocks. More recent work has produced a real range rather than a single number: some updates push the safe rate higher for shorter retirements or more flexible spending, while others — citing today's higher starting valuations — argue for something closer to 3.5% for a retirement expected to last 40 years or more rather than the traditional 30. There is no single "correct" answer, which is precisely why this calculator treats the withdrawal rate as an input to adjust, not a fixed constant.
What the number does not capture
A single FIRE number is a useful target, but it compresses a lot of real complexity into one figure.
- Sequence of returns risk — a market downturn in the first few years of retirement does far more damage than the same downturn happening later, because withdrawals are being taken from a shrinking balance — the 4% rule's historical testing already reflects this risk, but any individual retirement is still just one possible path, not a guarantee.
- Spending flexibility — a retiree willing to cut spending in a down market can typically sustain a higher initial withdrawal rate than the rule assumes, since the original studies model a rigid, inflation-adjusted withdrawal regardless of market conditions.
- Healthcare and major one-off costs — a country's healthcare system, and known future costs like a child's education or a home's major repairs, are not part of a simple annual-expenses figure unless deliberately budgeted in.
What this assumes, and where it stops
Assumptions
- Annual expenses are assumed to stay constant in real (inflation-adjusted) terms throughout retirement.
- The years-to-FI projection uses a constant real rate of return, which real markets never deliver in a straight line — actual years will vary, sometimes substantially, from this smoothed estimate.
- The withdrawal rate you choose is assumed sustainable for your actual retirement length; a rate validated for a 30-year retirement is not automatically validated for a 50-year one.
Limitations
- Does not model sequence-of-returns risk — the specific danger of a market downturn occurring early in retirement, which the original historical backtesting captures implicitly but a simple average-return projection like this one does not.
- Does not account for other income in retirement (Social Security, a pension, part-time work), which would reduce how much the portfolio itself needs to cover.
- Tax treatment of withdrawals is not modelled — a portfolio in tax-advantaged accounts may need to be somewhat larger to net the same after-tax spending as one in a taxable account, or vice versa, depending on account mix.
Common questions
Is the 4% rule guaranteed to work?
No — it is a historical backtest, not a guarantee. It describes what would have worked across past US market history, not a mathematical certainty about the future. Many practitioners treat it as a reasonable starting point to stress-test and adjust, particularly by staying willing to spend somewhat less in years following poor market returns.
Should I use a lower withdrawal rate for an early retirement?
Generally yes — the original studies modelled a 30-year retirement horizon. Someone retiring in their 30s or 40s under a FIRE plan is often planning for a 50-year-plus horizon, and most researchers who have examined longer horizons recommend a correspondingly lower withdrawal rate, commonly cited in the 3.25%–3.75% range rather than 4%.
Sources
- Safe withdrawal rates for retirement and the Trinity Study — Retirement Researcher
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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