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MathFIRE Number Calculator
FIRE number calculator: find how much you need to retire early, your years to FIRE from your savings rate, Coast FIRE, 25x rule and SWR bands.
Have feedback? Report bugs, suggest features, or share your thoughts — we read them allWhat is the FIRE Number?
Your FIRE number is the amount of invested capital that, withdrawn at a safe rate, will fund your living expenses indefinitely. The classic 4% Trinity Study rule gives FIRE Number = Annual Spending × 25. Withdraw 4% of a balanced portfolio each year and historical US data suggests roughly a 95% chance it lasts 30+ years. This calculator extends that with inflation-adjusted compounding, Coast FIRE (the lump you need today so passive growth alone reaches FIRE by your target age), and Lean/Fat bands so you can plan multiple lifestyle scenarios.
Key Features
- Headline FIRE number using your custom withdrawal rate (default 4%)
- Years-to-FIRE calculation using real (inflation-adjusted) returns
- Monthly contribution required to hit FIRE by your target age
- Coast FIRE: the lump-sum you need today with zero further contribs
- Trinity Study sensitivity table for 3.0% / 3.5% / 4.0% / 4.5% / 5.0% SWR
- Lean / Regular / Chubby / Fat FIRE bands based on lifestyle multiples
- Multi-currency display (USD, EUR, GBP, JPY, VND, BRL)
- 100% browser-side: no data leaves your device
How to Use
- Enter your current age and the age you want to retire
- Enter your invested savings today and your expected annual spending in retirement
- Adjust expected return (7% real US equities historical), inflation (3% is typical) and withdrawal rate (4% is the classic Trinity Study figure)
- Click Calculate to see your FIRE number, years to FIRE and required monthly contributions
- Compare Coast FIRE, Lean / Fat bands and Trinity sensitivity rows to find the plan that fits your risk tolerance
Frequently Asked Questions
The 4% rule comes from the 1998 Trinity Study, which back-tested historical US stock and bond returns to find the highest withdrawal rate that survived 30-year retirements in at least 95% of historical periods. With a 60/40 stock/bond portfolio, withdrawing 4% of the initial balance (inflation-adjusted thereafter) succeeded in nearly all rolling periods 1926-1995. Multiplied by 25 (the inverse of 4%) it gives the familiar 25× annual spending FIRE number. Subsequent updates (Bengen, Kitces) suggest 3.5-4.0% is still robust for longer 40-50 year retirements typical of early retirees, while 3.0% is ultra-conservative and 4.5-5.0% leans aggressive.
Coast FIRE is the amount you need invested TODAY so that, with zero additional contributions, compound growth alone reaches your full FIRE number by your target retirement age. Once you hit Coast, you can stop saving and only earn enough to cover current expenses. Lean FIRE uses a tighter budget (~half your current spend) for a frugal early retirement. Regular FIRE matches your current spending. Chubby FIRE adds comfort buffer (~1.5×). Fat FIRE targets a luxurious lifestyle (~2.5× current spend) with travel, multiple homes and dining freedom. This calculator computes the capital needed for each.
Use nominal (face-value) return and inflation separately — the calculator combines them via the Fisher equation: real return = (1 + nominal) / (1 + inflation) − 1. For US equities a common assumption is 10% nominal / 3% inflation = ~6.8% real. Vanguard's long-run capital markets assumption is closer to 5% real for a balanced 60/40 portfolio. Lower the rate if you want a conservative plan or assume higher market valuations going forward.
No — this is a pre-tax / pre-healthcare estimate. In practice you should add a tax buffer (10-20% on top of expenses depending on jurisdiction and account types), and US early retirees should budget separately for ACA premiums or self-funded health insurance until Medicare eligibility at 65. Bake those into the Annual Spending input rather than the SWR. International users in countries with public healthcare may need a smaller buffer.
For a 50-60 year retirement horizon most planners suggest 3.0-3.5% rather than the classic 4%. Bengen's later work showed 4% has a small but real failure risk when retirement exceeds 35 years, particularly if you retire into a bad sequence of returns. Many in the FIRE community use 3.25% as a 'forever rule' — that bumps the FIRE multiple from 25× to ~31× annual spending but dramatically reduces depletion probability. Use the sensitivity table above to see the dollar impact.
Enter your Current Monthly Contribution and the calculator compares it to the monthly amount required to hit your target retirement age. If you meet or beat it you get an 'On track' badge plus the monthly surplus; if you fall short it shows 'Off track' with the exact shortfall and the percentage you are below the required contribution (a warning under 25% short, a stronger flag beyond that). Separately, 'Years to FIRE' is now a genuine projection solved from what you actually invest each month — it is no longer just an echo of your target age. Raise your contribution, trim spending, or push the target age out to watch the verdict flip to on-track.
Yes. The FIRE math (the 25x rule, safe withdrawal rate, real compounding) is currency-agnostic, so the tool works for any country — pick USD, EUR, GBP, JPY, VND or BRL for display. The assumptions you should localise are the expected return (use your home-market equity/bond history rather than US figures), inflation (enter your country's rate), and the spending input (fold in local taxes and whether you have public healthcare). The withdrawal-rate logic and Trinity sensitivity table apply globally; only the input values change.
Three usual culprits: (1) your real return is low — inflation eats nominal gains, so check that your nominal return assumption is at least 6-7% if you expect 3% inflation; (2) your annual spend is high relative to your contributions — the FIRE number is 25× spend, so cutting $1k/year of spending cuts $25k off the target; (3) you're starting from a small base — early years compound less, so even modest extra savings shave years off the back end. Try halving spending or doubling contributions in the form to see how each lever moves the number.
Not directly. This is a deterministic constant-rate model. Real markets cluster bad years together, so retiring into a downturn can permanently impair a portfolio even if average returns hit the assumption. The 4% Trinity figure already accounts for historical worst cases, but if you want to model sequence risk explicitly use Monte Carlo tools like cFIREsim or Portfolio Visualizer alongside this calculator. A common defensive tactic is the 'bond tent' — holding more bonds in the 5 years before and after retirement to dampen sequence shocks.
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