FIRE number calculator
Calculate your Financial Independence / Retire Early target based on your annual spending.
MSc Finance, Chartered Accountant (ICAEW)
Financial analyst with 12 years experience in mortgage advisory, investment planning and personal finance education.
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About the FIRE number calculator
The FIRE movement — Financial Independence, Retire Early — is built on a deceptively simple mathematical insight: if you accumulate enough invested assets, you can live indefinitely off the returns without ever depleting the principal. The key tool is the Safe Withdrawal Rate (SWR), and the most famous version is the 4% rule, which emerged from the landmark Trinity Study (1998) by researchers Cooley, Hubbard, and Walz at Trinity University, Texas.
The 4% rule works as follows: withdraw 4% of your portfolio in year one of retirement, then increase that amount each year with inflation. According to the Trinity Study, this strategy had a 95%+ success rate (the portfolio survived 30 years) across all historical market conditions including the Great Depression, WWII, and the 1970s stagflation. Your FIRE number is therefore: annual expenses ÷ 0.04 = 25× your annual expenses. Spending £30,000/year requires a £750,000 portfolio.
The rule has important caveats. It was based on a 30-year retirement horizon — many FIRE adherents retire at 35–45 and need 50–60 year portfolios, for which a 3.25–3.5% SWR is more conservative. It also assumed a US stock/bond portfolio (roughly 60/40). Lower expected returns in today's environment lead many financial planners to recommend 3.3–3.5% as a more robust SWR. This calculator lets you model different SWRs so you can see the impact of being more or less conservative.
How it works
FIRE Number = Annual Expenses ÷ Safe Withdrawal Rate Standard (4% rule): Annual expenses × 25 Conservative (3.5%): Annual expenses × 28.6 Aggressive (5%): Annual expenses × 20 Time to FIRE: solve for n where: Portfolio(n) = current × (1+r)^n + monthly × ((1+r)^n - 1) / r and Portfolio(n) ≥ FIRE Number
Where
SWRSafe Withdrawal Rate — the percentage of portfolio withdrawn annually; 4% is the standard benchmark25×The "multiply by 25" rule — the inverse of the 4% SWRrMonthly portfolio growth rate (annual return ÷ 12)Worked example
Annual expenses: £35,000
SWR: 4%
FIRE Number = £35,000 ÷ 0.04 = £875,000
Current portfolio: £100,000
Monthly investment: £1,500
Expected return: 7%/year → 0.583%/month
Solving iteratively: reaches £875,000 in approximately 21 years
Each £1,000 less in annual expenses reduces FIRE number by £25,000
Each 0.5% lower SWR increases FIRE number by 12.5%
Tips to improve your result
- 1.
The single most powerful lever in FIRE is your savings rate, not your income. Someone earning £80,000 and saving 50% reaches FIRE in ~17 years from zero. The same person earning £50,000 and saving 50% reaches FIRE in the same time — because both have the same ratio of savings to expenses.
- 2.
Geographic arbitrage — retiring to a lower cost-of-living country — can dramatically reduce your FIRE number. £20,000/year in Portugal or Thailand provides a higher standard of living than £35,000 in the UK.
- 3.
The sequence of returns matters more than the average return. Two bad years at the start of retirement (when the portfolio is largest) are far more damaging than two bad years near the end. This is why many FIRE adherents use a cash buffer of 1–2 years' expenses to avoid selling in a down market.
- 4.
A FIRE number for a 35-year-old should use a more conservative SWR (3–3.5%) than the classic 4%, because the 4% rule was validated for 30-year retirements, not 60-year ones.
- 5.
Tax-efficient withdrawal sequencing matters: typically use ISAs first (tax-free), then SIPPs/pensions (taking advantage of the annual tax-free amount), then any taxable accounts. Getting the order wrong can add years to your working life.