About this tool
The FIRE movement, in two words
FIRE stands for Financial Independence, Retire Early. The idea is simple to state and demanding to execute: instead of depending on a salary until the State Pension age, you build a large enough investment portfolio that the income it generates — interest, dividends, capital gains — durably covers your living expenses. Once that's true, work becomes optional rather than mandatory. Some people stop entirely; others cut back to part-time, switch to a lower-paid but more fulfilling career, or alternate stretches of work and time off.
The movement took shape in the US in the 2010s, built on two pillars: an unusually high savings rate, often well above what's considered normal, and broadly diversified long-term investing, typically low-cost global equity index funds. In the UK the usual vehicles are a Stocks & Shares ISA (tax-free gains, £20,000 annual allowance), a workplace or personal pension (SIPP) with tax relief, and a general investment account for anything beyond those wrappers. The calculator above deliberately doesn't model specific account types: it works from a total balance and an overall return, to stay simple and comparable across savers.
The 25x rule and the 4% withdrawal rate
The 25x rule is the core of the calculation. It says the capital needed for financial independence equals 25 times your annual expenses. Twenty-five is simply the inverse of 4%: withdrawing 4% of a portfolio each year means spending 1/25th of its starting value. If you spend £30,000 a year, the target is £750,000.
Where does the 4% figure come from? From US academic research published in the 1990s, most famously the so-called "Trinity study" — Cooley, Hubbard and Walz, finance professors at Trinity University in San Antonio, published in the AAII Journal in 1998. The study tested, against historical US stock and bond market data going back to 1926, how often a diversified portfolio survived 15-, 20-, 25- and 30-year sequences of annual withdrawals at various rates. A 4% real (inflation-adjusted) withdrawal rate from a portfolio with 50-75% stocks had a very high historical success rate over 30-year periods — but it's a statistical result on past US data, not a guarantee, and UK savers should note that sterling returns, inflation and tax treatment differ from the US study's assumptions.
How the calculator works
The calculation happens in two steps. First the target: annual expenses × 25. Then the trajectory: the simulator starts from your current savings, adds your contribution at the end of each month, and applies a monthly return equivalent to the annual return you entered. It advances month by month until capital reaches the target, then displays how long that takes and charts the growth curve. The chart shows the classic shape of long-term saving: a nearly straight line early on, when contributions dominate, then acceleration as compound returns take over.
Amounts are shown in today's pounds: inflation, withdrawal taxes and fund fees are not modelled. That's a deliberate simplification that keeps results comparable across scenarios. In practice, using an inflation-adjusted return — say 4-5% instead of a 6-7% nominal average — gives a more conservative, purchasing-power-based read.
Worked example
Sam spends £30,000 a year, already has £50,000 invested, saves £1,000 a month, and assumes a 6% annual return.
- Target capital: 30,000 × 25 = £750,000
- Estimated time to reach it: about 22 years and 8 months
- Total contributed over that period: about £272,000, with the rest coming from compound returns
The most powerful lever shows up immediately when you change the inputs. Raising monthly savings to £1,500 drops the timeline under eighteen years. Cutting annual expenses to £25,000 lowers the target to £625,000 and shortens the timeline further. Reducing spending works twice: it increases available savings and lowers the capital needed. That's why the savings rate, more than income, is the decisive variable in a FIRE calculation.
The limits worth knowing
The 4% rule rests on historical US market data over roughly 30-year horizons. Three caveats matter. First, a very early retirement implies a withdrawal horizon well beyond 30 years, which makes 4% less comfortable — many practitioners use 3% to 3.5% instead, equivalent to targeting 28-33x expenses. Second, sequence-of-returns risk matters as much as the average return: a sharp market downturn in the first few years of withdrawals can permanently damage a portfolio you're drawing from. Third, tax treatment varies sharply by wrapper in the UK: an ISA's gains are entirely tax-free, while a general investment account is subject to Capital Gains Tax (£3,000 annual exempt amount for 2026/27) and dividend tax above the £500 dividend allowance.
Add to that the ordinary uncertainties of life: a health expense, a family change, a move can durably shift the annual spending figure the whole calculation rests on. Treat the number you get as an order of magnitude to revisit regularly, not a fixed target carved in stone.
Lean FIRE, Fat FIRE, Coast FIRE
Several variants have become popular. Lean FIRE targets independence on a deliberately frugal budget: a lower target reached sooner, but less cushion for the unexpected. Fat FIRE instead assumes a comfortable spending level, pushing the target and timeline out but offering more security. Coast FIRE describes an intermediate milestone: the capital already saved will, through compound growth alone with no further contributions, reach the target by traditional retirement age — you still need to cover current living costs, but the saving effort itself can stop. Barista FIRE, finally, means covering part of your expenses with part-time or lower-stress work while the portfolio funds the rest.
Disclaimer
This simulation relies on return assumptions and is not regulated financial advice. Past performance does not guarantee future returns, and no withdrawal rate guarantees a portfolio will never run out. Consult an FCA-authorised financial adviser before making major investment decisions.