The maths, month by month
The calculator runs your money forward one month at a time. Each month it does three things in order: grows the balance by one month of your expected real return, adds your contribution, then checks whether the balance has reached your target.
That target, your FIRE number, is the pot at which a "safe" withdrawal covers a full year of spending. With £30,000 of annual spending and a 4% withdrawal rate the number is £750,000, because 4% of £750,000 is exactly £30,000. Lower the withdrawal rate to 3.5% and the same spending needs £857,000, which is why that single slider moves your date more than almost anything else on the page.
The return deserves two clarifications, because both trip people up elsewhere. It is real, already adjusted for inflation, so every figure the projection reports is in today's money: a projected £750,000 means £750,000 of today's spending power, not a larger future number that buys less. If you want a feel for how much work that adjustment does over decades, the Bank of England's inflation calculator will show you what any past sum is worth today. And monthly compounding uses the twelfth root of the annual rate rather than dividing by twelve, which keeps a 5% assumption genuinely 5% a year instead of quietly becoming 5.12%.
It is also the projection's one big fiction. A deterministic model assumes the same return every single year, and real markets never oblige; the order of returns matters enormously, since a crash in your first year of retirement does far more damage than the same crash twenty years in, even though the average is identical. Treat the date this page gives you as a centre of gravity, and run the plan through the Monte Carlo simulator before trusting it.
One input question comes up constantly: what belongs in "current savings"? Everything earmarked for financial independence, which means ISAs, brokerage accounts, pensions and workplace schemes. Leave out your emergency fund and the equity in the home you live in, since neither can fund your spending. The walkthrough of every input takes the remaining fields in order, along with the four mistakes that flatter a plan.
Priya's twenty-two years
Priya is 34. She has £62,000 invested, adds £1,150 a month, expects 5% real growth and wants £28,000 a year in retirement at a 3.75% withdrawal rate. Her FIRE number is £746,667 (£28,000 ÷ 0.0375), and running the months forward, her balance crosses that line at age 56 and a half, around 22 years away.
Add £150 a month and the date pulls in to just past 55, seventeen months earlier. Assume 6% growth rather than 5% and it lands near 54 and a half, two years earlier. But trim spending to £26,000 and the date drops below 54, thirty-one months earlier, because lower spending shrinks the target and frees money to contribute. Nothing else pulls twice.
The spending cut wins outright, and notice what that means: the strongest lever on the whole page is the one input entirely within Priya's control, while the runner-up, the market's return, is the one she has no say over at all. That asymmetry is the most useful thing this tool teaches, and it is why the sensitivity analyzer exists as its own page.
If your own date looks impossibly far away, the culprit is usually the savings rate rather than the return: the share of income you keep sets the timeline far more than the percentage you earn on it, and the savings rate calculator shows that relationship directly. For a longer walk through the arithmetic, including three worked household examples and the assumptions that decide whether the number holds, see what a FIRE number is and how to calculate yours.
Reading the date honestly
Tax never appears in the projection. Contributions and withdrawals are treated as net figures, which is fine for a first estimate and misleading the moment your money sits in wrappers with different tax treatment; the pension bridge view exists for exactly that split.
Spending is assumed flat for life, and real retirees rarely behave that way. Most spend less as they age, which is why the spending smile option is there to adjust the curve; the flat default is deliberately the conservative choice, so a surprise is more likely to be pleasant. Along the same lines, the state pension and one-off events (an inheritance, a house downsize, a year off) only enter the projection if you add them yourself.
And the withdrawal rate is a rule of thumb wearing the costume of a guarantee. The 4% figure comes from one specific 30-year US study, the Trinity study, so retiring at 45 means planning for a 50-year horizon it never tested; most researchers suggest 3.25–3.5% for horizons that long. The tool lets you set any rate because the honest answer depends on your horizon and your flexibility, and the safe withdrawal rate explorer maps where the rule holds and where it breaks.