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FIRE number calculator
Spending times the inverse of the withdrawal rate you trust: that is the number. Then the age your current saving reaches it, and the levers that move it.
Your real number, not a fantasy frugal one. The plan only works if you would actually live on it.
4% is the classic study; 3 to 3.5% is the cautious modern reading for long retirements. Lower rate, bigger target, safer plan.
Two honest notes. The withdrawal rate is the whole argument: 4% came from a US study of 30-year retirements, and retiring at 40 means funding maybe 50 years, which argues for 3 to 3.5% and a bigger target. And spending is the lever nobody markets: cutting £200/mo of permanent spending shrinks the target by £60,000 at your chosen rate, which usually beats any realistic return improvement.
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Straight answers
Where does the 4% withdrawal idea come from?
From historical simulations of US stock and bond portfolios, popularised by the Trinity study. It found that an initial 4% withdrawal, adjusted for inflation, survived most historical 30 year periods. It is a planning heuristic built on one country's past, not a law of nature, and longer retirements or worse markets can break it.
Why does my FIRE number swing so much when I nudge the return assumption?
Because compounding stretches small differences over decades. One percentage point of annual return, held for 30 years on £500 a month, is the difference between roughly £502,000 and £610,000: a swing of over £100,000 from a single point of return. That sensitivity is the honest finding: treat the output as a range, not a target etched in stone.
Is the number this calculator gives me a guarantee?
No. It is arithmetic on your assumptions about savings, returns and spending, all of which will be wrong in some direction. The useful part is watching how the answer moves when you change one input at a time, which tells you which lever actually matters for you.
Educational information, not financial advice. Figures current as of July 2026 where dated; allowances and rates change, so check the source before acting.