Your “FIRE number” is calculated using the common 4% rule — 25 times your annual spending — a widely used estimate for how much you’d need invested to live off safely without earning a salary. To keep things meaningful, we adjust your expected return for inflation (assumed at 2.5% a year, the UK’s long-term average), so every figure here is shown in today’s money rather than inflated future pounds. This is an illustration only, based on a fixed rate of return applied consistently; real returns vary year to year. This is not financial advice.