FIRE calculator

Everything is in today’s pounds. The return you enter is a real return, after inflation and fees, so a figure for age 60 means what that money buys now.

Your FIRE number is a year’s spending divided by the withdrawal rate: £28,000 at 3.5% is £800,000. That is the classic figure. In the UK it isn’t enough to know the total, because the money sits in two kinds of pot that unlock at different ages.

• The bridge. From the day you stop work to the pension access age, everything comes out of ISAs and general investment accounts. The calculator adds up each year’s spending (less any part-time income), discounted at the real return, to find the bridge pot needed on your last working day.
• The pension. From access age the pension pays. Once part-time income has stopped and the State Pension has started, the yearly draw over the withdrawal rate gives the pot that has to last. The years before the State Pension starts draw more, and that extra is added on as a finite stream.

ISA money left over after the bridge counts towards the pension side. A big pension can’t make up for a short bridge, because it can’t be reached in time. The earliest FIRE age is the first whole age at which both pots are covered, with your monthly saving carrying on until then.

What this calculator assumes

• A steady real return every year. Real markets don’t do that, so treat the result as an illustration, not a forecast.
• With tax switched on, 25% of each pension withdrawal is tax-free and the rest is taxed as income at 2026/27 rates for England, Wales and Northern Ireland, alongside the State Pension. The lifetime cap on tax-free cash (£268,275) isn’t applied, and tax bands are held flat in real terms.
• ISA and GIA withdrawals are treated as tax-free. Capital gains and dividend tax on a GIA aren’t counted.
• Part-time income in the Barista option is take-home pay, and pension saving stops when full-time work does.
• The full new State Pension is £12,548 a year in 2026/27, held level in real terms. It rises with the triple lock today, which has often beaten inflation, but that isn’t assumed.

The 4% rule says you can draw 4% of your pot in the first year of retirement, raise the amount with inflation each year, and expect it to last 30 years. It comes from US studies of stock and bond returns in the 1990s.

Two things make it a looser fit here. UK markets have historically returned less than the US, and research using UK data from 1900 to 2008 put the highest rate that survived every 30-year period below 4%. And early retirees need the money for longer: someone stopping at 50 may be drawing for 40 years or more. Rates of 3% to 3.5% are often used for UK plans for that reason, which raises the FIRE number by about 14% to 33% over the 4% figure.

The State Pension changes the picture once it starts. At £12,548 a year in full, it covers a large part of a modest budget, so the pot only has to fund the rest from State Pension age.

A pension can’t be touched before the normal minimum pension age, which rises from 55 to 57 on 6 April 2028. If you stop work at 50, seven years of spending have to come from somewhere else, however large the pension is.

That is why the calculator splits the target into two. The bridge pot is sized to run down to roughly nothing by access age; the pension is sized to last. A plan that looks fine on the total can fail on the bridge, and the chart shows it as the ISA line reaching zero before the pension unlocks.

Pension contributions get tax relief at your marginal rate on the way in, and employer contributions usually come only through a pension. A quarter of the pot can come out tax-free and the rest is taxed as income, which can be at a lower rate than the relief you got. The cost is the lock: nothing comes out before access age.

ISAs get no relief on the way in but everything comes out tax-free, at any age. You can pay in up to £20,000 a year; pensions allow up to £60,000 including tax relief, less for some high earners.

For an early retiree, the split between the two decides the shape of the plan as much as the total. Too little in ISAs and the bridge fails; too little in pensions and you give up relief that would have made the same pot bigger. The calculator shows how a change to the monthly split moves both pots and the earliest FIRE age. Unused pensions also come into the estate for inheritance tax from April 2027, which changes the trade-off for money you don’t expect to spend.

The calculator uses the same return every year. Real returns arrive in a different order, and the order matters once you’re drawing money out. A fall in the first few years of retirement means selling more units to fund the same spending, leaving less to recover when markets do. Two people with the same average return can end up far apart.

The risk is highest around the day you stop work, which is also when the bridge pot is being spent. A lower withdrawal rate, a cash buffer for a year or two of spending, cutting back in bad years and part-time income are common ways people try to soften it. The calculator doesn’t model any of these; changing the withdrawal rate and the Barista option shows how much difference some of them make.