UK FIRE Planner — ISA, SIPP & Pension Bridge

What this FIRE calculator UK actually models

The Financial Independence, Retire Early (FIRE) movement asks one question: at what age can your portfolio replace your employment income? The UK FIRE Planner answers that question under UK rules — meaning it routes contributions through the three vehicles a UK saver uses (ISA, SIPP, LISA), applies the statutory caps and bonuses, and handles the part most international FIRE calculators miss: the pension access age gap. The math is a 60-year year-by-year simulation. Each year the model applies real (inflation-adjusted) growth to every pot, adds the year's contributions, and checks whether the total portfolio multiplied by your chosen safe withdrawal rate covers your retirement expenses. The first year that condition is met is your FIRE age.

The pension bridge — the UK-unique number

A US saver hitting FIRE at 53 can typically draw on a workplace retirement account at age 59½ with little fuss. A UK saver hitting FIRE at 53 cannot touch their SIPP until at least 57 (the minimum pension access age from April 2028 under Finance Act 2022). The gap between FIRE age and pension access age is the bridge — the period that has to be funded entirely from ISA, LISA (if you are over 60), cash, and any other accessible investments. Bridge years × retirement expenses gives the bridge funds needed. The calculator reports both that figure and the bridge funds available at your projected FIRE age, so the surplus or shortfall is visible before retirement, not after. One common scenario: UK savers maximise SIPP contributions for the tax relief, then find the bridge between FIRE age 52 and access age 57 needs five years of ISA-funded living — five years that the SIPP cannot fund.

ISA, SIPP, and LISA inside the simulation

The ISA contribution is capped at the statutory annual allowance (currently £20,000) per the gov.uk figure held in the planner's constants file. The SIPP contribution is grossed up by 25% to reflect basic-rate income tax relief — a £400 net monthly contribution becomes a £500 gross contribution inside the pension. The LISA contribution is capped at £4,000 a year, only counts before age 50 (statutory rule), and receives a 25% government bonus. LISA balances are locked until age 60 for retirement use, so the calculator only counts the LISA toward bridge funds when the FIRE age is at or above 60. The pension pot itself accumulates employer pension matching plus your own pension percentage of salary plus the grossed-up SIPP contributions. Cash savings are tracked in real terms (zero real growth) on the conservative assumption that interest on cash roughly matches inflation. Other investments compound at the real return rate.

Coast FIRE — the underused milestone

The Coast FIRE age is the earliest age at which the current pot, left untouched and compounding at the real return rate, will hit the FIRE number by your target retirement age. Hitting Coast FIRE means contributions become optional — the math works without them. Coast FIRE is often reached years before full FIRE — the exact gap depends on starting pot, contributions, and the return assumption. Reaching Coast FIRE opens the option of a career sabbatical, a lower-paid passion-project job, or a phased step-down without derailing the retirement plan. The calculator reports Coast FIRE age alongside FIRE age so the optionality is on the dashboard, not buried.

Three scenarios — bear, base, bull

A single return assumption hides the question "what if I'm wrong about returns?". The calculator runs the full simulation three times — once at the bear return (default 3% real), once at the base return (default 5% real), and once at the bull return (default 7% real). The bear-case FIRE age and bull-case FIRE age sit alongside the base-case to show how sensitive the plan is to return shocks. A plan that hits FIRE at 50 in the base case but slips to 62 in the bear case is materially different from a plan that lands at 50 vs 53 across the three scenarios. The first plan depends on returns coming through; the second tolerates a wide return range.

What the calculator does not do

It does not model tax on withdrawals (pension drawdown is taxed as income above the personal allowance; ISA withdrawals are tax-free). It does not model sequence-of-returns risk in retirement — the safe withdrawal rate (default 3.5%) is the lever for that. It does not include rental income, business income, or non-portfolio assets. It does not consider salary changes other than the steady growth rate input. It treats market returns as a smooth annual rate; real markets are volatile and the actual FIRE age depends heavily on the order in which good and bad years arrive (sequence risk).