FIRE Calculator

Most calculators ask for one savings rate and one return, then draw a straight line to a single number. Real financial independence rarely behaves like that. Your life has phases. You might spend fifteen years saving hard, drop to part-time work for five, then stop earning altogether.

This tool is built around those phases. For each one you set how long it lasts, the accounts you pay into or draw from, the growth and contribution rules for each account, your monthly spending, and any one-off events such as an inheritance or a house purchase. Press Calculate and the engine walks your whole plan month by month, then pressure-tests it three ways: a Monte Carlo simulation, a historical backtest, and a sensitivity analysis. It is free and needs no account.

If your finances are simple and stay in one country, a basic 4% calculator will do. This one earns its keep when your plan has more than one chapter.

Your FIRE number is the portfolio you need before paid work becomes optional. The quick version is the 25 times rule: take your expected annual spending in retirement and multiply by 25. Plan to spend £40,000 a year and your target is roughly £1,000,000. That multiple is just the inverse of a 4% withdrawal rate.

The catch sits inside the words ‘expected annual spending’. Your costs at 45 are not your costs at 75. Mortgages end, children grow up, healthcare creeps in. That is why this calculator lets you set different expenses for each phase instead of one flat figure, and why the FIRE number it gives you reflects your actual plan rather than a rule of thumb. If you want a realistic starting figure, how much you really need to retire early works through the numbers for UK, EU and US savers, and net worth and FIRE covers what counts towards the total.

The 4% rule comes from the Trinity study, which looked at US stocks and bonds across the twentieth century and asked which withdrawal rate would have survived a thirty-year retirement. The answer, most of the time, was about 4%.

Two things get dropped when people repeat it. It was a thirty-year rule, so someone retiring at 45 and planning for fifty years needs to be more cautious. And it rests on US market history, US tax wrappers, and dollars. Markets elsewhere delivered different returns in a different order, ISAs, SIPPs and general accounts behave nothing like 401(k)s and IRAs, and 4% quietly assumes you can reach your money when you need it. Apply the rule unaltered to a UK or European plan and you can be wrong by years. We go into this in why the 4% rule breaks outside the US. This calculator hard-codes none of it. You set the withdrawal and tax rules that match where you live.

A straight-line projection shows what happens if markets return their average every year. They never do. And the order of those returns matters as much as the average. A crash in your first few years of withdrawals does far more harm than the same crash twenty years later, even when the long-run average is identical. That is sequence-of-returns risk, and it sinks plans that look fine on paper. The plain-English version is here.

The calculator tests for it two ways.

A Monte Carlo simulation runs your plan a thousand times against randomised return sequences and reports how often it succeeds, plus the gap between a lucky run (the 90th percentile) and an unlucky one (the 10th). A plan that works on average but fails one run in four is telling you something a single line never could.

A historical backtest runs your plan against every starting point in more than a century of real market data, drawn from the Shiller US dataset, the standard long-run series for this kind of work. You see how you would have fared retiring into 1929, 1966, or 2000. If your success rate looks shaky, how to stress-test your FIRE number covers what to do next.

The sensitivity analysis then ranks which assumptions move your retirement date most, so you can see whether your plan hinges on returns, inflation, or your savings rate. To watch one of those play out, what if inflation stays at 4% for a decade runs the scenario in full.

FIRE is not a single finish line. Coast FIRE is the point where your investments, left alone, will compound their way to your number by your target age, so you can coast on a smaller income and stop adding new money. Barista FIRE is semi-retirement, where part-time or passion work covers some of your spending while the portfolio handles the rest. Lean FIRE is independence on a deliberately modest budget. Fat FIRE is the same idea on a generous one.

Each is just a different shape of plan, which is what phases are for. Model Coast FIRE by setting contributions to zero in a later phase and checking whether you still reach your number. Model Barista FIRE by adding part-time income to a mid-life phase. Coast, Barista and Lean FIRE: pick your target helps you work out which one you are actually aiming for.

Retire at 45 and you may not touch a private pension until 57, or the state pension until later still. That leaves a gap to fund: years of spending drawn entirely from accessible accounts before tax-advantaged money unlocks. Plans that skip this look healthy and then fail in practice. Build the bridge openly, with a phase that draws from taxable and ISA-style accounts first and a pension phase that starts only when you can reach it. The bridge problem goes deeper on funding those years.

Two people with identical portfolios can run dry years apart, purely on the order they spend their accounts and the tax they pay doing it. This calculator applies an effective tax rate to each phase and lets you choose which accounts to draw from first, so you can test a sequence rather than assume one. For the principles, see tax-efficient withdrawal order in early retirement, and for the wrappers themselves, tax-advantaged accounts compared sets the SIPP, ISA, 401(k), Roth IRA and PEA side by side. US savers weighing where the next dollar goes will find Roth IRA vs 401(k) vs brokerage useful too.

Plainly, so you can decide whether it is worth half an hour:

• Multiple phases, not one. Accumulation, semi-retirement and full retirement, each with its own accounts, contributions, withdrawals and spending.
• Any currency, not just dollars. Set the one you actually budget in. If your money spans borders, managing money across multiple currencies is worth a read.
• No US assumptions baked in. You choose the withdrawal and tax rules.
• Three stress tests. Monte Carlo, a historical backtest, and a sensitivity analysis that ranks which inputs move your date most.
• Free, no account, nothing stored. Type your numbers and go.