FIRE Calculator: Work Out Your Financial Independence Number

Use the FIRE calculator above

Enter your annual spending, your current invested savings, your monthly contribution and an expected growth rate. The tool estimates your FIRE number, the years until you hit it, and the pot you would have at your target date. Treat the result as a planning compass, not a promise - markets do not move in straight lines.

What FIRE actually means

FIRE is a savings strategy, not a product you can buy. The plan is simple to state and hard to do: spend less than you earn, invest the difference into low-cost funds, and keep going until your investments could cover your living costs indefinitely. At that point paid work becomes optional. Some people quit entirely. Many shift to part-time, lower-stress or self-employed work - often called Barista FIRE or Coast FIRE.

The appeal is obvious if you have ever felt trapped by a payslip. The maths behind it, though, is what separates a daydream from a plan. That is where knowing your FIRE number changes everything.

Your FIRE number and the 4% rule

Your FIRE number is the total invested pot you need so that a safe yearly withdrawal covers your spending. The most common rule of thumb is the 4% rule, drawn from US research on how long a portfolio of shares and bonds tends to last. It says you can withdraw 4% of your starting pot in year one, then adjust that amount for inflation each year, with a strong chance the money lasts roughly 30 years.

Flip that around and the formula is short:

FIRE number = annual spending ÷ 0.04, which is the same as annual spending × 25.

So if you need £30,000 a year to live, your FIRE number is £30,000 × 25 = £750,000. Spend £40,000 a year and the target jumps to £1,000,000. The figure scales directly with your lifestyle, which is why cutting recurring costs does double duty: it lowers the pot you need and frees up cash to invest.

The 4% rule is a guide, not a guarantee. It was built on a specific historical dataset and a 30-year horizon. If you retire at 40 and might draw for 50 years, a more cautious 3% to 3.5% withdrawal (a multiplier of 28 to 33 times spending) gives a wider safety margin. Build that judgement into the target you set in the FIRE calculator.

How the FIRE calculator works

The calculator runs two linked sums. First it sets your target, then it projects how long your contributions and growth take to reach it.

Step 1 - the target: annual spending × 25 (or your chosen multiplier).

Step 2 - the timeline: it grows your current pot plus your monthly contributions at your chosen annual return until the balance hits the target. In plain words:

Future pot = (current savings grown by compound returns) + (monthly contributions grown by compound returns).

Compounding is the engine. Each year's growth earns its own growth the next year, so the curve starts slow and steepens sharply later on. To see that effect on its own numbers, our compound interest calculator shows how a single sum snowballs over time, and the investment calculator projects regular monthly contributions over the long run.

One honest caveat on returns: nobody knows the future. A 5% real (after-inflation) return is a common, fairly conservative planning assumption for a diversified global equity portfolio over decades, but some years are negative and some are double-digit. Run the calculator at a few different rates so you see the range, not a single tidy outcome.

Worked example: Priya, 34, aiming for 50

Priya is a project manager in Leeds. She spends about £28,000 a year and wants to know whether early retirement is realistic.

• FIRE number: £28,000 × 25 = £700,000.
• Starting point: £60,000 already invested across an ISA and a workplace pension.
• Contributions: £1,200 a month (including employer pension contributions).
• Assumed return: 5% a year after inflation.

Growing £60,000 plus £1,200 a month at 5% real returns, Priya's pot reaches roughly £700,000 after about 18 years - so around age 52. If she stretches her monthly contribution to £1,500, she shaves a couple of years off and lands closer to her target of 50. If returns come in nearer 4%, it takes longer; nearer 6%, sooner. That spread is exactly why you test a range rather than fixate on one number.

A second look: Coast FIRE for a younger saver

Coast FIRE is a softer version. The idea is to invest enough early that, even if you never add another penny, compounding alone carries the pot to your FIRE number by your normal retirement age. After that you only need to earn enough to cover today's bills.

Take Marcus, 28, who manages to invest £90,000 by his early thirties. Left untouched at 5% real growth for 35 years, that £90,000 could grow to comfortably over £450,000 in today's money without a single further contribution. He has not reached full FIRE, but he has reached "coast" - he can ease off saving hard and let time finish the job. This is why starting young matters so much: the first decade of contributions does the heaviest lifting.

How monthly investing gets you there

The single biggest lever after your spending is how much you invest each month, and how early. Three habits move the needle:

• Automate it. Set a standing order on payday so investing happens before you can spend the money.
• Increase contributions with every pay rise. Direct part of each rise straight to investments and you raise your savings rate without feeling poorer.
• Keep costs low. A 1% annual platform-and-fund fee can quietly swallow a large share of your gains over 20 years. Cheap, broad index funds are the FIRE community's default for a reason.

Your savings rate - the percentage of take-home pay you invest - is the headline figure. The higher it is, the shorter your journey, because saving more both grows the pot faster and shrinks the lifestyle you need to fund. To see what is actually landing in your account each month, the salary calculator shows your take-home pay after Income Tax and National Insurance, which is the real number your savings rate comes out of.

The role of ISAs and pensions

Where you hold your FIRE pot matters as much as how big it is, because tax wrappers change how much you keep. In the UK, two accounts do most of the work.

Stocks and shares ISAs let you invest up to the annual ISA allowance of £20,000 (2026/27) with no tax on growth or withdrawals, and you can access the money at any age. That flexibility makes ISAs the natural home for the years between early retirement and the age you can touch a pension. Our ISA calculator projects how an ISA pot could grow within the allowance.

Pensions add tax relief on the way in, which is a powerful boost, but you generally cannot access a private pension until the normal minimum pension age. A common FIRE structure is a "bridge": build an ISA pot large enough to fund the gap years, then let the pension carry you through later life. A pension calculator helps you size that second pot.

To see your whole position in one place - investments, pension, property and debts - a net worth calculator gives you the starting balance the FIRE calculator builds on.

Inflation and your target

A FIRE number is only meaningful in today's money. If you need £30,000 a year now, you will need more in cash terms in 20 years to buy the same things, because prices rise. There are two clean ways to handle this:

• Work in real (after-inflation) returns. If you assume, say, 5% real growth, your £750,000 target already represents today's spending power and you do not need to inflate it separately.
• Or work in nominal returns and then inflate your target spending. This is fiddlier and easier to get wrong, so most people stick with real returns.

Either way, revisit the calculation every couple of years. A pay rise, a house move, a new child or a paid-off mortgage can all reshape your annual spending - and your spending is what sets the whole target.

Common mistakes people make with FIRE

• Underestimating real spending. Annual budgets often miss the lumpy costs: car replacement, home repairs, holidays, insurance renewals. Use a true 12-month figure, not a good month × 12.
• Forgetting the pension access gap. If you retire at 45 but cannot draw your pension for years, an all-pension pot leaves you cash-poor in between. Plan the ISA bridge first.
• Treating 4% as gospel. For very long retirements it can be too aggressive. A lower withdrawal rate means a bigger number, but a far smaller chance of running out.
• Ignoring tax on the way out. Pension withdrawals beyond the tax-free portion are taxed as income, and large investment gains outside an ISA can trigger Capital Gains Tax. Sequencing withdrawals to stay within allowances matters.
• Assuming returns are smooth. A bad run of markets in the first few years of retirement (sequence risk) does more damage than the same run later. A cash buffer of a year or two of spending softens the blow.
• Counting the State Pension too early. It can reduce the private pot you need, but only from State Pension age, and the amount depends on your National Insurance record. Treat it as a later top-up, not a foundation.

One quick regional note: FIRE planning is UK-wide, but the tax you pay along the way is not identical across the UK. Income Tax bands differ in Scotland, which affects your take-home pay and therefore how much you can invest each month. The savings strategy is the same; the cash flowing into it depends on where you live and earn.

These figures are estimates for guidance only and not personal financial advice. Investment values can fall as well as rise, and your own plan should reflect your circumstances. For a clear, independent overview of investing in the UK, see the MoneyHelper guide to investing.

Plan the rest of your journey

Once you have a FIRE number, the next questions are about getting there and holding it tax-efficiently. Project your monthly contributions with the investment calculator, see the snowball effect with the compound interest calculator, and shelter your growth using the ISA calculator and pension calculator.