FI Number Calculator UK

Find the exact portfolio size you need to be financially independent - and how long it will take to get there.

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Your numbers

Annual expenses in retirement£25,000

How much you expect to spend each year in retirement, including housing, bills, food, and leisure. Exclude mortgage if it will be paid off.

Current invested portfolio (ISAs, SIPPs, GIAs)£0

The total value of all your invested assets today. Include ISAs, pensions (SIPPs), and general investment accounts. Exclude property, cash savings, and emergency funds.

Annual savings / contributions£10,000

How much you add to your investments each year, including pension contributions (employee + employer) and ISA deposits.

Expected annual return (7%)

0%15%

The nominal (before inflation) return you expect. Global equity trackers have broadly delivered 7-10% per year over long historical periods, but past returns are not a guide to future performance.

Inflation rate (2.5%)

0%8%

The Bank of England targets 2%. Higher inflation means your money buys less each year, so you need a larger pot.

Real return: 4.4% · All figures in today's money.

What happens to my data?

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Your FI number

£625,000

Years to FI

31 yrs

Progress

Based on £25,000/year at a 4% withdrawal rate.

Timeline to FI

years

Age 61

target retirement age

Portfolio progress

Current: £0Target: £625,000

£625,000 still to go

Portfolio projection

How your savings rate affects the timeline

Saving more shortens your path to FI - often faster than chasing higher returns.

Annual savings	Monthly	Years to FI	Retire at
£5,000	£417	44	Age 74
£7,500	£625	36	Age 66
£10,000 current	£833	31	Age 61
£15,000	£1,250	25	Age 55
£20,000	£1,667	21	Age 51

Want to understand the maths?

Read: The Ransom Price - Calculating Your Financial Independence Number

The article behind the calculator. How the 4% rule works and why it matters.

Year-by-year breakdown

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The complete guide

FI Number Calculator: Your Independence Target

Calculate exactly how much you need to retire early. Our free FI number calculator shows your target portfolio size and time to financial independence.

Financial independence starts with a single number. Not a vague hope, not a rough guess, but a hard target you can plan around and track over time. Your FI number is the portfolio size at which your investments can cover your living costs indefinitely, freeing you from the need to work for money.

Our FI number calculator does the maths for you. Enter your annual expenses, current savings, and expected returns, and it tells you exactly how much you need and how long it will take to get there.

What Is a FI Number?
How to Use the Calculator
Common Use Cases
How Savings Rate Affects Time to FI
Adjustments for UK Investors

What Is a FI Number?

Your FI number is the total amount of invested capital you need before you can live off your portfolio's returns. It is based on the 4% rule. The original Trinity Study finding suggested that, over a 30-year US retirement, a 4% starting withdrawal adjusted for inflation each year had a high historical success rate in the data tested. The "roughly 95% success" headline is a simplification of that backtest, not a forward guarantee, and outcomes for any individual depend on real-world returns, sequencing, fees, and tax.

The calculation is simple: multiply your expected annual expenses in retirement by 25.

FI Number = Annual Expenses x 25

If you plan to spend £30,000 per year, your FI number is £750,000. If you need £40,000, you are targeting £1,000,000. The 25x multiplier is just the inverse of the 4% withdrawal rate - nothing more complicated than that.

For a deeper look at how this rule was derived and what its limitations are, read our full breakdown on calculating your FIRE number. And if you want to understand why UK investors may need to adjust that 4% figure, our review of Beyond the 4% Rule covers the evidence.

FI number calculator showing target portfolio size, progress percentage, and years to financial independence

How to Use the Calculator

The FI number calculator is designed to give you a clear answer in under a minute. Here is how to use it step by step.

1. Enter Your Annual Expenses

Start with what you expect to spend each year in retirement. If you are not sure, look at your last 12 months of bank statements and subtract costs that disappear when you stop working (commuting, work lunches, professional clothing). Our budgeting 101 guide can help you build an accurate picture of your spending.

2. Add Your Current Portfolio Value

Enter the total value of your invested assets. This includes ISAs, SIPPs, GIAs, and any other investments you plan to draw from in retirement. If you are not sure of the total, use the net worth tracker to get an up-to-date figure.

3. Set Your Annual Savings

How much are you putting away each year? This is the amount going into investments, not just sitting in a savings account. The calculator uses this figure alongside your expected return rate to project how quickly your portfolio will grow.

4. Choose Your Expected Return Rate

A common modelling assumption is 7-8% nominal or 4-5% real (after inflation). The calculator lets you adjust this to match your own expectations. Global equity indices have delivered real returns broadly around 5% over long historical periods, though past performance is not a guide to future returns. Use the compound interest calculator to see how different return assumptions change your projections over time.

5. Review Your Results

The calculator shows your FI number, your current progress as a percentage, and the estimated number of years until you reach financial independence at your current savings rate. It also includes a reverse mode where you can enter a target retirement age and see what savings rate you would need to hit that deadline.

6. Choose Your Portfolio Type

The tool supports ISA, SIPP, and GIA portfolio types, so you can see how each wrapper affects your path to FI. This matters because SIPP contributions come with tax relief, while ISA withdrawals are tax-free. The right mix depends on your income, your planned retirement age, and whether you want access to funds before 57.

7. Save to Your Profile

If you are logged in, you can save your inputs to your financial profile. This means you can come back and update your numbers as your situation changes without starting from scratch each time.

Common Use Cases

Early Retirement Planning

The most common reason to calculate a FI number is to plan for early retirement. If you want to stop working at 45 or 50, you need a clear target and a timeline. The calculator shows whether your current savings rate is enough, or whether you need to increase contributions, reduce expenses, or both.

Setting Savings Targets

Once you know your FI number, you can work backwards to a monthly savings target. If you need £750,000 in 15 years and you have £100,000 today, the calculator will show you exactly how much to save each month to close that gap at a given return rate.

Comparing Scenarios

Run the calculator multiple times with different inputs. What happens if you cut expenses by £5,000 a year? What if you increase your savings rate by 5%? What if returns are lower than expected? Comparing scenarios helps you build a plan that works even if things do not go perfectly.

How Savings Rate Affects Time to FI

Your savings rate is the single most important variable in reaching financial independence. It matters more than investment returns, more than income, and more than clever tax planning. A higher savings rate works in two directions at once: it increases the money flowing into your portfolio and it reduces the expenses your portfolio needs to cover.

Here is how savings rate affects the number of years to reach FI, assuming a 5% real return and starting from zero.

Savings Rate	Years to FI
10%	51
20%	37
30%	28
40%	22
50%	17
60%	12.5
70%	8.5
80%	5.5

Plotted out, the curve makes the point more forcefully than the table does.

Years to financial independence by savings rate

Source: Mr Money Mustache - The Shockingly Simple Math Behind Early Retirement (5% real return, 4% withdrawal)

The relationship is not linear. Moving from a 10% to a 20% savings rate shaves off 14 years. Moving from 70% to 80% only saves 3 years. The biggest gains come from getting your savings rate above 30-40%, where the timeline starts to compress dramatically.

Adjustments for UK Investors

The standard FI number formula works globally, but UK investors have two major advantages worth building into their plans.

State Pension Bridging

If you retire early, you will not receive the State Pension until age 66 (rising to 67 and eventually 68). But once it kicks in, the full new State Pension pays around £11,500 per year. That reduces your required portfolio withdrawals significantly.

new state pension - www.gov.uk

This means your true FI number has two phases. Before State Pension age, your portfolio needs to cover all your expenses. After State Pension age, it only needs to cover the gap between your pension income and your total spending. The calculator helps you plan for both phases.

ISA and SIPP Sequencing

UK investors have access to two powerful tax-advantaged wrappers, and the order in which you draw from them matters.

SIPPs offer upfront tax relief (20% or 40% depending on your marginal rate) and tax-free growth, but you cannot access funds until age 57 (from April 2028). Withdrawals are taxed as income.
ISAs offer no upfront tax relief, but growth and withdrawals are completely tax-free with no age restriction.
GIAs (General Investment Accounts) have no tax advantages but no restrictions either.

A common strategy is to live off ISA and GIA funds in early retirement, then switch to SIPP withdrawals once you reach pension age. This keeps your taxable income low in the early years while your SIPP continues to grow tax-free. Running separate scenarios for each wrapper in the calculator helps you see how this sequencing affects your timeline.

Author's Take

Michael McGettrick

Founder, Freedom Isn't Free

The maths in this calculator is the easy part. The honest, uncomfortable part is the input you put into it. Most people I have spoken to about FIRE quote an annual-expense figure that comes from one of two places: either their current spending, or their current spending plus a comfortable cushion. Neither is the right answer. Your current spending is shaped by the constraints of your current life - the commute, the work clothes, the lunches, the fatigue-driven Deliveroo. None of those continue post-retirement, so the figure is wrong on the high side. Your comfortable cushion, on the other hand, is wrong on the low side because retirement is when you finally have the time to spend money on the things you have been deferring.

The two-bound framing I have settled on is to run the calculator twice: once with a deliberate floor (the minimum life I would still call good), and once with a deliberate ceiling (the maximum I would actually use given an honest accounting of my interests and energy). The gap between the two numbers is the decision space. Inside that gap is where the genuinely interesting trade-offs live - shorter hours, a less profitable job, a year off, a different city - and most of them are invisible if you only run the calculator once and stop. The Rule of 25 will give you a number whatever you put in. The harder work is being honest about what life that number is funding.

A Note on Risk and Advice

This guide and calculator are general information for UK readers, not personal financial advice or a recommendation to invest in any specific product. Investing puts your capital at risk; the value of investments can fall as well as rise and you may get back less than you put in. The 4% rule, the 25x multiplier, and the savings-rate scenarios shown here are modelling tools based on historical data, not promises about future outcomes. Tax rules, State Pension entitlement, and pension access ages change over time. For advice tailored to your circumstances, speak to an FCA-regulated financial adviser.

Start Calculating

Your FI number is not a fantasy figure. It is a concrete, calculable target. Once you know what it is, every pound you save and invest moves you measurably closer.

Try the FI number calculator and find out exactly where you stand.

Further Reading:

Quit Like a Millionaire - Kristy Shen - A practical guide to reaching financial independence and retiring early, with clear worked examples on calculating your FI number and optimising your savings rate. (Affiliate link - we may earn a small commission at no extra cost to you.)

The Psychology of Money - Morgan Housel - Explores why behaviour and mindset matter more than spreadsheets on the path to financial independence. (Affiliate link - we may earn a small commission at no extra cost to you.)

Frequently asked questions

How do you calculate your FI number?

Multiply your expected annual expenses in retirement by 25. The result is the size of the invested portfolio that, drawing down at 4% per year, should cover your spending indefinitely. £30,000 of expenses gives £750,000. £40,000 gives £1,000,000. Use a lower multiplier (around 30x, equivalent to a 3.25% withdrawal rate) if you plan to retire before age 50.

Is the 4% rule safe for early retirees in the UK?

The original 4% research was based on a 30-year US retirement. UK and global investors retiring in their forties or fifties are planning for a 40 to 50-year drawdown, where many FIRE planners model a more conservative 3.25% to 3.5% withdrawal rate. The calculator lets you adjust the rate to see how the assumption changes the portfolio size required. This is general information, not personal advice; capital is at risk and a regulated adviser can model your specific position.

Should I include my house in my FI number?

No. The FI number only counts liquid invested assets you can draw an income from. Your home keeps a roof over your head but does not generate spendable returns unless you sell it or rent rooms. Equity in a paid-off home reduces your annual expenses (no mortgage payment), which is how it shows up in the maths.

How does the State Pension change my FI number?

Once you reach State Pension age (currently 66, rising to 67 by 2028), the full new State Pension pays around £12,000 a year. That reduces what your portfolio has to cover. If your expenses are £30,000 and the State Pension covers £12,000, your portfolio only needs to fund £18,000 a year from that point on. That's a target of £450,000 instead of £750,000. Enter the State Pension amount and age in the calculator to see the two-phase number.

Does the FI number account for inflation?

If you use a real (inflation-adjusted) return rate, the FI number is already in today's pounds and the result reflects purchasing power. A 5% real return means 5% after inflation. The calculator defaults to a 7% nominal return and 2.5% inflation, giving roughly 4.4% real, but both fields are editable.

What is a good savings rate to aim for?

Modelling suggests rates above 30% of take-home pay compress the timeline meaningfully. Below 20% the projection stretches past 35 years from a zero start. The inflection points in the standard model fall around 30% (28 years), 50% (17 years), and 60% (12.5 years), assuming a 5% real return. Most UK workplace pensions default to a combined 5% to 8% employer plus employee contribution; an ISA or SIPP top-up can be considered alongside this to lift the overall rate, depending on your circumstances.