FI Number Calculator
Your Financial Independence Number is the portfolio size you need to retire and live off investments indefinitely. Based on the Trinity Study 4% rule, adjusted for UK inflation.
Your Details
How much do you spend per year?
4% is the classic 'rule'. Lower = more conservative.
BoE target: 2% | Historical CAGR since 1975 (inc. 70s/80s): 4.9%
The FI Number is calculated using the 4% rule ( Trinity Study): your portfolio needs to be 25× your annual expenses. At a 4% withdrawal rate, your portfolio historically survives 30+ years.
Your Financial Independence Number
FI Number (today's £)
£750,000
4% withdrawal rate
FI Number at Retirement
£1,228,962
Inflation-adjusted (+20 yrs)
Monthly Passive Income
£2,500
In today's money
Years to Retire
20
~2046
FI Number by Withdrawal Rate
ISA Tip
Investing inside a Stocks & Shares ISA means all growth and withdrawals are 100% tax-free. With a £20,000/year limit, you can build your FI number without any capital gains or income tax. Model your ISA growth →
What is a Financial Independence Number?
Your FI Number (also called your FIRE number) is the total portfolio value that allows you to retire and withdraw a sustainable income indefinitely without ever running out of money. It's calculated as your annual expenses divided by your safe withdrawal rate (SWR).
The most commonly cited safe withdrawal rate is 4%, from the 1998 Trinity Study. At a 4% SWR, your FI Number is simply 25× your annual expenses. This means a £30,000/year lifestyle requires a £750,000 portfolio.
UK Inflation Adjustment
UK inflation averages around 3.5% historically (ONS CPI data 1975–2024), though the Bank of England targets 2%. Your FI number in today's money needs to be inflation-adjusted to reflect purchasing power at your actual retirement date.
ISA Advantage
If your FI portfolio is held inside a Stocks & Shares ISA, withdrawals are completely tax-free — no capital gains tax, no income tax. With £20,000/year to invest, you can build a significant ISA portfolio over time.