FIRE stands for Financial Independence, Retire Early. Here's the maths behind it, how to calculate the exact amount you need, and whether it's realistic for ordinary people.
FIRE — Financial Independence, Retire Early — is a movement built around one simple idea: save aggressively enough that your investments produce more income than you spend, and you never have to work again. The maths behind it is surprisingly straightforward.
The foundation of FIRE is the 4% Rule, which comes from the Trinity Study — a 1998 analysis of historical stock and bond market returns. The finding: if you withdraw no more than 4% of your portfolio in your first year of retirement (adjusting for inflation each year after), your money has an extremely high probability of lasting 30+ years.
This is sometimes called the 25x Rule. Your FIRE number is exactly 25 times your annual spending — because 4% of that sum equals one year of expenses.
The key variable isn't your income — it's your savings rate. Here's the counterintuitive truth: someone earning £30,000 and saving 50% of it will reach FIRE faster than someone earning £80,000 and saving 10%.
These assume 5% real investment returns (after inflation) — roughly what a globally diversified index fund portfolio has historically produced.
Step 1: Calculate your annual expenses. Not what you earn — what you actually spend. Track 3–6 months of spending and annualise it. Be honest; include irregular expenses like car repairs and holidays.
Step 2: Adjust for retirement. Some costs drop (commuting, work clothes, childcare) and some rise (healthcare, leisure). Many FIRE followers use 80–90% of current spending as their retirement estimate.
Step 3: Multiply by 25. That's your number.
Step 4: Factor in other income. If you'll receive a state pension, rental income, or any passive income, subtract that annual amount from your expenses before multiplying by 25. A £10,000 state pension reduces your FIRE number by £250,000.
The biggest legitimate risk to the 4% rule isn't average returns — it's a bad sequence. If markets crash 40% in your first two years of retirement and you're still withdrawing 4%, you deplete your portfolio so severely that it can't recover even when markets bounce back. This is why many FIRE practitioners use a slightly lower withdrawal rate (3–3.5%) or keep 1–2 years of expenses in cash as a buffer.
Honestly, full early retirement by 35 is only realistic for high earners or those willing to make major lifestyle sacrifices. But the financial principles behind FIRE are useful for everyone: living below your means, investing the difference consistently, and understanding the compounding effect of a high savings rate. Even if your goal is retiring at 58 instead of 38, the same maths applies.