What's your number to retire early?

What Is FIRE and How Do You Calculate Your Number?

FIRE — Financial Independence, Retire Early — is a movement built on a deceptively simple idea: save and invest aggressively enough that your portfolio generates more income than you spend, at which point work becomes optional. The "retire early" part is almost secondary; many FIRE adherents keep working after reaching financial independence, simply because they choose to. The goal is freedom, not idleness.

The calculation at the heart of FIRE is the 25× rule, derived from the academic 4% safe withdrawal rate. If you need £30,000 per year to live, you need £750,000 invested. At that level, historically — based on over 70 years of US market data analysed in the 1994 Trinity Study — withdrawing 4% per year (and increasing it with inflation annually) has an approximately 95–100% success rate over a 30-year retirement. For longer retirements common in early retirees, many adopt a 3–3.5% SWR for additional safety, which translates to a 28–33× multiple.

The most important variable in this equation isn't your investment return or your income — it's your annual expenses. Reducing your spending has a double effect: it lowers your FIRE number, and it frees up more income to invest. This is why the FIRE community places such emphasis on understanding where money actually goes — which is exactly what the budget tool on this page is designed to help with.

The Four Types of FIRE

FIRE isn't one-size-fits-all. The community has evolved several variants, each suited to different lifestyles, risk tolerances, and ambitions:

Type	Annual expenses	FIRE number (4% SWR)	Description
Lean FIRE	£15,000–£25,000	£375k–£625k	Highly frugal lifestyle, often with geographic flexibility (lower cost areas)
Standard FIRE	£25,000–£45,000	£625k–£1.1m	Comfortable middle-class lifestyle without major deprivations
Fat FIRE	£50,000–£100,000+	£1.25m–£2.5m+	Luxurious or high-cost lifestyle with travel, private schooling, etc.
Barista FIRE	Any	Reduced (partial coverage)	Portfolio covers part of expenses; part-time work covers the rest
Coast FIRE	Any	Grows to full FIRE by retirement age	Enough invested now to coast to full FIRE without more contributions

Understanding Coast FIRE

Coast FIRE is one of the most psychologically powerful milestones in the FIRE journey. It asks a different question: how much do I need invested today so that, with no further contributions, my portfolio will compound to my full FIRE number by my target retirement age?

The formula is: Coast FIRE = FIRE Number ÷ (1 + annual return)^{years to retirement}

For example, if your FIRE number is £750,000, you expect 7% annual returns, and you have 25 years until retirement age 60: Coast FIRE = £750,000 ÷ (1.07)²⁵ = £750,000 ÷ 5.43 = £138,200.

Once you hit Coast FIRE, you're in a fundamentally different position. You no longer need to save aggressively — you only need to earn enough to cover your living expenses. This unlocks the freedom to take lower-paid but more fulfilling work, reduce hours, or simply stop stressing about saving. Many people hit Coast FIRE years before hitting full FIRE, making it an important early victory to calculate and celebrate.

FIRE in the UK: ISAs, SIPPs, and the State Pension

Most FIRE content is US-centric, but the UK has its own tax-efficient wrappers and state pension system that significantly change the strategy.

Stocks and Shares ISA

The ISA is the cornerstone of UK FIRE planning for early retirees. You can contribute up to £20,000 per year, and all growth and withdrawals are completely tax-free — no capital gains tax, no income tax on dividends. Crucially, ISAs have no minimum access age: you can withdraw at any time, making them ideal for funding the years between early retirement and pension access age. A globally diversified index fund held inside an ISA is the most common UK FIRE vehicle.

SIPP (Self-Invested Personal Pension)

A SIPP is extraordinarily tax-efficient, but comes with an access constraint. Basic-rate taxpayers get 25% uplift on contributions (the government adds tax relief — put in £800, pension receives £1,000). Higher-rate taxpayers can claim an additional 25% back via self-assessment, making the effective cost of a £1,000 pension contribution just £600. The trade-off: you can't access a SIPP until age 57 (rising to 58 in 2028). For someone retiring at 45, this means the SIPP is inaccessible for over a decade. The SIPP should therefore be part of a broader strategy, not the sole vehicle.

The layered FIRE strategy for UK residents

The most tax-efficient approach layers both wrappers: maximise the ISA first (up to £20,000/year, freely accessible), then add to the SIPP with any remaining capacity (claiming the tax relief). In early retirement, you live off ISA withdrawals until pension access age, at which point the SIPP becomes accessible. This strategy minimises tax both while building wealth and while drawing it down.

The UK State Pension

The full new State Pension is approximately £11,502 per year (2025/26), received from age 67. You need 35 qualifying National Insurance years to receive the full amount, and 10 to receive anything. For an early retiree, the State Pension has two important implications. First, it reduces the income you need your portfolio to generate once it starts — effectively cutting your annual expenses by £11,502. Second, you'll accumulate fewer NI years if you stop working young, potentially reducing your entitlement. Consider making voluntary Class 3 NI contributions (£824/year currently) to fill gaps — at £11,502/year, a full State Pension is one of the best guaranteed-return "investments" available.

The Biggest Risk: Sequence of Returns

The 4% rule's near-100% historical success rate hides one critical scenario: retiring just before a major market crash. If your portfolio drops 40% in your first year of retirement and you're still withdrawing 4%, you're selling depressed assets permanently — those shares can never recover for you because they're gone. This is sequence-of-returns risk, and it's the primary danger in early retirement.

The standard mitigation strategies include:

• Cash buffer: Keep 1–2 years of expenses in cash. In a downturn, draw from cash instead of selling equities, giving your portfolio time to recover.
• Flexible spending: Reduce withdrawals by 10–15% in bad market years. The combination of a lower SWR and spending flexibility makes portfolio failure near-impossible in most historical scenarios.
• Lower SWR: A 3–3.5% SWR is highly conservative and has a near-perfect historical record over 40–50 year periods.
• Part-time income: Even £500–£1,000/month from part-time work during a downturn dramatically reduces pressure on the portfolio.

How to Reach FIRE Faster: Practical Strategies

Maximise your savings rate, not just your income

Your savings rate — the percentage of income you invest — is the most powerful lever you have. A household saving 50% of after-tax income can reach FIRE in approximately 17 years from scratch. At 70%, it's around 8.5 years. At 25%, it's over 40 years. Increasing income matters, but so does controlling the denominator. Many FIRE households maintain high savings rates by owning property outright (no rent or mortgage in retirement expenses), owning vehicles outright (no finance payments), and eliminating recurring subscription costs.

Invest in low-cost global index funds

Costs compound just as returns do. A fund charging 0.1% annual fee versus one charging 1% makes a staggering difference over 20+ years — the difference can amount to hundreds of thousands of pounds on a large portfolio. Vanguard's FTSE All-World ETF (VWRL/VWRP), iShares' equivalent, and similar products give UK investors broad global diversification at costs of 0.10–0.22%. There is no evidence that actively managed funds outperform over long periods after costs; extensive academic research supports low-cost passive index investing for long-term wealth building.

Optimise asset location

Hold growth assets (equities) inside your ISA and SIPP. Hold any bonds or cash outside wrappers if your wrapper capacity is limited, since these generate less taxable income. Keep tax-inefficient assets (high-dividend funds, REITs) inside wrappers where dividends won't be taxed. Good asset location can add 0.5–1% per year of after-tax return without changing your investment strategy at all.

Track spending relentlessly

Most people who track their spending for the first time discover significant "leakage" — small recurring costs that are individually insignificant but collectively substantial. The budget tool on this calculator is a starting point. Dedicated tools like Money Dashboard, Emma, or YNAB give you a live picture of where money goes. Knowing your actual annual spending — not an estimate — is the foundation of an accurate FIRE number.