FIRE Calculator: When Can I Retire Early?

Methodology and Assumptions

Author/editor: Starlight Tools finance calculator team. Last reviewed: 23 June 2026.

We assume monthly compounding. In “Plan by spending (SWR)” we compute a real FI number from after-tax portfolio withdrawal need: (spending + healthcare + active expenses − dependable income − active income) ÷ (1 − tax rate) ÷ SWR. To keep purchasing power, that real target is inflated to the projected FI date. We then project your portfolio from today’s balance using monthly investments, temporary income or expense streams, expected annual return, and investment fee drag. Time-to-FI is when the projection first meets or exceeds the nominal target for that month.

• Portfolio growth: FV = P₀(1+r)^n + PMT·((1+r)^n−1)/r where r is monthly return and n months.
• Inflation adjustment: target grows by (1+i)^{years} with annual inflation i.
• SWR: adjustable. Lower SWR = larger FI number. The 4% rule is a rule of thumb from historical research, not a guarantee.
• Limits: This model uses fixed long-run assumptions. It does not forecast market crashes, account-specific tax rules, state pension timing, benefits, debt amortisation, or adviser suitability.

References: William Bengen’s 1994 safe-withdrawal research, the Trinity Study by Cooley, Hubbard, and Walz, Bank of England inflation-target material, and retirement-spending research discussing safe-withdrawal caveats.

Useful starting points: Bengen on the 4% rule, Scott, Sharpe, and Watson on 4% rule caveats, and Bank of England inflation material.

Tip: Try a cautious SWR and a range of return assumptions. Real life is lumpy; this is a planning compass, not a promise.

Understanding FIRE (Financial Independence, Retire Early): A Friendly Guide

FIRE is a simple idea with a lot of jargon around it: build a portfolio large enough that a small, sustainable withdrawal can cover your living expenses. When your investments can reasonably fund your lifestyle, you’ve reached Financial Independence—whether you keep working, change careers, go part-time, or retire early. Think of FIRE as a design problem: match reliable resources to the life you want, with a healthy margin of safety.

Your FI Number (Rule of Thumb)

A common shortcut is the Safe Withdrawal Rate (SWR). Estimate your required annual spending in retirement, subtract any expected reliable income (e.g., state pension later, rental net, annuities), then divide by a chosen withdrawal rate. Many people test 3.5%–4% for long horizons. For example, £24,000 per year at a 4% SWR implies a ~£600,000 portfolio in today’s money. A lower SWR (e.g., 3.5%) increases the FI number but adds resilience.

Real Money vs. Nominal Money

Your spending power matters more than the pound figure. This tool shows a “today’s money” FI number and a nominal target at your projected FI date. The nominal target grows with the inflation setting, so you’re planning to maintain purchasing power, not just hit a number on a screen.

What Drives Time to FI?

• Spending: Every £1 of recurring spending requires ~£25–£30 in portfolio at a 3.3–4% SWR. Reducing needs is powerful.
• Savings rate: A higher savings rate shortens time to FI dramatically (you’re adding more and needing less).
• Investment returns: Long-term average returns compound; the range is uncertain year-to-year.
• Inflation: Higher inflation means a larger nominal target at the FI date for the same real lifestyle.
• Other income: Part-time work, small pensions, or guaranteed income can reduce the portfolio required.

Sequence-of-Returns & Flexibility

Markets are bumpy. The order of gains and losses, especially early in retirement, can alter outcomes (sequence risk). Flexibility helps: keep a modest cash buffer, trim discretionary spending in poor markets, and consider dynamic rules (e.g., skip inflation raises after a down year). The calculator keeps things approachable with fixed inputs, but real-life adjustments add resilience.

Common Pitfalls (and Easy Fixes)

• Anchoring to 4% blindly: Treat SWR as a starting point. Try a range (3.5%, 4.0%) and note the impact.
• Ignoring fees & taxes: They reduce net returns. Build a small margin or test lower return assumptions.
• Underestimating spending: Track a few months of “essentials vs. flex” to set realistic targets.
• No emergency buffer: Keep an easy-access cushion so surprises don’t force selling during downturns.

Your Levers (Practical, Calm Changes)

• Boost savings rate: Automate contributions the day after payday; redirect pay rises and windfalls.
• Right-size lifestyle: Big wins often come from housing, transport, and food systems, not tiny line items.
• Extend compounding: A little longer to FI can materially increase safety.
• Layer income: Part-time, freelance, or passion income post-FI reduces withdrawal pressure.

How to Use This Calculator

1. Enter current portfolio, monthly investments, expected return, and inflation.
2. Pick a mode: By spending (SWR) or By FI number.
3. Adjust SWR to your comfort; consider testing 3.5% and 4.0%.
4. Review your FI number, time to FI, FI age, and the live chart.
5. Experiment: nudge spending, contributions, returns, or other income to see what moves the needle most.

Educational only, not financial advice. Personal circumstances, taxes, account rules, benefits, fees, and market conditions vary. Use conservative settings, review yearly, and build a plan you can stick to calmly.

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