How to Calculate Your FIRE Number (And Actually Hit It)
Your FIRE number is the investment portfolio size that lets you live off returns indefinitely without working. It's calculated as your annual expenses multiplied by 25 — the inverse of the 4% safe withdrawal rate from the Trinity Study. Simple math. Profound implications.
This post covers how to calculate your number, how to model the path to hitting it, and the variables that most FIRE calculators ignore.
The Core Formula
FIRE Number = Annual Retirement Expenses × 25
Use your retirement spending, not your current spending. Will you have a paid-off house? No commuting costs? Healthcare through an employer? Your retirement budget may be meaningfully different from today's. Most people underestimate retirement spending by 15–20% — particularly healthcare, which averages $6,000–$12,000/year for early retirees before Medicare eligibility at 65.
The Variables That Actually Matter
Savings rate — the single biggest lever. Going from a 15% to a 30% savings rate doesn't just double your annual savings; it also reduces your FIRE number (because you're spending less). The compounding effect is dramatic. See the sensitivity table below.
Investment return assumption — most FIRE planners use 6–7% real (inflation-adjusted) returns for a diversified stock/bond portfolio. Stress-test your plan at 5% to see the conservative case.
Timeline — the 4% rule was designed for 30-year retirements. Retiring at 40 means a 50-year timeline; many use 3.5% withdrawal rate for longer horizons, which changes the multiplier from 25 to ~28.6.
Other income — Social Security, rental income, part-time work. Every $1,000/month in other income reduces your required portfolio by $300,000.
Sequence of returns risk — retiring into a bear market is the biggest threat to a FIRE plan. A 10–15% cash/bond buffer for the first 2–3 years of retirement significantly reduces this risk. For a deeper look at withdrawal sustainability, see How Long Will My Money Last in Retirement?
The Savings Rate Sensitivity Table
| Savings Rate | Years to FIRE (from $0) | Notes |
|---|---|---|
| 10% | ~43 years | Traditional retirement timeline |
| 20% | ~32 years | Solid but slow |
| 30% | ~25 years | Meaningful acceleration |
| 40% | ~19 years | Early retirement territory |
| 50% | ~15 years | Classic FIRE timeline |
| 65% | ~10 years | Aggressive FIRE |
| 75% | ~7 years | Extreme FIRE |
Assumes 7% real return, 4% withdrawal rate. The non-linearity is the key insight: early savings rate gains are enormous; later gains diminish.
How to Model Your Path
- Calculate your FIRE number (annual expenses × 25)
- Enter your current portfolio value
- Set your annual savings amount
- Choose your expected return (6–7% real is standard)
- See your projected FIRE date
- Stress-test at 5% return and 3.5% withdrawal rate
- Model the impact of increasing savings rate by 5% or 10%
Tracking Progress Monthly
Your FIRE number is a target. Your net worth is your progress toward it. Track your net worth monthly and calculate your FIRE progress percentage: current portfolio ÷ FIRE number. A net worth tracker makes this visible every month and shows the trend line that confirms your strategy is working.
The Roth IRA Connection
Tax-advantaged accounts — particularly the Roth IRA — are the most efficient vehicles for building your FIRE portfolio. Tax-free growth and withdrawals mean your Roth balance is worth more than a taxable account of the same size. Maxing out your Roth IRA every year is one of the highest-leverage actions in any FIRE plan.
The Bottom Line
Your FIRE number isn't a fantasy — it's a math problem. Calculate it, model the path, stress-test the assumptions, and revisit it annually. The people who retire early aren't lucky. They're specific.
Frequently Asked Questions
What is a FIRE number?
Your FIRE number is the total investment portfolio value needed to retire and live off investment returns indefinitely. It's calculated as annual retirement spending multiplied by 25, based on the 4% safe withdrawal rate from the Trinity Study.
How do I calculate my FIRE number?
Estimate your annual retirement spending (not current spending), then multiply by 25. Subtract your current portfolio to find the gap. Model how long it takes to close that gap based on your annual savings and expected investment return.
What savings rate do I need to retire early?
At a 50% savings rate, most people reach FIRE in approximately 15 years from zero. At 65%, roughly 10 years. The savings rate is the primary lever — income matters less than the gap between income and spending.
Is the 4% rule safe for a 40-year retirement?
The 4% rule was designed for 30-year retirements. For 40–50 year retirements, many FIRE planners use 3.5% (multiply annual spending by 28.6 instead of 25) for additional safety margin. The difference in required portfolio is significant but the additional security is meaningful for early retirees.
Should I include my home in my FIRE number?
Generally no — your FIRE number should be investable assets that generate returns. A paid-off home reduces your retirement spending (no mortgage), which lowers your FIRE number, but the home itself doesn't produce income unless you sell or rent it.
Ready to Put This Into Action?
The math behind freedom is simple. The FIRECalc Lite – FIRE Number Calculator calculates your FIRE number, projects your freedom date, and lets you stress-test every savings rate and return assumption — all in one pre-built Google Sheet. Instant download, yours forever.
Or browse the full Savings & Investing Templates collection to find the right tool for your situation.