How to Calculate Your FIRE Number

The internet will tell you that your FIRE number is 25 times your annual expenses. $60,000 × 25 = $1,500,000. Done. Go save $1.5 million and retire. This number is not wrong, exactly. It is incomplete. It assumes a 30-year retirement, no other income sources, historical US returns continuing indefinitely, and constant spending for the rest of your life. Change any of those assumptions — and you will change all of them — and the number moves significantly. Sometimes down. Often by hundreds of thousands of dollars.

The simple answer (and why it is incomplete)

The 25x rule comes from the 4% withdrawal rate. If you withdraw 4% of your portfolio in year one and adjust for inflation, the portfolio historically survives 30 years with a 95%+ success rate. 1 ÷ 0.04 = 25. Multiply your expenses by 25 and you have the portfolio size that sustains a 4% withdrawal.

The problems: 30-year assumption. If you retire at 55, you might need 40+ years. At 45, potentially 50+. The 4% rule was calibrated for 30. Longer retirements need lower withdrawal rates, which means higher multiples. 3.5% = 28.6x. 3.0% = 33.3x. No other income. Most people will eventually receive Social Security, pensions, or other income. Your portfolio does not need to cover those dollars. US exceptionalism. The research used US market returns, which were among the best in the world during the study period. Constant spending. Real spending changes over retirement. Healthcare costs rise. Travel costs fall. Mortgages get paid off.

Step 1: Honestly estimate annual expenses

Start with what you actually spend, not what you think you spend. Pull 12 months of bank and credit card statements. Add it up. Most people are surprised — usually on the high side.

Now adjust. Subtract work-related expenses: commuting ($3,000–$8,000/year for most people), work clothes, work lunches, professional development. These disappear in retirement. Add retirement-specific expenses: healthcare is the big one — budget $10,000–$25,000/year per couple before Medicare at 65. Add travel (most early retirees spend more, not less, in the first 5–10 years). Add hobbies and leisure that currently get crowded out by work.

The net adjustment? Most people’s retirement expenses land within 15–25% of their working expenses. Sometimes higher (healthcare), sometimes lower (no commute, paid-off mortgage). The common advice to plan for 80% of working income is often about right for traditional retirees but can be wildly off for early retirees who plan to travel extensively or who have dependent children.

Step 2: Identify all future income sources

This is where most 25x calculations go wrong. Your FIRE number is not 25 times your total expenses. It is 25 times the gap between your expenses and your other income.

Social Security: Create an account at SSA.gov and check your actual projected benefit. For most workers with 20+ years of earnings, expect $2,000–$3,500/month at full retirement age (67 for most readers). That is $24,000–$42,000/year. For a couple, potentially $48,000–$70,000/year combined. This is real money. Ignoring it inflates your FIRE number by $600,000–$1,750,000.

Pensions: Federal employees (FERS) get a defined benefit pension. Military veterans may have VA disability compensation (tax-free). Some state employees and union workers have pensions. UK citizens have State Pension. International readers may have government pension schemes. Add these up. They are guaranteed income that reduces portfolio requirements dollar for dollar.

Rental income: If you own rental property, net rental income (after expenses, maintenance reserves, and vacancy) reduces the portfolio gap.

Part-time work: If you plan to work part-time in early retirement (the Barista FIRE approach), include that income — but be conservative. Plan for 70% of expected part-time earnings.

Step 3: Determine retirement length

Retirement length drives withdrawal rate, which drives your FIRE number. This is not something to be optimistic about. Plan to live longer than you expect.

Retire at 65: plan for 30 years (age 95). Retire at 55: plan for 40 years. Retire at 50: plan for 45–50 years. Retire at 45: plan for 50 years. Retire at 40: plan for 55–60 years. Medical advances make planning for extended lifespans more important, not less.

The withdrawal rate scales with retirement length. 30-year retirement: 4% is defensible. 40-year retirement: 3.5% is safer. 50-year retirement: 3.0–3.25% is prudent. See the safe withdrawal rates guide for the detailed research behind these numbers.

Step 4: Apply the right withdrawal rate

Your withdrawal rate depends on retirement length, spending flexibility, backup income sources, and personal risk tolerance. Here is a decision framework:

Use 3.0–3.25% if: retiring before 45, no Social Security for 20+ years, no spending flexibility, no other income. Use 3.5% if: retiring at 45–55, some Social Security coming eventually, moderate spending flexibility. Use 4.0% if: retiring at 55–65, Social Security within 10–12 years, pension or other guaranteed income, willing to adjust spending. Use 4.5–5.0% if: retiring at 60+, Social Security within 5–7 years, using variable withdrawal strategies (Guyton-Klinger guardrails), significant backup income.

The withdrawal rate simulator lets you test your specific rate against historical data across different starting years.

Step 5: Build in safety margin

Do not retire at exactly your FIRE number. The day you hit $1,500,000, do not submit your resignation. Real life does not follow historical averages. Markets can drop 30% the month after you retire. Expenses can spike. Healthcare costs can surprise you.

Aim 15–25% above your minimum number. If your math says $1.5M, target $1.725M–$1.875M. The extra capital buys survival margin during poor sequences of returns.

Build a 1–2 year cash buffer. Keep $60,000–$120,000 in high-yield savings outside your investment portfolio. This is not part of your FIRE number — it is a separate buffer that prevents forced selling during market downturns.

Plan spending flexibility. Identify your essential expenses (housing, food, healthcare, utilities, insurance) and your discretionary expenses (travel, dining out, hobbies). In a bad market year, discretionary spending is your pressure valve. Know in advance where you would cut.

The integrated calculation

Here is a complete example that puts all five steps together. This is roughly how real FIRE planning works when you account for all the variables.

Household profile: Couple, both age 55. Annual expenses: $80,000. Social Security at 67: $30,000/year combined (taking at FRA, not early). One partner plans to consult part-time ages 55–62: $25,000/year conservatively.

Phase 1, ages 55–62 (7 years): Expenses $80,000. Part-time income $25,000. Portfolio must cover $55,000/year. These are the highest-draw years, but the shortest phase.

Phase 2, ages 62–67 (5 years): Expenses $80,000. No part-time work. No Social Security yet (waiting for FRA). Portfolio must cover $80,000/year. This is the peak vulnerability period.

Phase 3, ages 67+ (28+ years): Expenses $80,000. Social Security $30,000. Portfolio must cover $50,000/year. Medicare reduces healthcare costs. This is the longest phase but the lowest draw rate.

A simple 25x calculation would say $80,000 × 25 = $2,000,000. But that ignores $30,000/year of Social Security starting at 67 (present value roughly $500,000–$700,000) and $25,000/year of part-time income for 7 years ($175,000 total).

A phased calculation using 3.5% withdrawal rate for the 40-year horizon: the portfolio needs to sustain $55K for 7 years, then $80K for 5 years, then $50K for 28+ years. Accounting for reduced draws in Phase 3, the required starting portfolio is approximately $1.8M–$2.0M — similar to the simple calculation, but for different reasons. The Coast FIRE calculator can help you model when your current portfolio will grow to reach this target.

Add the 20% safety margin: $2.16M–$2.4M target. Plus a $100,000 cash buffer. Total target: $2.26M–$2.5M in investable assets.

Common mistakes

Not accounting for Social Security. Social Security is the single largest “asset” most Americans have. At $30,000/year for 28 years (67–95), the present value is roughly $500,000–$700,000 depending on discount rate. Ignoring it means saving $500K–$700K more than necessary. Create your SSA account and use your actual projected benefit.

Using current expenses without adjustment. Your expenses at 55 will not be your expenses at 75. Healthcare costs rise. Mortgage may be paid off. Children launch. Travel spending often peaks in the first decade of retirement and declines after. Model phases, not a single number.

Assuming a constant withdrawal rate for all phases. Someone retiring at 50 does not need a 3.0% rate for the entire retirement. They need a conservative rate for the first 15–17 years (before Social Security), then the portfolio requirement drops substantially. Phased planning captures this.

Ignoring flexibility. The FIRE number is not a cliff. If you have $1.4M and need $1.5M, you are not destitute — you are 93% of the way there with a slightly higher withdrawal rate for a few years until Social Security or market growth closes the gap. Rigid adherence to a single number creates unnecessary anxiety and often unnecessary extra working years.

Overly-optimistic return assumptions. Assuming 10% nominal returns when planning is dangerous. Use 7% nominal (roughly 4–5% real after inflation) for projections. If markets deliver more, you end up with surplus. If you plan for 10% and get 6%, you fall short. Conservative assumptions create pleasant surprises. Optimistic assumptions create catastrophic ones.

Run your own numbers through the Coast FIRE calculator and the withdrawal rate simulator. The calculators do the phased math automatically and let you adjust assumptions until the output matches your actual life.

This article is for educational purposes. It is not financial advice. FIRE numbers depend on assumptions about returns, inflation, spending, and income that may not hold. Verify Social Security projections at SSA.gov. Consult a fee-only financial planner for decisions specific to your situation.