FI Number Math: How to Calculate What You Actually Need

Your FI number is 25× annual expenses. That's the headline rule. For someone spending $70K/year, target is $1.75M. The math comes from the 4% safe withdrawal rate: $1.75M × 4% = $70K annual income, sufficient to cover expenses.

Reality is messier. The 25× rule assumes 30-year retirement, modest portfolio volatility, and flexibility to adjust spending. Early retirees need 30-33× to handle 50-year horizons. High spenders face sequence risk differently than modest spenders. Tax location matters. Let me walk through how to actually calculate YOUR FI number.

Step 1: Define Retirement Expenses

Not your current spending. Retirement-specific spending. This usually differs:

• Commuting costs: $0 (likely)
• Work clothing, meals: reduced
• Mortgage: paid off in most retirement plans, so housing costs drop
• Healthcare: potentially higher (pre-Medicare private insurance is expensive)
• Travel: typically higher for first 10-15 years of retirement
• Saving: zero (no more contributions)

The honest number is usually 70-90% of pre-retirement spending, not 100%.

Step 2: Adjust for Life Stage

Retirement spending isn't flat. The "retirement smile" shows:

• Years 1-10: high (travel, hobbies, new activities) — spend 110-120% of base
• Years 10-20: low (settled lifestyle, less travel) — spend 85-95% of base
• Years 20+: high (medical, long-term care) — spend 110-130% of base

Average over 30 years: roughly equal to your initial projection.

Step 3: Apply the Multiplier

Standard cases:

• 30-year retirement (age 65 onward): 25× expenses
• 40-year retirement (age 55): 28× expenses
• 50-year retirement (age 45): 30-33× expenses
• 60+ year retirement (age 40 or earlier): 33-40× expenses

These multipliers come from Monte Carlo simulations showing portfolio survival probability at different withdrawal rates.

Step 4: Account for Income Sources

Don't calculate FI number on total spending. Subtract guaranteed income first:

Target retirement spending: $70K

Expected Social Security (age 67): $24K

Pension (if applicable): $10K

Net portfolio withdrawal need: $70K - $24K - $10K = $36K

FI number = $36K × 25 = $900K

This is dramatically lower than calculating 25× of gross spending ($1.75M). Social Security in particular is often overlooked in FI planning but adds 30-40% to retirement income for most earners.

For FIRE adherents retiring before 62, you can't collect Social Security yet — so use higher multiplier for the pre-62 portion, lower multiplier once benefits kick in.

Step 5: Tax Considerations

FI number calculations use "spending" but withdrawals are taxed. A $40K Roth withdrawal equals $40K spending; a $40K Traditional IRA withdrawal equals $30-32K spending after tax.

If your FI number is calculated on tax-free assumption but your portfolio is 50% Traditional, you need more saved. Account for this by either:

1. Calculating FI number on post-tax required withdrawals (add 20-30% for Traditional accounts)
2. Planning Roth conversions in early retirement to reduce Traditional balance before RMDs
3. Using average effective tax rate (not marginal) for rough estimates

The Worked Example

Single person, age 40, targeting retirement at 55.

Current spending: $75K/year. Retirement-adjusted: $65K/year.

Guaranteed income at 67: $28K Social Security.

For 55-67 (12 years): need full $65K from portfolio.

For 67+: need $65K - $28K = $37K from portfolio.

Combined FI number calculation:

• Pre-Social Security (12 years × $65K × ~15x): $780K needed for bridge period
• Post-Social Security (33× $37K for 50-year window): $1.22M needed for long-term

Total FI target: ~$2M

Rough equivalent via simple rule: 30-33× annual expenses = $2M-$2.15M. Close enough.

The Trinity Study Refinements

The original 4% rule used US large-cap stocks and Treasuries. Modern FI number calculations improve on it:

• International diversification adds safety (not in original study)
• Current high valuations reduce expected returns (may lower safe rate)
• Dynamic withdrawal rules can support higher initial rates
• Flexible spending dramatically improves portfolio survival

Morningstar's 2024 analysis found 3.7-4.0% as safe starting rate for 30-year retirement in current market conditions. Slightly lower than historical 4%.

The Non-Financial FI Number

Reaching FI number matters only if you actually want to retire. Many FI achievers find retirement difficult because:

• Work provides social connection and identity
• Structure and purpose matter
• "Forever vacation" gets boring after 6 months

Planning for what you'll DO after hitting FI is at least as important as the number itself. Part-time consulting, volunteering, new career, travel — have a plan.

The Verification

Once you hit your FI number, stress test before actually retiring:

1. Run several simulation scenarios (cFIREsim, Portfolio Visualizer)
2. Consider what happens if market drops 40% in year 1
3. Verify healthcare plan for pre-Medicare years
4. Model unexpected expenses (family assistance, major repairs, medical)

The margin of safety matters enormously. Retiring at exactly FI number with no buffer is risky. Having 20-30% additional cushion dramatically improves success probability.

The Simple Formula

FI Number = (Annual Expenses - Guaranteed Income) × Multiplier

Where Multiplier ranges from 25 (traditional age 65 retirement) to 40 (ultra-early retirement).

Apply this to your own numbers. The answer is probably between $1M and $3M for most people. That's your target. Saving toward it is decades of discipline; the math to define it takes 30 minutes.