How do I calculate my current savings rate?

Savings rate = (Total Annual Savings + Investments) ÷ Gross Annual Income × 100. Include all savings: 401k contributions (including employer match), IRA contributions, taxable brokerage, emergency fund additions. Exclude principal paid on mortgage. Include gross income before taxes — the tax-deferred 401k contribution was earned before taxes. A 25% savings rate on $80,000 income = $20,000/year invested.

What is the difference between LeanFIRE and FatFIRE?

LeanFIRE targets early retirement with minimal lifestyle, typically under $40,000/year in expenses (FIRE number under $1M). Regular FIRE targets a comfortable middle-class retirement, $50,000–$80,000/year (FIRE number $1.25M–$2M). FatFIRE maintains a high-spending lifestyle, $100,000+/year (FIRE number $2.5M+). The formula is the same for all; the difference is the annual expense target.

How to Calculate Your FIRE Number

Q: What is the FIRE number formula?

FIRE Number = Annual Expenses ÷ Safe Withdrawal Rate. Using the standard 4% rule: FIRE Number = Annual Expenses × 25. Spend $50,000/year → FIRE number is $1,250,000. Spend $80,000/year → $2,000,000. For more conservative planning with 40–50 year horizons, use 3.5% (FIRE Number = Annual Expenses × 28.6).

Q: How does savings rate affect time to FIRE?

Savings rate is the single biggest lever. At 10% savings rate: ~43 years to FIRE. At 25%: ~32 years. At 50%: ~17 years. At 70%: ~8.5 years. These assume 5% real returns and a 4% withdrawal rate. Higher savings rate both accumulates wealth faster AND reduces your FIRE number (because lower spending means you need less to retire on). It has a double compounding effect.

Q: Does Social Security reduce my FIRE number?

Yes, significantly. Every $1,000/month in Social Security income reduces your required annual portfolio withdrawal by $12,000, which reduces the required portfolio by $300,000 at the 4% rule. If your FIRE number based on expenses is $1.5M but you expect $2,000/month in Social Security, your actual required portfolio is $900,000 — a $600,000 reduction. Very early retirees (40s) can't rely on Social Security for their first 20 years but should model it as a supplement starting at 62.

Calculate your exact FIRE number and timeline: FIRE Number Calculator →

Step 1: Calculate Your Annual Expenses

Your FIRE number is based on your spending, not your income. Before you can calculate your target, you need to know how much you actually spend in retirement. This isn't the same as your current expenses — it's your expected retirement spending, which may be higher or lower.

Common adjustments: housing costs often decrease (mortgage paid off, downsizing), but healthcare costs significantly increase before Medicare at 65. Travel often increases in early retirement. Work-related costs (commuting, professional clothing, convenience food) disappear. A reasonable starting estimate: 80%–90% of pre-retirement spending for most people, though FIRE practitioners often target 70%–80% through lifestyle optimization.

Step 2: Apply the 25× Formula

FIRE Number = Annual Retirement Expenses × 25

This comes from the 4% safe withdrawal rate (1 ÷ 4% = 25). At 25× your expenses, you can withdraw 4% annually (inflation-adjusted) with historically high odds of never depleting the portfolio over 30 years.

$40,000/year in expenses → FIRE number: $1,000,000
$60,000/year → $1,500,000
$80,000/year → $2,000,000
$100,000/year → $2,500,000

For early retirees with 40–50 year horizons, many practitioners use 28× (3.5% rate) or 31× (3.25% rate) for a larger safety buffer.

Step 3: Adjust for Social Security

Social Security income directly reduces how much your portfolio must generate. Every $1,000/month in Social Security reduces the required portfolio by $300,000 at the 4% rule. Even a modest $1,500/month benefit cuts $450,000 from your FIRE number.

Very early retirees (retiring at 35–45) typically ignore Social Security for their initial calculations — they'll need to self-fund for 20–30 years before claiming — but treat it as a floor that kicks in at 62–70. Someone retiring at 55 with a 15-year gap before Social Security at 70 needs a larger interim portfolio than their FIRE number at full retirement implies.

Step 4: Calculate Your Timeline

Time to FIRE is driven overwhelmingly by your savings rate. The math follows directly: your savings rate determines both how fast wealth accumulates and implicitly what your FIRE number is (lower spending = lower target). The dual effect is why savings rate is so powerful:

Years to FIRE by Savings Rate (5% real return, from $0, 4% SWR)
Savings Rate	Years to FIRE	Why It Compounds
10%	43 years	Spending 90%, needing 22.5× income to retire
25%	32 years	Spending 75%, needing 18.75× income
40%	22 years	Spending 60%, needing 15× income
50%	17 years	Spending 50%, needing 12.5× income
70%	8.5 years	Spending 30%, needing only 7.5× income

Raising your savings rate from 25% to 50% cuts your timeline from 32 years to 17 years — saving 15 years of work. That 15 years is bought by spending $1 less today (reducing your FIRE number) while simultaneously saving $1 more (growing the portfolio faster). Both effects compound together.

LeanFIRE, FIRE, and FatFIRE

The FIRE community uses three main variants: LeanFIRE (under $40K/year, FIRE number under $1M) requires aggressive frugality or geographic arbitrage. Regular FIRE ($50K–$80K/year, $1.25M–$2M) is the middle path. FatFIRE ($100K+/year, $2.5M+) maintains a high standard of living. All use the same 25× formula — the target just scales with your chosen lifestyle.

Once you hit your FIRE number, the Retirement Income Calculator shows exactly how much monthly income that portfolio can sustainably generate at different withdrawal rates. For the accumulation side, the FIRE Number Calculator shows your target, timeline, and year-by-year progress.

Frequently Asked Questions

What is the FIRE number formula?

FIRE Number = Annual Expenses × 25 (using the 4% rule). Spend $60,000/year → FIRE number is $1,500,000. For longer horizons (40–50 years), use Annual Expenses × 28.6 (3.5% rate) for a larger safety margin.

How does savings rate affect time to FIRE?

Savings rate is the single biggest lever — more so than investment returns. At 10%: ~43 years. At 50%: ~17 years. At 70%: ~8.5 years. Higher savings rate has a double effect: you accumulate faster AND lower your spending reduces your FIRE number simultaneously.

How do I calculate my savings rate?

Total annual savings and investments ÷ gross annual income × 100. Include all savings vehicles (401k, IRA, brokerage, etc.). Use gross income, not after-tax. A 25% savings rate on $80,000 gross income = $20,000/year invested across all accounts.

Does Social Security reduce my FIRE number?

Yes — directly and significantly. Every $1,000/month in Social Security reduces the required portfolio by $300,000 at the 4% rule. Early retirees need to self-fund until Social Security kicks in at 62–70, so they need a larger interim portfolio but can plan for reduced withdrawals later.

Is the 4% rule safe for early retirement?

For 40–50 year horizons, many practitioners use 3.5% (28× expenses) instead of 4%. Historical success rates at 4% over 40+ years are still 80%–90% across US market history, but not as robust as 30-year periods. Adding a flexible spending component (cutting 10% in bad market years) significantly improves long-term survival rates.