FIRE Number Calculator — How the 4% Rule Actually Works

Your FIRE number is the amount of money you need invested before you can stop working. It is calculated using the 4% rule: multiply your annual expenses by 25. If you spend $60,000 per year, your FIRE number is $1,500,000. Once you have $1,500,000 invested in index funds or similar assets, you can withdraw 4% annually ($60,000) and — based on historical data — your portfolio will last 30+ years without running out.

That is the entire concept. The free FIRE calculator takes this formula further: it also accounts for your current savings, your monthly savings rate, and your expected return to calculate exactly how many years it will take you to reach your FIRE number. This guide explains where the 4% rule comes from, when it is trustworthy, and when to adjust it.

What Is a FIRE Number — and How Is It Calculated?

Your FIRE number is the total invested portfolio value that generates enough annual income through the 4% safe withdrawal rate to cover your expenses indefinitely. The formula:

FIRE Number = Annual Expenses ÷ Safe Withdrawal Rate

Or equivalently: FIRE Number = Annual Expenses × 25 (since 1 ÷ 0.04 = 25)

Monthly Spending	Annual Spending	FIRE Number (4%)
$2,000	$24,000	$600,000
$3,000	$36,000	$900,000
$4,000	$48,000	$1,200,000
$5,000	$60,000	$1,500,000
$7,000	$84,000	$2,100,000
$10,000	$120,000	$3,000,000

Notice that the FIRE number is entirely driven by spending — not by income. Two people earning the same income can have dramatically different FIRE numbers based on how much they spend. This is why FIRE practitioners focus so heavily on the expense side of the equation.

Where the 4% Rule Comes From — The Trinity Study

The 4% rule originates from the 1998 Trinity Study, published in the American Association of Individual Investors Journal. Three finance professors (Philip Cooley, Carl Hubbard, and Daniel Walz) at Trinity University analyzed historical US stock and bond market data from 1926-1995 to answer one question: what withdrawal rate would have survived any 30-year period without depleting a portfolio?

Their finding: a 4% annual withdrawal rate from a portfolio of 50-75% stocks and 25-50% bonds had a 95%+ success rate across all historical 30-year periods tested. Even through the Great Depression, World War II, and the 1970s stagflation, a 4% withdrawal rate did not exhaust most portfolios.

Critical context most people miss:

• The study was based on 30-year retirement periods — not 40 or 50 years, which is realistic for FIRE practitioners retiring in their 30s or 40s
• It used historical US market data — other countries have had lower long-term returns
• It assumed fixed annual withdrawals — most real retirees adjust spending down in bad market years, which improves survival rates significantly
• It did not include Social Security income, which would allow lower withdrawal rates from the portfolio

Updated research (including the "Beyond the 4% Rule" studies) suggests 4% is still reasonable for 30-year periods, but FIRE practitioners retiring for 40-50 years should consider 3.3%-3.5% to improve portfolio survival odds.

Sell Custom Apparel — We Handle Printing & Free Shipping

How to Use Our FIRE Calculator to Find Your Number

The free FIRE calculator calculates your FIRE number and the years to reach it from four inputs:

1. Monthly income (after tax): Your current take-home pay. This determines how much you can potentially save.
2. Monthly expenses: What you plan to spend in retirement — this drives the FIRE number. Be honest. The calculator uses this as your target withdrawal amount.
3. Current savings: Your existing invested assets. This is the foundation the calculator builds on.
4. Expected annual return: Historically, a 60/40 stock/bond portfolio has returned 7-8% nominally, or 5-6% after adjusting for inflation. For a conservative estimate, use 6-7%.
5. Safe withdrawal rate: Default is 4%. You can lower this to 3.5% for a more conservative estimate if you plan to retire very early (under 45).

The calculator outputs: your FIRE number (expenses ÷ SWR), how many years until your portfolio hits that number, and your projected retirement year. It also classifies your FIRE type based on your spending level.

The most impactful input change: Reducing monthly expenses by $500 ($6,000/year) reduces your FIRE number by $150,000 (at 4% SWR) AND increases your monthly savings by $500 — a double compounding effect that can shorten your FIRE timeline by 1-3 years.

Which Safe Withdrawal Rate Should You Use — 3%, 3.5%, or 4%?

The right SWR depends on your retirement timeline and risk tolerance:

SWR	Best For	Portfolio Impact	Notes
4.0%	Retiring at 55-65, flexible spending	25x annual expenses	Standard; 95%+ success in 30-yr periods
3.5%	Retiring at 40-55, or wanting more cushion	28.6x annual expenses	Adds ~3.6 more years of expenses as buffer
3.0%	Retiring under 40, very conservative	33.3x annual expenses	Strong safety margin; extends timeline significantly
5.0%	Retiring after 60, flexible spending	20x annual expenses	Higher risk for long timelines; often includes SS

The FIRE community's current consensus leans toward 3.5% for early retirees (before 50) and 4% for those retiring at 55+. This reflects updated research on historical data including international markets and longer periods. In the free FIRE calculator, you can adjust the SWR slider — try 4% and 3.5% to see the difference in your FIRE number and timeline.

Spending flexibility matters more than the exact SWR. Someone who can reduce spending by 10-20% in a bad market year has a much higher practical success rate than the pure math suggests. If you can be flexible, 4% is fine. If you need fixed income with no flexibility, 3.5% provides meaningful extra safety.

Five Common FIRE Number Calculation Mistakes

• Using current spending instead of planned retirement spending. If you currently spend $7,000/month but plan to spend $5,000/month in retirement (no more mortgage, kids moved out, commuting eliminated), use $5,000 — not $7,000. Overestimating inflates your FIRE number unnecessarily.
• Forgetting healthcare. In the US, healthcare before Medicare (age 65) is a significant expense for FIRE practitioners. ACA marketplace plans can cost $500–$1,500/month depending on location and coverage level. Add this explicitly to your monthly expenses estimate.
• Not counting home equity. Your FIRE number refers to liquid invested assets — stocks, bonds, index funds. Home equity is not accessible without selling or borrowing. If you plan to sell your home after retiring, model that separately.
• Using pre-tax income and savings. The calculator uses monthly income after tax and monthly expenses. Make sure both are on the same post-tax basis.
• Ignoring sequence of returns risk. A 40% portfolio drop in your first two years of retirement is far more damaging than the same drop after 10 years. Retiring in a year with a strong market and then experiencing an immediate bear market is a real risk. Consider retiring with 1-2 years of expenses in cash to avoid selling during an early downturn.

Frequently Asked Questions

Is the 4% rule still valid in 2026?

Yes, with caveats. For 30-year retirement periods with a diversified stock/bond portfolio, the 4% rule still has a high historical success rate. For retirements lasting 40-50 years (common in early FIRE cases), many researchers recommend 3.3-3.5% for better long-term safety.

What is the 25x rule?

The 25x rule is another way to state the 4% rule: multiply your annual expenses by 25 to find your FIRE number. It comes from the fact that 1 ÷ 0.04 = 25. If you spend $50,000/year, your FIRE number is $1,250,000.

Does the FIRE number include a house?

No — the FIRE number refers to liquid invested assets that generate the 4% withdrawal. If you own a home, your home equity does not count toward the FIRE number unless you plan to sell and use the proceeds.

What if I have Social Security?

Social Security income reduces how much your portfolio needs to generate. If you expect $12,000/year from Social Security starting at 67, and your annual expenses are $60,000, your portfolio only needs to cover $48,000/year — cutting your FIRE number from $1.5M to $1.2M.