FIRE Calculator: Financial Independence and the 4% Rule

How to calculate your FIRE number with the 4% rule

Your FIRE number is your annual spending divided by the safe withdrawal rate: with the 4% rule you need 25 times your spending, or $1,125,000 for $45,000 a year. The tool then estimates how many years it takes to get there, growing your savings at the real return (return minus inflation).

The formulas used

• FIRE number = annual spending ÷ safe withdrawal rate (SWR). At 4% that is 25 years of spending; at 3.5%, about 28.6.
• Savings rate = (net income − spending) ÷ net income.
• Real return = nominal return − inflation (a simple Fisher approximation, floored at 0.1%).
• Years to FIRE: each year, portfolio = portfolio × (1 + real return) + annual savings, until it passes the target (70 years at most).
• Variants: Lean FIRE = 75% of spending, Fat FIRE = 140%, Barista FIRE = 50% (the other half is assumed to come from part-time work), all divided by the same SWR.
• Coast FIRE: the amount you need today to reach the target at 65 without saving more, i.e. target ÷ (1 + real return)^{65 − age}.

Worked example (default values)

Age 30, $65,000 invested, $90,000 net income, $45,000 spending, 4% SWR, 8% return, 2.5% inflation. The savings rate is 50% ($45,000 a year), the target $1,125,000 and the real return 5.5%. The portfolio passes the target after 15 years, at age 45. Lean FIRE: $843,750; Fat FIRE: $1,575,000; Barista FIRE: $562,500. The Coast FIRE threshold is $172,710, so $65,000 is not there yet. On a $60,000 income (25% savings rate) it would take 27 years.

Sustainable withdrawal over a fixed period (annuity)

The "Sustainable withdrawal over a set period" mode finds the level withdrawal that brings the capital down to zero after the chosen number of years: withdrawal = capital × t ÷ (1 − (1 + t)⁻ⁿ), where t is the rate per period and n the number of withdrawals. For monthly withdrawals, t = annual rate ÷ 12 and n = years × 12, as in a spreadsheet's PMT function; each withdrawal happens at the end of the period. The reverse calculation gives the capital needed: capital = withdrawal × (1 − (1 + t)⁻ⁿ) ÷ t. If the withdrawal is indexed to inflation, the annual rate used is the exact real rate, (1 + return) ÷ (1 + inflation) − 1, and amounts are in today's dollars. Default example: $500,000, 5% return, 2% inflation, 25 years, indexed monthly withdrawal: real rate 2.94%, sustainable withdrawal $2,355.79 a month, or $706,736.17 over 25 years. Without indexation, the fixed withdrawal rises to $2,922.95 a month; indexed and yearly, it is $28,526.06 a year. In reverse, $2,000 a month indexed for 25 years requires $424,486.55. Check against Microsoft's published examples: $10,000 at 8% over 10 months gives $1,037.03 a month (PMT), and $500 a month for 20 years at 8% is worth $59,777.15 (PV).

Assumptions and limits

• Same return every year, no early crash, no fees or taxes.
• All amounts are in today's money, so annual savings are assumed to rise with inflation.
• The 4% rule was tested on retirements of 30 years or less. For a 40- to 50-year early retirement, a lower rate (the slider goes down to 2.5%) cuts the risk of running out: at 3.5% the target rises to $1,285,714 and the timeline to 17 years.
• This is a simulation and does not replace advice from a financial planner.

Sources and dates: William P. Bengen, "Determining Withdrawal Rates Using Historical Data", Journal of Financial Planning, October 1994 (30-year retirements tested on US data from 1926); Cooley, Hubbard and Walz, "Retirement Savings: Choosing a Withdrawal Rate That Is Sustainable", AAII Journal, February 1998 (the Trinity study). Annuity formula checked against Microsoft's examples for the PMT and PV functions. Default return and inflation are indicative, unverified values (checked 22/10/2026).

Frequently Asked Questions

How much money do I need to reach FIRE?

With the 4% rule, 25 times your annual spending: $45,000 of spending requires $1,125,000. With a more cautious 3.5% withdrawal rate you need about 28.6 times, or $1,285,714.

What is Coast FIRE?

It is the amount that, with no further contributions, will grow to your target by age 65 through compounding alone. At 30, with a $1,125,000 target and a 5.5% real return, you need $172,710 invested today.

What savings rate do I need to retire in 15 years?

With the default values ($65,000 already invested, $45,000 spending, 5.5% real return), a 50% savings rate reaches the target in 15 years. At 25% it takes 27 years, because you save less and spend more.

Does the 4% rule hold for a 40-year retirement?

The Bengen (1994) and Trinity (1998) studies tested retirements of 30 years or less. For a longer horizon, a lower withdrawal rate reduces the risk of running out; the tool accepts 2.5% to 5%. At 3.5%, the default example takes 17 years instead of 15.

How much can I withdraw each month for 25 years from $500,000?

With a 5% annual return and a withdrawal indexed to 2% inflation, $500,000 lets you withdraw $2,355.79 a month in today's dollars for 25 years, after which the capital is used up. Without indexation, the fixed withdrawal rises to $2,922.95 a month. It is an annuity calculation, like a spreadsheet's PMT function, with withdrawals at the end of each month.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.