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FIRE Number Calculator

Computes the portfolio needed to fund annual expenses at a safe withdrawal rate (the FIRE number, 25× expenses at 4 %), your progress towards it, and the years until current savings plus annual saving reach it at an assumed return.

Annual expenses in retirementYearly spending the portfolio must cover, in today's money. Safe withdrawal rate (%) (optional)Percent of the portfolio withdrawn in the first year; 4 % is the Trinity-study rule, 3–3.5 % is more conservative for long retirements. Current investable savings (optional)Amount already invested towards the goal. Annual savings (optional)Amount saved and invested per year; takes precedence over annual_income × savings_rate_percent. Annual income (optional)Yearly income, used with savings_rate_percent when annual_savings is omitted. Savings rate (%) (optional)Percent of annual_income saved each year. Annual return (%) (optional)Expected annual return in percent; use a real (after-inflation) return since expenses are in today's money. Current age (years) (optional)Optional; enables the projected FI age.

When to use

You want to know how much you need to retire early or become financially independent, and how many years of saving at your current rate it will take.

Do not use when: You want to project a balance to a fixed retirement age (use retirement-savings) or test how long a balance lasts (use savings-withdrawal).

Formula

fire_number = annual_expenses / (withdrawal_rate_percent/100); with r = annual_return_percent/100, S = annual_savings (end of year) and C = current_savings: years_to_fi = ln((fire_number + S/r) / (C + S/r)) / ln(1 + r) (r ≠ 0), or (fire_number − C) / S when r = 0

Annual compounding with savings added at the end of each year and constant returns. The 4 % rule comes from the Trinity study (Cooley, Hubbard & Walz 1998) for 30-year horizons with a 50–75 % stock allocation; longer retirements argue for a lower rate.

Inputs

Parameter	Type	Unit	Required	Description
`annual_expenses`	number		yes	Yearly spending the portfolio must cover, in today's money. Range: > 0, ≤ 1000000000
`withdrawal_rate_percent`	number	%	default 4	Percent of the portfolio withdrawn in the first year; 4 % is the Trinity-study rule, 3–3.5 % is more conservative for long retirements. Range: > 0, ≤ 20
`current_savings`	number		default 0	Amount already invested towards the goal. Range: ≥ 0, ≤ 1000000000000
`annual_savings`	number		no	Amount saved and invested per year; takes precedence over annual_income × savings_rate_percent. Range: ≥ 0, ≤ 1000000000
`annual_income`	number		no	Yearly income, used with savings_rate_percent when annual_savings is omitted. Range: ≥ 0, ≤ 1000000000
`savings_rate_percent`	number	%	no	Percent of annual_income saved each year. Range: ≥ 0, ≤ 100
`annual_return_percent`	number	%	default 7	Expected annual return in percent; use a real (after-inflation) return since expenses are in today's money. Range: ≥ -50, ≤ 50
`current_age`	number	years	no	Optional; enables the projected FI age. Range: ≥ 0, ≤ 100

Outputs

Output	Type	Unit	Description
`fire_number`	number		annual_expenses / (withdrawal_rate_percent / 100).
`fire_multiple`	number		100 / withdrawal_rate_percent (25 at 4 %).
`progress_percent`	number	%	current_savings / fire_number × 100.
`remaining_to_save`	number		max(0, fire_number − current_savings), before any growth.
`annual_savings_used`	number		annual_savings, or annual_income × savings_rate_percent / 100.
`years_to_fi`	number	years	Years until savings plus growth reach the FIRE number (0 if already reached).
`fi_age`	number	years	current_age + years_to_fi (only when current_age is given).

Example

40,000 expenses at 4 %, 100,000 saved, 30 % of 100,000 income, 7 %, age 30: {"annual_expenses":40000,"withdrawal_rate_percent":4,"current_savings":100000,"annual_income":100000,"savings_rate_percent":30,"annual_return_percent":7,"current_age":30} → {"fire_number":1000000,"fire_multiple":25,"progress_percent":10,"remaining_to_save":900000,"annual_savings_used":30000,"years_to_fi":14.7,"fi_age":44.7}

60,000 expenses at 3.5 %: {"annual_expenses":60000,"withdrawal_rate_percent":3.5} → {"fire_number":1714285.71,"fire_multiple":28.57,"progress_percent":0}

GET https://tttkmbb.com/api/v1/calculate/fire-number?annual_expenses=40000&withdrawal_rate_percent=4&current_savings=100000&annual_income=100000&savings_rate_percent=30&annual_return_percent=7&current_age=30

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/fire-number (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/fire-number · Markdown: https://tttkmbb.com/finance/fire-number.md · JSON definition: https://tttkmbb.com/finance/fire-number.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="fire-number"
OpenAPI operationId: calculate_fire_number
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Wikipedia – Trinity study (safe withdrawal rates) (reference)
Wikipedia – FIRE movement (reference)
Investopedia – What Is the 4% Rule for Withdrawals in Retirement? (reference)

FAQ

Why 25 times expenses?

Withdrawing 4 % of a portfolio each year means the portfolio must be 1 / 0.04 = 25 times the annual withdrawal. At 3.5 % it is 28.6×, at 3 % it is 33.3×.

Should expenses include taxes?

Yes. Withdrawals from pre-tax accounts are taxable, so annual_expenses should be the gross amount needed including income tax and health insurance.

Related calculators

Retirement Savings Calculator — Project the balance to a specific retirement age.
Savings Withdrawal Calculator — How long the portfolio lasts at a given withdrawal.
Rule of 72 Calculator — Doubling time at the assumed return.