What is a FIRE number?

It's an estimated portfolio size intended to sustain your annual spending indefinitely through withdrawals, without depleting the principal, at least under the assumptions of a chosen model. FIRE stands for Financial Independence, Retire Early. The number is calculated as annual expenses ÷ withdrawal rate (expressed as a decimal).

Where does the withdrawal rate assumption come from?

It's typically based on historical portfolio simulations, most famously the 'safe withdrawal rate' research suggesting that withdrawing around 4% of a diversified portfolio annually, adjusted for inflation, has historically had a strong chance of lasting 30 years without depletion. This calculator uses whatever withdrawal rate you enter — it doesn't assume 4% by default.

Why does a small change in withdrawal rate move the target so much?

Because the FIRE number is expenses divided by the rate, and division amplifies small changes in a small denominator. Dropping the assumed withdrawal rate from 4% to 3% doesn't increase the target by 25% — it increases it by 33%, because you're dividing the same expenses by a smaller number.

Does the calculator account for how quickly I'll actually reach my FIRE number?

It provides an illustrative years-to-target estimate using a fixed assumed growth rate on your current savings plus contributions, but that estimate is highly sensitive to the growth rate assumed and doesn't reflect actual market volatility, sequence-of-returns risk, or changes to your contribution rate over time.

Is a 30-year safe withdrawal rate the right assumption for an early retirement?

Not necessarily. Safe withdrawal rate research is often modeled around a roughly 30-year retirement horizon. Retiring significantly earlier extends the period the portfolio needs to last, which is one reason some FIRE planning uses a more conservative withdrawal rate than 4% for very early retirements.

Retirement guide · Reviewed 28 August 2026

A FIRE number is one division — and division amplifies small assumptions.

A FIRE target looks like a single clean number, but it's entirely a function of the withdrawal rate you assume — and small changes to that rate move the target far more than intuition suggests.

The FIRE number formula

FIRE number = annual expenses ÷ withdrawal rate. If you spend $40,000 a year and assume a 4% withdrawal rate, the target is $40,000 ÷ 0.04 = $1,000,000. The formula is a direct rearrangement of the question "how large a portfolio, withdrawn from at this rate, would cover this spending level" — it's essentially the inverse of a withdrawal-rate calculation.

Why the withdrawal rate assumption dominates the result

Because expenses are divided by the rate, and the rate is a small number (typically 3-5%), small absolute changes in the rate produce large percentage changes in the target. Moving the assumed rate from 4% to 3% — a one-point change — increases the required portfolio by a full third, not by 25%. This is why the choice of withdrawal rate is the single most consequential assumption in the entire model; it deserves more scrutiny than any other input.

Where the withdrawal rate assumption comes from

Widely-cited withdrawal rate research examined how various fixed withdrawal rates, adjusted annually for inflation, would have performed against historical market returns over multi-decade periods — with roughly 4% frequently cited as a rate that historically survived a 30-year horizon in most historical scenarios studied. That research is backward-looking and horizon-specific; it isn't a guarantee, and a materially longer retirement horizon (as FIRE often implies) is one reason some planners use a more conservative rate than 4%.

What the FIRE calculator computes

Enter your annual expenses, an assumed withdrawal rate, your current savings, and an annual contribution amount. The calculator returns your FIRE number and an illustrative years-to-target estimate based on a fixed assumed growth rate — useful for seeing directionally how the target and timeline move as your assumptions change.

Calculate your FIRE number ↗ Model retirement savings with contributions ↗

Frequently asked questions

What is a FIRE number?: It's an estimated portfolio size intended to sustain your annual spending indefinitely through withdrawals, without depleting the principal, at least under the assumptions of a chosen model. FIRE stands for Financial Independence, Retire Early. The number is calculated as annual expenses ÷ withdrawal rate (expressed as a decimal).
Where does the withdrawal rate assumption come from?: It's typically based on historical portfolio simulations, most famously the 'safe withdrawal rate' research suggesting that withdrawing around 4% of a diversified portfolio annually, adjusted for inflation, has historically had a strong chance of lasting 30 years without depletion. This calculator uses whatever withdrawal rate you enter — it doesn't assume 4% by default.
Why does a small change in withdrawal rate move the target so much?: Because the FIRE number is expenses divided by the rate, and division amplifies small changes in a small denominator. Dropping the assumed withdrawal rate from 4% to 3% doesn't increase the target by 25% — it increases it by 33%, because you're dividing the same expenses by a smaller number.
Does the calculator account for how quickly I'll actually reach my FIRE number?: It provides an illustrative years-to-target estimate using a fixed assumed growth rate on your current savings plus contributions, but that estimate is highly sensitive to the growth rate assumed and doesn't reflect actual market volatility, sequence-of-returns risk, or changes to your contribution rate over time.
Is a 30-year safe withdrawal rate the right assumption for an early retirement?: Not necessarily. Safe withdrawal rate research is often modeled around a roughly 30-year retirement horizon. Retiring significantly earlier extends the period the portfolio needs to last, which is one reason some FIRE planning uses a more conservative withdrawal rate than 4% for very early retirements.

Source note: This guide describes a general FIRE-number model based on a user-entered withdrawal rate. All calculations happen in your browser — nothing you enter is sent to a server or stored. This is educational information, not retirement or investment advice.