FIRE Calculator: How Much You Need to Retire Early — and Why Your Savings Rate Sets the Date

Two people earning the same salary can retire fifteen years apart, and the reason isn’t luck or returns — it’s the share of their pay they save. Your savings rate, not your income, sets your timeline to financial independence, because the percentage you save does two things at once: it builds your portfolio faster and it shrinks the portfolio you need. A credible FIRE number is roughly 25 times your annual spending — but that “25” rests on a 4% withdrawal assumption worth stress-testing before you trust it with the next 50 years of your life.

One disclaimer governs every figure below: these numbers are illustrative and depend on assumptions about returns, inflation, taxes, and behavior no calculator can predict. Past performance does not guarantee future results.

Your FIRE number is your annual spending times about 25

Your FIRE number — the portfolio that lets you stop working — is roughly 25 times what you spend in a year, because 25 is simply the inverse of a 4% withdrawal rate. If you can safely draw 4% of a portfolio each year, the portfolio you need equals your annual spending divided by 0.04, and 1 ÷ 0.04 = 25. The multiplicand is your spending, not your income — the withdrawal funds your life, not your paycheck.

So a $40,000-a-year lifestyle implies a target near $1,000,000 (illustrative).

The 4% traces to William Bengen’s 1994 paper in the Journal of Financial Planning, which tested a 30-year retirement against U.S. market history and found that a retiree could withdraw about 4% of the starting balance in year one, raise that amount with inflation each year after, and survive every historical period he tested 1. Bengen’s own figure was closer to 4.15%; the round number became the shorthand. The Trinity Study (Cooley, Hubbard & Walz, 1998) reached a similar place from a different angle: 4% inflation-adjusted withdrawals over 30 years survived the large majority of historical periods — roughly 95% or higher for stock-heavy balanced portfolios 4.

Treat 25x as a working assumption, not a guarantee — a historical worst-case for one country over one period, which an early retiree has good reason to stress-test. That is the job of the last section.

Two people, same income, retire fifteen years apart

Your savings rate beats your income because it works both ends of the equation at once — it pours more money into the portfolio every year and it lowers the portfolio you need. Savings rate is just dollars saved divided by take-home pay. That single fraction, not your salary, is the lever.

On the numerator side, a higher savings rate means more dollars invested each year, so the portfolio grows faster. On the denominator side, it means you live on less — and since your target is spending times 25, living on less lowers your FIRE number at the same time. That second effect is the half most calculators hide.

Make it concrete. Two colleagues each take home $80,000 (illustrative; 5% real return and a 4% withdrawal rate assumed).

• Saver A puts away 15%, living on about $68,000. Her target is roughly $1.7 million.
• Saver B puts away 50%, living on about $40,000. His target is roughly $1.0 million.

Saver B invests far more every year toward a smaller number. He wins on both axes at once: a bigger annual contribution and a finish line $700,000 closer. That gap compounds into a decade-plus difference in retirement dates — between two people the IRS sees as identical earners.

This is why “earn more” is the weaker lever. A raise helps only if your savings rate rises with it; if your spending climbs to match, your FIRE number climbs too, and you have run in place.

What your savings rate does to the clock

A simple model popularized by Mr. Money Mustache shows that your savings rate alone — not your salary — predicts how many years you have left to work: save 50% and you’re looking at roughly 17 years; save 75% and it’s about 7. The model comes from his 2012 post “The Shockingly Simple Math Behind Early Retirement” 3, and it rests on three stated assumptions: about 5% return after inflation while saving, a 4% withdrawal rate afterward, and a starting net worth of zero.

From those assumptions, the savings rate maps cleanly to a working-years count (all figures illustrative):

• Save 5% → about 66 years
• Save 30% → about 28 years
• Save 50% → about 17 years
• Save 75% → about 7 years

The deltas are the point: move from saving 30% to saving 50% and you cut roughly 11 years off your working life — without earning an extra dollar.

Treat this as a model, not a promise. It assumes a steady real return and a constant savings rate that real life rarely delivers, and Karsten Jeske of Early Retirement Now has published a rebuttal arguing the math oversimplifies once you add valuations, variable returns, and taxes 8. The table illustrates the accumulation half of the last section’s lever; it does not guarantee a date.

The 4% rule was built for a 30-year retirement — yours might last 55

The 25x target leans on a 4% rule calibrated to a 30-year retirement, and an early retiree facing 45 to 60 years should treat that as a starting point to stress-test, not a settled number — which is exactly why the experts who study it most closely disagree about it. Bengen and the Trinity Study both modeled roughly 30 years 1; someone retiring at 40 may need the money to last 50. The longer the horizon, the more the math strains: Jeske’s simulations show historical success rates falling sharply as it stretches — for a balanced portfolio at a 4% rate, the roughly 95% success over 30 years drops to roughly 65% over 60 years (approximate figures, attributed to his work) 6.

The mechanism behind that decay is sequence-of-returns risk, which Michael Kitces locates in the first decade of withdrawals 9. A bad run of returns early in retirement, paired with the spending you keep taking out, can permanently impair the portfolio even when long-run averages turn out fine — the order of returns matters, not just the average. A 50-year retirement exposes you to more chances of starting into a bad decade.

Here the experts genuinely split, and it’s worth seeing why. Bengen himself later raised his figure — to 4.5% in 2006 and 4.7% in 2021 — but that 4.7% is still a 30-year worst-case, reached by broadening the portfolio, not by extending the horizon 10. Jeske moves the opposite way, down to about 3.5% — “3.5% is the new 4%” — by holding the portfolio and stretching the horizon to 45–60 years 6. They diverge because they hold different variables fixed: Bengen the horizon, Jeske the portfolio. For an early retiree, the long horizon is the binding constraint, so lean conservative.

Because the multiple is the inverse of the rate, your FIRE target is a range, not a point: about 21x at 4.7%, 25x at 4%, 28.6x at 3.5%, 30.8x at 3.25%. Re-run the earlier example at the conservative end: the $40,000-a-year spender whose number was about $1,000,000 at 25x now faces roughly $1,144,000 at 28.6x — about $144,000 more for the same lifestyle, purely because the runway is longer (illustrative). A long-horizon early retiree should plan toward that higher multiple.

Run your own number — then pressure-test it

Computing your own FIRE number takes three steps — nail down your real annual spending, multiply by a multiple you’ve chosen with your horizon in mind, then check it against your actual savings rate and timeline.

First, find your true annual spending — the last twelve months, not your salary — because spending is the multiplicand for everything above. Your starting point matters too; the net worth calculator establishes the stash you’re building from.

Second, pick your multiple deliberately: 25x as the conventional anchor, leaning toward 28.6x or higher if your horizon runs 45 years or more.

Third, model the contributions that get you there, and pressure-test the assumptions — returns, inflation, taxes, sequence of returns — that no single number captures. The retirement savings calculator handles the contribution modeling. (Which expenses to trim, how to allocate, and the tax tactics that speed the trip are each their own article.)

Your FIRE number isn’t a fact you look up; it’s a personalized, assumption-dependent projection you build from your own spending and savings rate, then stress-test. Build yours now with the Retirement Readiness Report, which turns this framework into a projection with every assumption made explicit.

Sources

• William P. Bengen — Determining Withdrawal Rates Using Historical Data (1994)
• Mr. Money Mustache — The Shockingly Simple Math Behind Early Retirement (2012)
• Cooley, Hubbard & Walz — the Trinity Study (AAII Journal, 1998)
• Early Retirement Now — The Ultimate Guide to Safe Withdrawal Rates, Part 1
• Karsten Jeske — Safe Withdrawal Rates: A Guide for Early Retirees (SSRN)
• Early Retirement Now — rebuttal to the “Shockingly Simple Math”
• Michael Kitces — Understanding Sequence of Return Risk & Safe Withdrawal Rates
• Advisor Perspectives — Bill Bengen Boosts the “4% Rule” to 4.7% (2025)
• Financial Samurai — The 4% Rule: Clearing Up Misconceptions With Bill Bengen