CalculateY
How the FIRE movement calculates early retirement
The FIRE number is the amount of savings required to retire and live indefinitely off investment returns. The formula is straightforward:
FIRE Number = Annual Expenses / Safe Withdrawal Rate
At the most common withdrawal rate of 4%, the formula simplifies to Annual Expenses multiplied by 25. Someone spending $40,000 per year needs approximately $1,000,000 saved. Someone spending $80,000 per year needs approximately $2,000,000.
The 4% figure comes from the Trinity Study, published in 1998 by three finance professors at Trinity University. They analyzed historical market data from 1926 onward and found that a portfolio of 50% stocks and 50% bonds could sustain a 4% annual withdrawal for 30 years with a roughly 95% success rate. The study has been updated and debated since, but the 4% rule remains the standard starting point for FIRE calculations.
What counts as annual expenses
The denominator in the formula matters a great deal. Underestimate your expenses and your FIRE number is too low; overestimate and you work longer than necessary.
Annual expenses for FIRE purposes should include everything you actually spend: housing, food, transportation, healthcare, insurance, travel, subscriptions, and irregular costs like car repairs or home maintenance. A common approach is to track spending for 12 months, then add 10 to 15% for costs that are easy to forget (annual fees, irregular repairs, the occasional emergency).
Healthcare is often the largest underestimated expense for early retirees who leave employer-sponsored coverage. In the United States, private health insurance before Medicare eligibility at 65 can cost $400 to $800 or more per month for an individual, depending on age and plan.
How savings rate determines years to FIRE
The time it takes to reach the FIRE number depends almost entirely on your savings rate: the percentage of income saved and invested. A higher savings rate compresses the timeline from two directions simultaneously. You accumulate the target balance faster, and a higher savings rate implies lower expenses, which means a smaller FIRE number to begin with.
This table shows approximately how many years it takes to reach the FIRE number at different savings rates, assuming a 7% average annual return on investments and starting from zero savings:
| Savings rate | Approximate years to FIRE |
|---|---|
| 10% | 43 years |
| 20% | 33 years |
| 30% | 26 years |
| 40% | 21 years |
| 50% | 16 years |
| 60% | 12 years |
| 70% | 8 years |
| 80% | 5 years |
At a 10% savings rate, the math leads to a traditional retirement timeline. At 50%, most people reach FIRE in their early to mid 40s (assuming they start in their mid-20s). At 70% savings, roughly 8 years is the estimated timeline, which is how some FIRE practitioners retire in their early 30s.
The returns assumption matters. Using 5% instead of 7% adds roughly 3 to 5 years at most savings rates. Using 9% shaves off a similar amount. For planning purposes, 6% to 7% (reflecting long-run real stock market returns after inflation) is a reasonable estimate, but actual returns will vary year to year, and some years will be negative.
Worked example: $50,000 income, $30,000 expenses
Take someone earning $50,000 per year after tax and spending $30,000 per year. Their savings rate is ($50,000 - $30,000) / $50,000 = 40%.
Their FIRE number at 4% withdrawal rate: $30,000 / 0.04 = $750,000.
They save $20,000 per year and invest it at an estimated 7% annual return. Using the future value of an annuity formula, reaching $750,000 takes approximately 21 years. If they start at 28, their estimated FIRE date is around age 49.
If they cut expenses to $25,000 per year, two things change: the FIRE number drops to $625,000, and their savings rate rises to 50%. The revised estimated timeline drops to roughly 16 years, with an estimated FIRE date around age 44. A $5,000 reduction in annual spending saves approximately 5 years of working.
Lean FIRE, regular FIRE, and Fat FIRE
FIRE practitioners often break into three groups based on target spending levels.
Lean FIRE targets annual expenses below $40,000, often around $25,000 to $35,000. This requires a FIRE number in the $625,000 to $875,000 range. It is achievable faster but leaves little margin for unexpected expenses, lifestyle changes, or healthcare cost increases.
Regular FIRE (often just called FIRE) typically targets $40,000 to $80,000 in annual expenses, with FIRE numbers of $1,000,000 to $2,000,000. This is the range most commonly discussed in FIRE communities.
Fat FIRE targets $80,000 or more per year, sometimes $150,000 to $200,000. FIRE numbers of $2,000,000 to $5,000,000 are typical. This approach preserves a higher standard of living and provides a larger buffer against sequence-of-returns risk, but the required savings are substantially larger.
There is no objectively correct category. The right number depends on where you live, what you value, whether you plan to generate any income in retirement, and how much uncertainty you are comfortable accepting.
Inflation adjustment
The 4% rule is expressed in real (inflation-adjusted) terms. Withdrawing 4% in year one and then adjusting that dollar amount upward each year for inflation is the standard assumption in the Trinity Study methodology.
In practice, someone with a $1,000,000 portfolio withdrawing 4% takes $40,000 in year one. If inflation runs 3% in year two, they take $41,200. By year 20 at 3% average inflation, that initial $40,000 withdrawal grows to approximately $72,000 in nominal dollars, but its purchasing power remains equivalent to the original $40,000.
Some FIRE practitioners use a variable withdrawal strategy instead. In down market years, they spend less; in strong years, they spend more. This flexibility significantly improves the odds of a portfolio lasting indefinitely, though it requires a degree of spending flexibility that not everyone can accommodate.
Sequence-of-returns risk
The biggest threat to a FIRE plan is not average returns over 40 years; it is what happens in the first decade. A prolonged market decline early in retirement, combined with ongoing withdrawals, can permanently deplete a portfolio even if later decades produce strong returns. This is called sequence-of-returns risk.
The 4% rule accounts for this to a degree: the Trinity Study’s 95% success rate includes scenarios where the market dropped significantly in the early years. But a longer retirement horizon (40 or 50 years rather than 30) reduces the success rate. Some FIRE practitioners use 3.5% or 3.25% as a more conservative withdrawal rate to account for longer time horizons.
Having one or two years of expenses in cash or short-term bonds provides a buffer: in a market downturn, you draw from cash rather than selling equities at depressed prices.
Running the numbers
The calculations in this post use fixed assumptions. Your actual FIRE number depends on your specific expenses, expected Social Security or pension income (which reduces the required portfolio), tax situation, and withdrawal strategy. The retirement calculator lets you model different savings amounts, rates of return, and time horizons to see how the estimates change with your specific inputs.
The arithmetic of FIRE is not complicated. The challenge is the savings rate required to make the timeline short enough to matter.