FIRE and the 4% rule, explained

What FIRE means

FIRE stands for Financial Independence, Retire Early. Financial independence (FI) means having enough invested that the money could pay your living costs for the rest of your life, so a paycheck becomes optional. Some people then stop working entirely; many switch to part-time work or work they enjoy more.

The movement grew through blogs and forums in the 2010s, but its arithmetic comes from retirement research. Two numbers do most of the work: how much you need, your FI number, and how fast you get there, which depends mostly on your savings rate.

The FI number: yearly spending × 25

Your FI number is a rough target for your invested savings. The common shortcut is your yearly spending multiplied by 25. Multiplying by 25 is the same as dividing by 4%, which is where the 4% rule comes in: take out 4% of your savings in the first year of retirement, raise that dollar amount with inflation every year after, and history suggests the money has usually lasted at least 30 years.

Where the 4% rule comes from

In 1994, financial planner William Bengen tested US stock and bond returns going back to 1926. He asked how much a retiree could withdraw each year, raised with inflation, without running out of money in any historical period. A first-year withdrawal of about 4% lasted at least 33 years in every period he tested, for portfolios with 50% to 75% in stocks (Bengen, 1994).

In 1998, three finance professors at Trinity University, Philip Cooley, Carl Hubbard and Daniel Walz, ran a similar test with large US company stocks and high-grade corporate bonds from 1926 to 1995. Their paper became known as the Trinity study. With half in stocks and half in bonds, a 4% withdrawal raised with inflation lasted 30 years in 95% of the historical periods they tested (Cooley, Hubbard and Walz, 1998 and 1999).

Neither study claimed 4% was safe for everyone. The Trinity authors wrote that there is no single best withdrawal rate: it depends on how long you need the money and how much risk of running short you accept.

Sequence risk: when the bad years come

Two retirees can earn the same average return over 30 years and end up in very different places, depending on the order of the returns. Losses in the first years of retirement do the most damage: you sell investments to live on while prices are down, and those shares miss the recovery. This is called sequence-of-returns risk. The historical periods that came closest to failing generally began just before a deep market fall or years of high inflation, such as the mid-1960s.

Early retirees face more of it, because a bad start has more years to compound. That is one reason many people planning a very long retirement use a withdrawal rate of 3% to 3.5%, which means a larger FI number, or plan to spend flexibly: trimming withdrawals after a bad year or earning some income in the first years.

Updated estimates move with interest rates and market prices. Morningstar’s late-2025 research, for example, put a safe starting rate at 3.9% for a 30-year retirement with a 90% chance of the money lasting, and higher for retirees willing to adjust their spending (Morningstar, 2025).

Your savings rate is the main lever

How quickly you reach an FI number depends mostly on the share of your take-home pay you save. A higher savings rate works twice: more goes into investments, and lower spending means a smaller target.

Real returns jump around, and their order matters, so treat these as rough. The point is the shape, which the blogger Mr. Money Mustache popularized in 2012: going from saving 10% to 50% cuts the wait from about half a century to under 20 years. Your savings rate is the part you control most.

Lean, Fat, Barista and Coast FIRE

These are informal community labels, not official categories, and people draw the lines in different places.

• Lean FIRE: reaching FI on a deliberately small budget. The target is smaller, but there is less room for surprises such as a large medical bill.
• Fat FIRE: reaching FI with a generous budget, which means a much larger FI number and usually a higher income or more years of work.
• Barista FIRE: leaving a full-time career once savings cover most costs, and working part-time for the rest, sometimes partly for health insurance. The name comes from a part-time coffee-shop job.
• Coast FIRE: having enough invested early that, with no more contributions, growth alone could reach your FI number by a traditional retirement age. You still work to pay today’s bills, but you no longer need to save for retirement.

For Coast FIRE with a $1,500,000 FI number at 65 and a hypothetical 5% yearly return after inflation, someone aged 35 would need about $347,000 invested now to coast there over 30 years. A lower return would need more.

Criticisms and open questions

• History isn’t a forecast. The 4% rule describes what worked in past US markets. Future returns could be lower.
• Too cautious or too aggressive? Over 30 years, many historical periods left retirees with large unspent balances, so some call 4% too cautious; for a 50-year early retirement, others call it too aggressive.
• Health care and access to money. In the US, people who stop working before 65 usually need their own health insurance until Medicare, and withdrawals from retirement accounts before age 59½ generally owe an extra 10% tax unless an exception applies.
• It is easier on a high income. Saving half your pay is far easier on a large salary, and very high savings rates can mean giving up a lot today.

Key takeaways

• Your FI number is roughly the yearly spending your other income won’t cover, divided by a withdrawal rate: × 25 at 4%.
• The 4% rule comes from tests of past US returns over about 30 years. It is a starting point, not a promise.
• Bad early years do the most harm; long retirements often plan on 3% to 3.5%.
• Your savings rate does more than anything else to shorten the road.

Retirement Ready on Wenlo uses the same ideas: its FI number divides the spending your other income doesn’t cover by your withdrawal rate (4% unless you change it), and it shows the share of historical periods since 1928 in which your plan would have lasted. The figures here are hypothetical and simplified, not a plan or a recommendation.