What Is the FIRE Movement — The 25x Rule

FIRE stands for Financial Independence, Retire Early—meaning "achieve financial independence and retire early." It's a financial movement that spread in the early 2010s, centered on U.S. bloggers (Mr. Money Mustache, J.L. Collins, etc.).

The core idea is this: cut spending and greatly raise your savings rate to amass assets quickly, and once you can live on the income from those assets alone, you become "free from labor." The goal isn't necessarily to quit working but to have the "option of not having to work."

The skeleton of FIRE calculation is the "25x rule." Assets needed for retirement = annual spending × 25.

For example, if you live on $30,000 a year, your target assets are $30,000 × 25 = $750,000.

This number 25 is the reciprocal of the "4% rule" (1 ÷ 0.04 = 25). The 4% rule is a rule of thumb from William Bengen's 1994 research and the Trinity Study: if you withdraw 4% of assets in the first year of retirement and thereafter increase withdrawals only by inflation, assets were not depleted over 30 years based on past U.S. data.

In other words, "amass 25 times your annual spending, and you can withdraw 4% each year and last 30 years" is the starting point of FIRE.

Mr. Money Mustache's famous post 'The Shockingly Simple Math Behind Early Retirement' compressed the time to retirement into a single table. It's the savings rate, not the absolute amount of income, that determines the retirement point.

Assuming a real return of 5% and a safe withdrawal rate of 4%, a savings rate of 25% reaches financial independence in about 32 years, and a savings rate of 50% in about 17 years. Doubling the savings rate roughly halves the time to retirement.

Why is it so dramatic? Because a high savings rate means two things at once: (1) assets accumulate faster, and (2) since spending is low, the target assets you need are themselves smaller. When these two effects multiply, the time shrinks greatly.

FIRE is a powerful framework but not a cure-all. There are three representative criticisms.

First, the problem of market assumptions. The 4% rule is based on a 30-year U.S. past sample. But early retirement requires withdrawing for 40-60 years, an out-of-sample domain, and if future returns are lower than the past, plans wobble.

Second, sequence risk. If a large crash comes right after retirement, you must keep withdrawing from reduced assets, making recovery hard. Even for the same average return, a bad decline order changes the outcome greatly.

Third, real-world variables. Medical costs, inflation, unexpected expenses, and the risk of living long (longevity risk) push against plans. U.S.-style FIRE weighs the health-insurance burden heavily, but Korea's National Pension and health-insurance structure differs, so it's hard to apply directly.

In conclusion, FIRE's numbers are not a "guarantee" but a "goal based on assumptions." This article does not recommend a specific plan but helps you understand the principle of savings rate and compounding.