FIRE — Financial Independence, Retire Early — comes down to two numbers: how much you need invested to live off indefinitely, and how many years of saving it takes to get there. Both come from the same handful of inputs, run through two connected calculations.
Step 1: The FIRE Number
FIRE Number = Annual Expenses ÷ Safe Withdrawal Rate
The safe withdrawal rate (commonly 4%, based on the Trinity Study) is the percentage of a portfolio that can reasonably be withdrawn each year without running out of money over a long retirement. On $40,000 in annual expenses at a 4% withdrawal rate, that's a FIRE number of $1,000,000.
Step 2: Years to Get There
From current savings, an annual contribution amount, and an expected return, the calculator simulates growth month by month until the balance reaches the FIRE number. On $50,000 in current savings, $20,000 contributed annually, and a 7% return, reaching a $1,250,000 target (50,000 in annual expenses at 4%) takes about 21.8 years.
Why Both Numbers Matter Together
The FIRE number alone doesn't say anything about feasibility — a $2 million target means nothing without knowing whether it's 10 years away or 40. And a timeline alone doesn't mean anything without knowing what it's actually building toward. Seeing both side by side turns "I want to retire early" into an actual plan with a specific target and a specific date.
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