Coast FIRE Calculator

Coast FIRE is the financial independence milestone where your existing investments, left untouched, will grow into your retirement goal on their own. You no longer need to save. You only need to earn enough to cover what you spend today.

Coast FIRE Formula: Coast FIRE Number = Target / (1 + r)^n

Variable definitions:

• Target = total amount needed at retirement (often 25× annual expenses, based on the 4% rule)
• r = expected annual real investment return (historically 7% for a diversified index fund portfolio)
• n = years from now until target retirement age

The 4% Safe Withdrawal Rule: If you withdraw 4% of your portfolio per year in retirement, historical data shows the portfolio survives 30+ years in almost all market conditions. Retirement Target = Annual Expenses × 25

Worked example:

• Current age: 32, target retirement 65, so n = 33 years
• Annual retirement expenses: $50,000, so Target = $1,250,000
• Expected return: 7% annually
• (1.07)^33 = 9.325
• Coast FIRE = $1,250,000 / 9.325 = $134,043

Once you have $134,043 invested, you can stop contributing entirely and the portfolio still reaches $1.25M by 65.

Notice how brutally that number depends on n. The same person at 42 instead of 32 needs $263,700, nearly double, because they lost ten doublings-worth of runway. Ten years of delay costs more than a decade of contributions would have.

Why this matters: Reaching Coast FIRE buys options rather than retirement. You can go part-time, take the job you actually want, or move somewhere cheaper, because the retirement piece is already handled by compound growth.

Inflation note: Use a real, inflation-adjusted return of about 4-5% if your expense figure is in today’s dollars. Use 7% nominal only if you have also inflated the target into future dollars. Mixing the two is the most common way people badly underestimate this number.