Example: If you’re 30 years old, plan to retire at 65, and expect to spend $50,000 per year in retirement, you’ve completed step one.
Step 2: Calculate Required Retirement Savings
Once you know your annual spending, you need to determine how much money you’ll need saved by retirement to sustain that lifestyle.
Use the
4% Rule (or 25x Rule) established by the Trinity Study (1998) as a starting point: multiply your annual spending by 25. This assumes you can safely withdraw 4% of your portfolio annually without running out of money.
Formula: Retirement Savings Needed = Annual Spending × 25
Example: $50,000 × 25 = $1,250,000
This means you’ll need $1.25 million saved by age 65 to support $50,000 in annual spending. You can adjust this multiplier based on your risk tolerance (use 30x for more conservative, 20x for more aggressive).
Step 3: Calculate Years Until Retirement
Subtract your current age from your target retirement age to find how many years your money has to grow.
Formula: Years to Retirement = Retirement Age - Current Age
Example: 65 - 30 = 35 years
This is crucial because compound interest works magic over longer time periods. The more years you have, the less you need to save today to reach your Coast FIRE number.
Step 4: Determine Your Investment Return Rate
Your expected annual investment return significantly impacts your Coast FIRE calculation. Be realistic and conservative in your estimates.
Historical averages: The S&P 500 has returned about 10% annually before inflation (1926-2024), or roughly 7% after inflation. Most financial planners recommend using 6-8% for long-term projections.
Conservative approach: Use 6-7% to account for market volatility, fees, and taxes. It’s better to be pleasantly surprised than disappointed.
Example: We’ll use 7% annual return for our calculations, which is reasonable for a diversified stock portfolio.
Step 5: Calculate Your Coast FIRE Number
Now comes the main calculation. You need to work backwards from your retirement goal to find how much you need today.
Formula: Coast FIRE Number = Retirement Savings Needed / (1 + Return Rate)^Years
This formula uses the present value calculation to determine what amount today will grow to your target amount over the specified time period.
Example:
- Retirement Savings Needed: $1,250,000
- Return Rate: 7% (0.07)
- Years: 35
Coast FIRE Number = $1,250,000 / (1.07)^35 Coast FIRE Number = $1,250,000 / 10.68 Coast FIRE Number =
$117,000