Coast FIRE Calculator Formula

Understand the math behind the coast fire calculator. Each variable explained with a worked example.

Use the Coast FIRE Calculator

Formulas Used

Coast FIRE Number

coast_fire_number = annual_return > 0 ? target_amount / pow(1 + annual_return / 100, years_to_retire) : target_amount

Surplus/Deficit

is_coast_fire = current_savings - (annual_return > 0 ? target_amount / pow(1 + annual_return / 100, years_to_retire) : target_amount)

Projected Value at Retirement

projected_value = current_savings * pow(1 + annual_return / 100, years_to_retire)

Years Until Coast FIRE

years_to_coast = annual_return > 0 ? max(log(target_amount / pow(1 + annual_return / 100, years_to_retire) / max(current_savings, 1)) / log(1 + annual_return / 100), 0) : 999

Variables

VariableDescriptionDefault
current_savingsCurrent Retirement Savings(USD)200000
target_amountRetirement Target(USD)1500000
annual_returnExpected Annual Return(%)7
retirement_ageTarget Retirement Age(years)65
current_ageCurrent Age(years)35
years_to_retireDerived value= max(retirement_age - current_age, 0)calculated

How It Works

Coast FIRE

Coast FIRE means your existing savings will compound to your retirement goal WITHOUT any additional contributions.

Coast FIRE Number

Coast Number = Target / (1 + Return)^Years

This is the present value of your retirement target. If your current savings exceed this number, you have reached Coast FIRE.

After Coast FIRE

Once you hit Coast FIRE, you only need to earn enough to cover current expenses. You can:

  • Work part-time
  • Take a lower-paying but more fulfilling job
  • Reduce work stress knowing retirement is secured

    • Worked Example

    $200,000 saved at age 35, targeting $1,500,000 at age 65, 7% return.

    current_savings = 200000target_amount = 1500000annual_return = 7retirement_age = 65current_age = 35
    1. 01Coast FIRE number = $1,500,000 / (1.07)^30 = $197,100
    2. 02Current savings ($200,000) > Coast number ($197,100)
    3. 03Surplus = $200,000 - $197,100 = $2,900
    4. 04You have reached Coast FIRE -- existing savings will compound to $1.52M

    Frequently Asked Questions

    What is the difference between Coast FIRE and regular FIRE?

    Regular FIRE means you have enough to retire now. Coast FIRE means you have enough that compounding will get you there by a normal retirement age. You still need to work to cover current expenses, but you no longer need to save.

    What return rate should I use?

    A 7% real return (after inflation) is reasonable for a stock-heavy portfolio. For a more conservative estimate, use 5-6%. The higher the return assumption, the lower your Coast FIRE number appears.

    Can I coast with all my money in the market?

    Coast FIRE assumes your investments continue growing, so keeping most in stocks makes sense since you have a long time horizon. As you approach retirement age, gradually shift to bonds per your asset allocation plan.

    Ready to run the numbers?

    Open Coast FIRE Calculator