Coast FIRE Calculator Formula
Understand the math behind the coast fire calculator. Each variable explained with a worked example.
Formulas Used
Coast FIRE Number
coast_number = fire_number / pow(1 + annual_return / 100, years_to_grow)Growth Multiple
growth_multiple = pow(1 + annual_return / 100, years_to_grow)Years for Compound Growth
years_growth = years_to_growVariables
| Variable | Description | Default |
|---|---|---|
fire_number | FIRE Number (Target)(USD) | 1000000 |
current_age | Current Age(years) | 30 |
retirement_age | Target Retirement Age(years) | 60 |
annual_return | Expected Annual Return(%) | 7 |
years_to_grow | Derived value= retirement_age - current_age | calculated |
How It Works
Coast FIRE
Coast FIRE means you have saved enough that compound growth alone will reach your retirement goal -- you can "coast" without additional savings.
Coast Number = FIRE Number / (1 + return)^years
Once you reach your Coast number, you only need to earn enough to cover current expenses, not save for retirement.
Worked Example
$1M FIRE target, age 30, retire at 60, 7% return.
- 01Years to grow = 60 - 30 = 30
- 02Growth multiple = 1.07^30 = 7.6x
- 03Coast number = $1,000,000 / 7.6 = $131,367
- 04If you have $131K invested at 30, it grows to $1M by 60
Frequently Asked Questions
What is Coast FIRE?
Coast FIRE means having enough invested that compound growth alone will fund your retirement. You still need to work to cover living expenses, but you no longer need to save.
Why is Coast FIRE popular?
It allows people to take lower-stress or more fulfilling jobs without worrying about savings rate. Once you hit your coast number, you just need to cover expenses.
How reliable is the 7% return assumption?
The US stock market has historically returned about 10% nominal (7% real). Using 7% is reasonable but not guaranteed. Consider running scenarios at 5-8%.
Ready to run the numbers?
Open Coast FIRE Calculator