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_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_grow

Variables

VariableDescriptionDefault
fire_numberFIRE Number (Target)(USD)1000000
current_ageCurrent Age(years)30
retirement_ageTarget Retirement Age(years)60
annual_returnExpected Annual Return(%)7
years_to_growDerived value= retirement_age - current_agecalculated

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.

fire_number = 1000000current_age = 30retirement_age = 60annual_return = 7
  1. 01Years to grow = 60 - 30 = 30
  2. 02Growth multiple = 1.07^30 = 7.6x
  3. 03Coast number = $1,000,000 / 7.6 = $131,367
  4. 04If you have $131K invested at 30, it grows to $1M by 60

Ready to run the numbers?

Open Coast FIRE Calculator