Calculate your Coast FIRE number today or the age when current retirement savings can grow to the full FIRE target without more contributions by retirement.
Coast FIRE Formula
Coast FIRE is reached when today's invested balance can grow to the full retirement target without additional contributions.
First calculate the retirement target:
FIRE\ Target=Annual\ Spending/Withdrawal\ Rate
Discount the target to today:
Coast\ Number=FIRE\ Target/(1+r)^n
To solve for the coast age:
n=ln(FIRE\ Target/Current\ Balance)/ln(1+r)
Variables:
- r is expected real annual return
- n is years of compounding
- the withdrawal rate is entered as a decimal
- amounts remain in today's purchasing power with a real return
Coast-number mode finds the amount needed today for a selected retirement age. Coast-age mode finds when the current balance becomes sufficient.
Reaching Coast FIRE funds the future retirement target under the assumptions, not current living expenses.
Lower returns, higher spending, or a lower withdrawal rate increase the requirement.
Coast FIRE Illustration
Amount needed today to grow to a $1.5 million target at a 5 percent real return.
| Years until retirement | Coast FIRE number |
|---|---|
| 10 | $920,870 |
| 20 | $565,334 |
| 30 | $347,066 |
| 35 | $271,032 |
| 40 | $213,068 |
Example Problems
Example 1: Find the Coast FIRE number.
A 35-year-old targeting $60,000 spending at a 4 percent withdrawal rate needs $1.5 million at age 65; discounting 30 years gives about $347,000 today.
Example 2: Find the coast age.
With $250,000 invested, solve the compound-growth equation for years to $1.5 million.
Frequently Asked Questions
Can contributions stop immediately?
The concept says they can under the assumptions, but continued saving creates margin for lower returns or changing goals.
Does the model include inflation?
Yes when spending is in today's dollars and the return is a real return.
What if the balance already exceeds the coast number?
The result shows a surplus that could support earlier retirement, more spending, or more conservative assumptions.
