Calculate your Coast FIRE number: how much you need invested today to reach your retirement target with no more contributions, or the age you reach FIRE.
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:
To solve for the age the current balance reaches the full FIRE target:
n=ln(FIRE\ Target/Current\ Balance)/ln(1+r)
Variables:
- r is expected real annual return
- n is years of compounding
- the formula uses withdrawal rate and real return as decimals; the calculator inputs use percentages
- amounts remain in today's purchasing power with a real return
Coast-number mode finds the amount needed today for a selected retirement age. Target-age mode finds when today’s balance reaches the full FIRE target, assuming contributions stop today; it is not a future date at which contributions can first stop.
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 | $271,935 |
| 40 | $213,069 |
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 age the balance reaches the full FIRE target.
At age 35 with $250,000 invested and a constant 5% real return, $1.5 million is reached after about 36.72 years, at age 71.72, with no further contributions.
Frequently Asked Questions
Can contributions stop immediately?
The coast-number calculation assumes contributions stop once the required amount is invested. Future returns, spending and withdrawal sustainability are uncertain; reaching the modeled number does not guarantee that retirement is funded.
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 relative to the selected assumptions. Recalculate any different retirement age, spending or return scenario; a surplus does not itself establish that earlier retirement is affordable.
