Quick Summary
How Coast FIRE works - the math behind letting compound growth finish the job, what your number looks like at different ages, and how to calculate when you can stop actively saving for retirement.
Coast FIRE answers a specific question: at what point have you saved enough that compound growth alone gets you to retirement - without contributing another dollar?
It is not about retiring now. It is about reaching a savings milestone where retirement is handled and you can redirect your income however you want.
The Coast FIRE Calculator shows your number based on current age, target retirement age, and expected expenses. No signup required.
The Formula
Coast FIRE starts with two pieces:
- Retirement target - annual expenses x 25 (the standard 4% rule math)
- Present value - what that target is worth today, discounted by expected growth
The formula: Coast FIRE Number = Retirement Target / (1 + r)^n
Where r is the expected annual return and n is years until retirement.
In plain terms: if you need $1,250,000 at age 65 and expect 7% annual returns, how much do you need today for compound growth to bridge the gap?
Worked Examples
Scenario: $50,000/year retirement spending, so target is $1,250,000. Assuming 7% annual growth.
| Current Age | Years to 65 | Coast FIRE Number |
|---|---|---|
| 25 | 40 | $83,500 |
| 30 | 35 | $117,000 |
| 35 | 30 | $164,000 |
| 40 | 25 | $230,000 |
| 45 | 20 | $323,000 |
| 50 | 15 | $453,000 |
At 25, roughly $83,500 invested could grow to $1.25 million over 40 years at 7%. At 45, you need nearly four times that amount because there are only 20 years of compounding left.
Why the Numbers Change So Fast
Compound growth is exponential. Each decade of compounding roughly doubles the money at 7%. Remove a decade and you lose half the final value.
A 25-year-old with $83,500 and a 35-year-old with $164,000 are both on track for the same endpoint. The difference - $80,500 - is the cost of ten fewer years of growth. That is the real price of waiting.
What Changes After Reaching Coast FIRE
Once your invested assets hit the Coast FIRE number, retirement savings are - mathematically - handled. This opens up options:
- Reduce hours - work part-time and cover current expenses only
- Switch careers - take a lower-paying job that is more meaningful
- Start something - pursue a business idea without risking retirement
- Take a gap - travel or take time off, then return to work
The key constraint: you still need enough income to pay for life right now. Coast FIRE eliminates the need to save for the future, not the need to earn in the present.
The Variables That Matter Most
Return assumptions. The difference between 6% and 8% is enormous over 30 years. At 6%, a 30-year-old needs about $145,000. At 8%, about $93,000. Running both gives you a realistic range.
| Growth Rate | Coast FIRE at Age 30 (for $1.25M at 65) |
|---|---|
| 5% | $227,000 |
| 6% | $173,000 |
| 7% | $117,000 |
| 8% | $93,000 |
Target retirement age. Moving from 65 to 60 cuts five years of growth and raises the number by roughly 40%. Moving to 55 raises it even more. Early retirement targets make Coast FIRE harder to reach.
Spending level. This is the same lever as in all FIRE calculations. Reducing expected retirement spending from $60,000 to $40,000 cuts the Coast FIRE number by a third.
Coast FIRE vs. Full FIRE
| Coast FIRE | Full FIRE | |
|---|---|---|
| Goal | Stop saving for retirement | Stop working entirely |
| Portfolio needed | Fraction of final target | Full 25x expenses |
| After reaching it | Keep earning for current expenses | Optional work only |
| Timeline | Reachable in 5-15 years for many | Often 15-25+ years |
| Risk | Market returns may differ from projections | Sequence of returns, inflation |
Some people find Coast FIRE more motivating because the milestone is closer. Others see it as a waypoint on the path to full financial independence.
Running the Numbers
The Coast FIRE Calculator lets you plug in your age, savings, expected retirement spending, and growth assumptions. The FIRE Calculator shows the full financial independence timeline. For deeper compound growth exploration, the Compound Interest Calculator helps visualize how different rates and timeframes affect outcomes.