Coast FI Number: When You Can Stop Saving Early
There is a milestone most retirement calculators never show you: the point where you can stop contributing to retirement entirely and still arrive at full financial independence on schedule — just by leaving your existing balance alone. That milestone has a name: your coast FI number. This post explains what it means, how to calculate it yourself, and how to decide whether hitting it should change what you do with your paycheck right now.
What the coast FI number actually means
Standard retirement math asks: "How much do I need at age 65?" Coast FI asks a different question: "How much do I need right now so that compound growth gets me there, with zero additional contributions?"
The logic is straightforward. Money invested today has decades to grow. A dollar atundefineddoes more heavy lifting than a dollar at 55. At some point, your current balance is already "enough" to coast to a full retirement number — you just have to leave it alone and wait.
Once you cross that line, every dollar you earn stops being obligated to future-you. You can redirect it toward present-day quality of life: shorter hours, a career pivot, a sabbatical, or simply breathing room in your monthly budget. You are not retired, but you are no longer racing against time to fund retirement.
This is why coast FI sits somewhere between "still grinding" and full financial independence. It is the moment the runway is long enough that the plane will take off on its own.
The formula, plain and simple
To find your coast FI number, you need three inputs:
- Your full FI target — the portfolio size that would let you retire today (commonly 25× your annual spending, though your own number may differ based on your lifestyle and timeline).
- Your years until target retirement age — how many years compounding has to work.
- Your expected annual real return — growth after inflation. A conservative 5–6% real return is a reasonable planning assumption for a diversified portfolio.
The formula:
Coast FI Number = FI Target ÷ (1 + real return rate)^years
Worked example
Say your FI target is $1,500,000, you are 35, and you plan to retire at 60. That gives youundefinedyears of compounding. Using a 6% real return:
- (1.06)^25 = approximately 4.29
- Coast FI Number = $1,500,000 ÷ 4.29 = ~$349,650
If your current invested balance is $349,650 today, you could stop contributing entirely and — assuming 6% average real growth — arrive at $1,500,000 by age 60. Every dollar you add beyond that accelerates your timeline or gives you a buffer; it is no longer a hard requirement.
At 40, the same math withundefinedyears gives you: $1,500,000 ÷ (1.06)^20 = $1,500,000 ÷ 3.21 = ~$467,290. Waiting five years costs you roughly $117,000 in coast FI credit — which is why starting early matters so much.
How your coast FI number changes with age
| Age | Years to 60 | Coast FI Number (target: $1.5M, 6% real) |
|---|---|---|
| 30 | 30 | ~$261,000 |
| 35 | 25 | ~$350,000 |
| 40 | 20 | ~$467,000 |
| 45 | 15 | ~$625,000 |
| 50 | 10 | ~$838,000 |
| 55 | 5 | ~$1,121,000 |
Two things stand out. First, the coast FI number roughly doubles everyundefinedyears — a direct consequence of the rule of 72. Second, hitting coast FI atundefinedrequires less than a quarter of the final target. That gap is entirely filled by time and compounding, not your labor.
If your target retirement age is different, or you prefer a 5% real return assumption (more conservative), run your own version of the formula. The financial independence calculator on enoughmoneyplan lets you adjust these variables and see how your current balance stacks up against your coast FI threshold.
What to do once you hit coast FI
Reaching your coast FI number does not mean you must stop investing. It means you can — and that changes the decision from obligation to choice.
A few paths people take:
Redirect savings to present-day goals. Coast FI is popular among people who want to downshift careers, go part-time, or take a lower-paying job they actually enjoy. If retirement is funded, your current income only needs to cover current expenses.
Keep contributing but slow down. Some people hit coast FI and simply reduce their savings rate from 25% to 10%, using the freed-up cash to pay off a mortgage faster, fund a child's education, or rebuild a depleted emergency fund. Use the emergency fund calculator to make sure that base layer is solid before you redirect anything.
Use it as a confidence check. Even if you plan to keep saving aggressively, knowing you have already crossed the coast FI line removes a specific kind of financial anxiety. You are not behind. You have margin.
Barista FIRE variation. A close cousin of coast FI, barista FIRE means you stop full-time work but pick up part-time income that covers current expenses — letting your invested balance compound untouched. The coast FI number is the same calculation; the lifestyle choice is different.
The honest caveats
Coast FI math depends on assumptions that may not hold. Real returns fluctuate. Spending in retirement may be higher than expected (healthcare is the usual wildcard). Inflation can surprise. A 6% real return is not guaranteed.
Two ways to build in margin:
- Use a 5% real return instead of 6% when calculating your coast FI number. This raises the bar and gives you a buffer.
- Add 10–15% to your FI target before running the calculation, to account for unexpected late-career expenses.
Coast FI is a planning milestone, not a guarantee. Treat it as a strong signal, not a permission slip to stop thinking about money entirely.
It is also worth running a periodic financial check-in once a year to confirm your actual balance, updated return assumptions, and any changes to your target spending. Life changes — the number should update with it.
Thinking about career alongside money
One underrated benefit of understanding your coast FI number: it reframes career decisions. When you know retirement is funded, job changes become less financially fraught. If you are evaluating a lower-paying role you find more meaningful, mypassionjob.com can help you think through how passion and income intersect — a useful complement to the financial math you are doing here.
Key takeaways
- Your coast FI number is the invested balance you need today so compounding alone reaches your retirement target, with no further contributions.
- The formula: FI Target ÷ (1 + real return)^years.
- Hitting coast FI does not mean you stop saving — it means saving becomes optional, not obligatory.
- The earlier you hit it, the lower the number, because time does the heavy lifting.
- Use conservative return assumptions (5–5.5%) and add a buffer to your FI target to avoid planning on a razor's edge.
- Run an annual check-in to keep the calculation current as your life evolves.
If you want to see where you stand against your own coast FI threshold, the financial independence calculator on enoughmoneyplan lets you plug in your balance, target, and timeline to find out whether you are already coasting — or how close you are.