The One Number That Changes Everything
Your Coast FIRE number is the amount of money you need invested right now so that compound growth alone will fund your retirement — without saving another dollar. Once you know it, every financial decision gets simpler.
The good news: the math isn't complicated. The bad news: most online explanations make it harder than it needs to be. Let's fix that.
The Coast FIRE Formula
Here's the formula in plain English:
Coast FIRE Number = Retirement Target ÷ (1 + Real Return Rate) ^ Years Until Retirement
And broken down:
- Retirement Target = Annual retirement spending × 25 (the 4% rule)
- Real Return Rate = Investment return rate minus inflation rate
- Years Until Retirement = Retirement age minus your current age
That's it. Three inputs, one division, one exponent. Let's walk through it step by step.
Step 1: Determine Your Retirement Target
Your retirement target is how much total money you need invested when you actually retire. The standard approach uses the 4% rule (also called the 25x rule):
Retirement Target = Annual Expenses in Retirement × 25
| Annual Retirement Spending | Retirement Target |
|---|---|
| $30,000 | $750,000 |
| $40,000 | $1,000,000 |
| $50,000 | $1,250,000 |
| $60,000 | $1,500,000 |
| $80,000 | $2,000,000 |
How to estimate retirement spending:
- Start with your current annual spending
- Subtract costs that disappear in retirement (commuting, work clothes, retirement contributions)
- Add costs that may increase (healthcare, travel, hobbies)
- Most people land at 70-85% of their current spending
Common mistake #1: Using your current income instead of expenses. If you earn $100,000 but spend $50,000, your retirement target is based on $50,000 — not $100,000.
Step 2: Choose Your Real Return Rate
The "real" return rate is your investment return after inflation. This is critical because we want our Coast FIRE number in today's purchasing power.
Real Return Rate = (1 + Nominal Return) ÷ (1 + Inflation Rate) - 1
Or the simplified approximation: Nominal Return - Inflation Rate
| Scenario | Nominal Return | Inflation | Real Return |
|---|---|---|---|
| Conservative | 6% | 3.5% | ~2.4% |
| Moderate | 7% | 3% | ~3.88% |
| Aggressive | 10% | 2.5% | ~7.3% |
Which should you use?
- Moderate (7% / 3%) is the most commonly used and aligns with long-term U.S. stock market averages. This is what most FIRE calculators default to.
- Conservative (6% / 3.5%) adds a safety margin. Use this if you're risk-averse or want to be extra confident.
- Aggressive (10% / 2.5%) assumes a heavily equity-weighted portfolio with lower-than-average inflation. Only use this if you have high risk tolerance and a long time horizon.
Common mistake #2: Using nominal returns (7-10%) without adjusting for inflation. This makes your Coast FIRE number look artificially low and could leave you short in retirement.
Step 3: Calculate Years Until Retirement
This one's simple:
Years = Retirement Age - Current Age
Most Coast FIRE calculations use age 65 as the default retirement age, but you can use any age. Earlier retirement means fewer years of compounding, which means a higher Coast FIRE number.
| Current Age | Retire at 60 | Retire at 65 | Retire at 67 |
|---|---|---|---|
| 25 | 35 years | 40 years | 42 years |
| 30 | 30 years | 35 years | 37 years |
| 35 | 25 years | 30 years | 32 years |
| 40 | 20 years | 25 years | 27 years |
Putting It Together: Real Examples
Example 1: Alex, Age 28
Situation: Software developer earning $95,000. Spends about $45,000/year. Wants to retire at 62.
Step 1 — Retirement Target: $45,000 × 25 = $1,125,000
Step 2 — Real Return Rate: Using moderate assumptions: 3.88%
Step 3 — Years: 62 - 28 = 34 years
Calculation: $1,125,000 ÷ (1.0388)^34 = $1,125,000 ÷ 3.693 = $304,614
Result: Alex needs about $305,000 invested today to reach Coast FIRE. If Alex has $180,000 invested now, the gap is $125,000 — roughly 2-3 more years of aggressive saving.
Example 2: Maria, Age 35
Situation: Marketing manager earning $75,000. Spends $38,000/year. Plans to retire at 65.
Step 1 — Retirement Target: $38,000 × 25 = $950,000
Step 2 — Real Return Rate: Using moderate assumptions: 3.88%
Step 3 — Years: 65 - 35 = 30 years
Calculation: $950,000 ÷ (1.0388)^30 = $950,000 ÷ 3.141 = $302,388
Result: Maria needs about $302,000. If she's been saving in her 401(k) since age 25 with an employer match, she may already be close. Time to check those balances.
Example 3: David & Sarah, Both Age 42
Situation: Combined income $130,000. Household spending $65,000/year. Want to retire at 67.
Step 1 — Retirement Target: $65,000 × 25 = $1,625,000
Step 2 — Real Return Rate: Using moderate assumptions: 3.88%
Step 3 — Years: 67 - 42 = 25 years
Calculation: $1,625,000 ÷ (1.0388)^25 = $1,625,000 ÷ 2.577 = $630,578
Result: They need about $631,000 combined. If they have $450,000 across their retirement accounts, they need approximately $181,000 more — which at $3,000/month in contributions (with returns) could take about 4 years.
Example 4: Priya, Age 25
Situation: Just started her career at $55,000. Frugal lifestyle, spends $28,000/year. Wants to retire at 60.
Step 1 — Retirement Target: $28,000 × 25 = $700,000
Step 2 — Real Return Rate: Using moderate assumptions: 3.88%
Step 3 — Years: 60 - 25 = 35 years
Calculation: $700,000 ÷ (1.0388)^35 = $700,000 ÷ 3.837 = $182,434
Result: Priya needs just $182,000. At 25 with 35 years of compounding, even a modest savings target becomes powerful. Saving $1,500/month, she could reach Coast FIRE by age 33.
How Different Variables Change Your Number
Understanding the sensitivity of each variable helps you make better decisions:
Impact of Age (Starting Point)
For $40,000/year retirement spending at 65, moderate returns:
| Current Age | Coast FIRE Number | Difference from Age 25 |
|---|---|---|
| 25 | $222,000 | — |
| 30 | $270,000 | +$48,000 |
| 35 | $329,000 | +$107,000 |
| 40 | $401,000 | +$179,000 |
| 45 | $489,000 | +$267,000 |
Every 5 years you wait costs roughly $50,000-$90,000 more in required savings.
Impact of Spending Level
For a 30-year-old retiring at 65, moderate returns:
| Annual Spending | Coast FIRE Number |
|---|---|
| $30,000 | $203,000 |
| $40,000 | $270,000 |
| $50,000 | $338,000 |
| $60,000 | $406,000 |
| $80,000 | $541,000 |
Each additional $10,000 in annual spending adds roughly $67,000 to your Coast FIRE number.
Impact of Return Assumptions
For a 30-year-old with $40K spending, retiring at 65:
| Scenario | Coast FIRE Number |
|---|---|
| Conservative (2.4% real) | $567,000 |
| Moderate (3.88% real) | $270,000 |
| Aggressive (7.3% real) | $79,000 |
The difference between conservative and aggressive is 7x. This is why your return assumption matters more than almost any other variable.
Common Mistakes to Avoid
Mistake 1: Ignoring Inflation
If you use 7% returns without subtracting inflation, your number looks great — but it's in nominal dollars. When you actually retire, your purchasing power will be far less than you calculated.
Fix: Always use real (inflation-adjusted) returns. Our calculator handles this automatically.
Mistake 2: Forgetting About Healthcare
If you plan to coast (work a lower-paying job) before traditional retirement age, employer healthcare may not cover you. Self-funded health insurance can add $5,000-$15,000/year to your expenses.
Fix: Add estimated healthcare costs to your retirement spending estimate, especially if you plan to leave corporate employment.
Mistake 3: Not Accounting for Taxes
The 4% rule withdrawal is pre-tax for traditional retirement accounts. If your $40,000 annual spending comes from a traditional 401(k), you'll need to withdraw more to cover taxes.
Fix: Either gross up your spending estimate by 15-25% for taxes, or plan to use a Roth ladder conversion strategy.
Mistake 4: Using Today's Expenses Without Adjustment
Your spending at 65 won't be identical to today. Kids will be grown, mortgage may be paid off, but healthcare and hobbies may cost more.
Fix: Project your actual retirement lifestyle costs rather than using your current spending as-is.
Mistake 5: Treating Coast FIRE as Guaranteed
Market returns are averages, not guarantees. Your investments could underperform for a decade — look at 2000-2010 for U.S. stocks.
Fix: Use moderate or conservative assumptions. Check your progress annually. If you're behind, consider contributing a small amount to close the gap.
Beyond the Basic Formula: Advanced Considerations
Social Security
The standard Coast FIRE calculation ignores Social Security entirely. If you do receive Social Security benefits (even reduced ones), your actual retirement need is lower. Think of it as a safety margin.
Multiple Income Sources
If you have rental income, a pension, or other reliable passive income, subtract that annual amount from your retirement spending before calculating.
Example: $40,000 spending - $12,000 rental income = $28,000 effective spending. Your retirement target drops from $1,000,000 to $700,000.
Sequence of Returns Risk
Coast FIRE assumes average returns over time. But a market crash early in your coasting period hurts more than one near the end. Consider maintaining a 6-12 month emergency fund beyond your invested Coast FIRE number.
Try It Yourself
Skip the spreadsheet. Our Coast FIRE Calculator lets you plug in your exact numbers and see results instantly — including sensitivity analysis across conservative, moderate, and aggressive scenarios.
Want to work backward from a target age? The Reverse Calculator shows exactly how much you need to save per month to hit Coast FIRE by any age.
And if you're considering part-time work in your coasting phase, the Barista FIRE Simulator shows how even modest income can dramatically lower your savings target.
Frequently Asked Questions
Should I include my home equity in the calculation? No. Coast FIRE is about invested, liquid assets that generate returns. Your home is shelter, not an investment portfolio — unless you plan to sell and downsize.
What about 529 plans or HSAs? These don't count toward your Coast FIRE number because they're earmarked for specific purposes (education, healthcare). Keep them separate.
I have a pension. How does that change things? Subtract your expected annual pension income from your retirement spending. If your pension covers $20,000/year and you need $40,000, your effective need is $20,000/year — and your retirement target drops to $500,000.
What if inflation is higher than expected? Use conservative assumptions (higher inflation, lower real returns). The difference between 3% and 4% inflation over 30 years is significant. Our calculator lets you adjust both return and inflation rates independently.
How often should I recalculate? Once a year is plenty. Check your actual portfolio value against your Coast FIRE target. If you've crossed it — congratulations. If not, adjust your savings plan accordingly.