How to Calculate Your Coast FIRE Number
The Complete Step-by-Step Guide
The Coast FIRE Formula
Coast FIRE Number = FV ÷ (1 + r)ⁿ
Future Value
Your target retirement number (Annual Expenses × 25)
Return Rate
Expected annual return (typically 5-7%, inflation-adjusted)
Years
Years until retirement (Retirement Age - Current Age)
4-Step Calculation Guide
Follow these steps to calculate your Coast FIRE number by hand:
Determine Your Target Retirement Number
Multiply your annual expenses by 25. This uses the 4% safe withdrawal rate rule.
Why 25x? Because $1.2M × 4% = $48,000/year in retirement income.
Choose Your Expected Return Rate
Select a rate based on your investment strategy. Always use inflation-adjusted returns.
⚠️ Always use inflation-adjusted returns, not nominal.
7%
Mostly stocks, long time horizon
6%
Balanced portfolio
5%
Risk-averse, shorter horizon
Calculate Years Until Retirement
Subtract your current age from your target retirement age.
Apply the Formula
Plug your values into the formula and calculate.
Complete Worked Example
Let's walk through a complete calculation:
Alex
32 years old
Expenses
$48,000/year
Retirement
Age 65
Return Rate
7% real return
Calculation
Alex needs $128,685 invested today to Coast FIRE. If Alex has this amount, they can stop saving for retirement and let compound interest do the rest.
Verify with Our CalculatorCoast FIRE Numbers by Age & Goal
Quick reference: How much you need based on your age and target.
| Current Age | Years to 65 | For $1M Goal | For $1.5M Goal | For $2M Goal |
|---|---|---|---|---|
| 25 | 40 years | $67,000 | $100,000 | $134,000 |
| 30 | 35 years | $94,000 | $141,000 | $188,000 |
| 35 | 30 years | $131,000 | $197,000 | $262,000 |
| 40 | 25 years | $184,000 | $276,000 | $368,000 |
| 45 | 20 years | $258,000 | $387,000 | $516,000 |
| 50 | 15 years | $362,000 | $543,000 | $724,000 |
$1M Goal
$67,000
$1.5M Goal
$100,000
$2M Goal
$134,000
$1M Goal
$94,000
$1.5M Goal
$141,000
$2M Goal
$188,000
$1M Goal
$131,000
$1.5M Goal
$197,000
$2M Goal
$262,000
$1M Goal
$184,000
$1.5M Goal
$276,000
$2M Goal
$368,000
$1M Goal
$258,000
$1.5M Goal
$387,000
$2M Goal
$516,000
$1M Goal
$362,000
$1.5M Goal
$543,000
$2M Goal
$724,000
Assumes 7% real return, retiring at age 65.
Find Your Exact NumberUnderstanding the Key Variables
Choosing Your Target Number (The 25x Rule)
The 25x rule comes from the 4% safe withdrawal rate. If you can withdraw 4% of your portfolio annually without running out of money, you need 25 times your annual expenses saved.
Based on the Trinity Study (1998) and Bengen's original 1994 research: Trinity Study , Bengen (1994)
What Return Rate Should You Use?
The S&P 500 has historically returned about 10% nominally, or roughly 7% after inflation. However, future returns are uncertain. Here's how to choose:
S&P 500 historical data (1926-2023): Investopedia
- 7% — Historically reasonable for mostly-stock portfolios
- 6% — More conservative, accounts for fees and lower-growth periods
- 5% — Very conservative, adds a safety margin
💡 The lower the rate you assume, the higher your Coast FIRE number—but also the more margin of safety you have.
How Retirement Age Affects Your Number
Each additional year until retirement significantly reduces your Coast FIRE number. Time is your greatest asset in compound growth.
Retiring at 65 vs 55 could cut your Coast FIRE number by 50% or more.
5 Common Calculation Mistakes to Avoid
Using nominal returns instead of real returns
❌ Overstates growth, leads to undersaving
✅ Use 7% real return, not 10% nominal
Wrong years calculation
❌ Using birth year or wrong formula
✅ n = Retirement Age - Current Age
Overestimating return rate
❌ 10%+ is unrealistic long-term
✅ Stick to 5-7% for conservatism
Ignoring inflation in target
❌ Target too low in future dollars
✅ Use today's dollars with real returns
Forgetting other retirement income
❌ Oversaving when pension/Social Security exists
✅ Subtract expected benefits from target
Special Scenarios
Calculating with Social Security
Social Security reduces how much you need from investments:
Expenses $48K - SS $18K = $30K gap → Target = $750K (not $1.2M!)
Calculate with PensionCalculating for Couples
Couples can calculate jointly or separately:
- Shared expenses mean lower per-person targets
- Combined savings accelerates reaching Coast FIRE
- Diversified careers reduce risk
Ready to Calculate?
Now that you understand the math, let our calculator do the heavy lifting.
Frequently Asked Questions
Historically, yes. The S&P 500 has averaged about 10% nominal or 7% real (after inflation) since 1926. However, future returns may differ. Using 5-6% is more conservative and gives you a safety buffer.
Use today's dollars with real (inflation-adjusted) returns. This automatically accounts for inflation. If your expenses are $40K today, use $40K in the formula with a 7% real return.
Compare your current investments to your Coast FIRE number. If you have $150K and your Coast number is $128K, you've already reached Coast FIRE! Use our calculator to check your status.
Earlier retirement means fewer compounding years and a higher Coast FIRE number. For example, retiring at 55 instead of 65 roughly doubles what you need today. Our calculator lets you adjust retirement age.
Technically yes, but it's not recommended. High-interest debt (credit cards) should be paid first. Low-interest debt (mortgage) is more nuanced—some prefer paying it off for peace of mind, others invest instead.
"The best time to start was yesterday. The second best time is now."
Start your Coast FIRE journey today 🚀