Coast FIRE Formula: FI ÷ (1+r)^n Explained + Lookup Table (2026)

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The Formula at a Glance

Coast FIRE Number = FI Number ÷ (1 + r)ⁿ

Where r = expected return (7% typical) and n = years until retirement

Example: 30-year-old targeting $1.5M at 60 → needs $197,000 today. Then compound interest does the rest.

If you have read about what Coast FIRE is, you know the concept: save enough that compound interest grows your money to retirement without additional contributions.

But how do you actually calculate that number? This post breaks down the formula, explains each variable, and provides examples so you can calculate your own.

The Coast FIRE Formula

The formula comes from rearranging the compound interest equation. Instead of asking "how much will my money grow to?", we ask "how much do I need today to reach my goal?"

COAST FIRE FORMULA

Coast FIRE Number = FI Number ÷ (1 + r)ⁿ

FI Number = Your target retirement portfolio

r = Expected annual return (as a decimal)

n = Years until retirement

This is the present value formula from finance. It calculates how much a future amount is worth today, given a growth rate.

Variables Explained

FI Number (Target Retirement Portfolio)

This is how much you need invested to retire. The standard approach uses the 4% rule: multiply your expected annual retirement expenses by 25.

FI Number = Annual Expenses × 25

$40,000/year → $1,000,000
$50,000/year → $1,250,000
$60,000/year → $1,500,000
$80,000/year → $2,000,000
$100,000/year → $2,500,000

Use our FIRE Number Calculator if you need help with this step.

r (Expected Annual Return)

This is your assumed investment growth rate. Common choices:

7% - Historical stock market average (inflation-adjusted)
6% - More conservative estimate
5% - Very conservative / bond-heavy portfolio

Use 7% for a stock-heavy portfolio. Use 5-6% if you want extra safety margin or plan to shift to bonds over time.

Important: These should be real (inflation-adjusted) returns. If you use nominal returns (like 10%), you also need to inflate your FI number to future dollars.

n (Years Until Retirement)

The time between now and when you plan to retire. This variable has the biggest impact because it is an exponent - small changes create big differences.

Same FI goal ($1.5M), same return (7%), different timelines:

40 years → $100,000 needed today
30 years → $197,000 needed today
20 years → $388,000 needed today
10 years → $763,000 needed today

This is why starting young matters. The longer your timeline, the less you need to save.

Step-by-Step Calculation

Here is how to calculate your Coast FIRE number by hand:

Example: You are 32, want to retire at 60, expect to spend $50,000/year

Step 1: Calculate your FI Number

$50,000 × 25 = $1,250,000

Step 2: Determine years until retirement

60 - 32 = 28 years

Step 3: Choose your return rate

Using 7% = 0.07

Step 4: Calculate (1 + r)^n

(1.07)^28 = 6.649

Step 5: Divide FI Number by result

$1,250,000 ÷ 6.649 = $187,960

Coast FIRE Number: $187,960

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Worked Examples

Let us look at three different scenarios to see how the formula plays out.

Example 1: The Early Starter

Age: 25
Target retirement: 60
Years: 35
Annual expenses: $40,000
FI Number: $1,000,000
Return rate: 7%

$1,000,000 ÷ (1.07)^35 = $1,000,000 ÷ 10.68

Coast FIRE Number: $93,663

Save $94K by 25, never save for retirement again.

Example 2: The Mid-Career Professional

Age: 35
Target retirement: 60
Years: 25
Annual expenses: $60,000
FI Number: $1,500,000
Return rate: 7%

$1,500,000 ÷ (1.07)^25 = $1,500,000 ÷ 5.43

Coast FIRE Number: $276,342

Hit $276K by 35 to coast to retirement.

Example 3: The Late Starter (Conservative)

Age: 45
Target retirement: 65
Years: 20
Annual expenses: $50,000
FI Number: $1,250,000
Return rate: 6% (conservative)

$1,250,000 ÷ (1.06)^20 = $1,250,000 ÷ 3.21

Coast FIRE Number: $389,408

Higher number due to shorter timeline and conservative rate.

Coast FIRE Reference Table

Quick reference for common scenarios. All assume 7% real returns and retirement at age 60.

Current Age	Years to 60	$1M FI Goal	$1.5M FI Goal	$2M FI Goal
25	35	$94K	$140K	$187K
30	30	$131K	$197K	$263K
35	25	$184K	$276K	$368K
40	20	$258K	$388K	$517K
45	15	$362K	$544K	$725K
50	10	$508K	$763K	$1,017K

Note: These are approximations. Use the Coast FIRE calculator for exact numbers based on your situation.

Adjusting Your Assumptions

The formula is only as good as your inputs. Here is how to think about adjustments:

If you want more safety margin

Use 5-6% returns instead of 7%
Use 3.5% withdrawal rate (multiply expenses by 28.5 instead of 25)
Add 10-20% buffer to your FI number

If your situation changes

Recalculate when your expected expenses change
Update if your target retirement age shifts
Factor in Social Security if you are counting on it (reduces FI number needed)

Common mistakes to avoid

Mixing real and nominal returns (be consistent)
Forgetting healthcare costs in retirement expenses
Using overly optimistic return assumptions (10%+ is too aggressive)
Not updating the calculation as you age

Calculate your exact Coast FIRE number

Plug in your real numbers and see where you stand.

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Frequently Asked Questions

What is the Coast FIRE formula?

The Coast FIRE formula is: Coast FIRE Number = FI Number ÷ (1 + r)^n. FI Number is your retirement goal (typically 25x annual expenses), r is your expected annual return rate (usually 7%), and n is years until retirement. This calculates how much you need invested today so compound interest grows it to your goal.

How much do I need for Coast FIRE at 30?

A 30-year-old targeting retirement at 60 with $1.5M needs approximately $197,000 (at 7% returns). For a $1M goal, you need about $131,000. For a $2M goal, about $263,000. The key is the 30 years of compounding time.

What rate of return should I use in the Coast FIRE formula?

Most people use 5-7% for inflation-adjusted (real) returns. 7% is historically reasonable for a stock-heavy portfolio. Use 5-6% for extra safety margin or if you plan to hold more bonds. Never use 10%+ as that is overly optimistic.

Does the Coast FIRE formula account for inflation?

Yes, if you use inflation-adjusted returns. The standard 7% is already a "real" return (roughly 10% nominal minus 3% inflation). This means your Coast FIRE number is in today's dollars, making planning easier.

How do I calculate my FI number for the formula?

Your FI number is 25 times your expected annual expenses in retirement (based on the 4% rule). For example: $40,000/year = $1M, $50,000/year = $1.25M, $60,000/year = $1.5M, $80,000/year = $2M.

Why does age matter so much in the Coast FIRE formula?

Because time (n) is an exponent in the formula, not a multiplier. Each additional year of compounding dramatically reduces what you need today. A 25-year-old (35 years to 60) needs only $94K for a $1M goal, while a 45-year-old (15 years) needs $362K for the same goal.

Is Coast FIRE the same as Barista FIRE?

No. Coast FIRE means you have enough invested to stop saving for retirement but still work to cover living expenses. Barista FIRE means you have partial investments plus work part-time for income and benefits. Coast FIRE focuses on future retirement; Barista FIRE focuses on current lifestyle.