Calculating Coast FIRE comes down to one question:
The calculation itself is not complicated. The difficult part is making sure the inputs are consistent.
You first estimate the portfolio you want at retirement. Then you work backward to calculate how much would need to be invested today.
A Coast FIRE result can change substantially depending on your retirement spending, retirement age, withdrawal-rate assumption, return assumption, inflation treatment, and investment fees.
Step 1: Estimate Your Retirement Spending
Start with the annual spending you expect your portfolio to help cover in retirement.
For this example, we will use:
This should be a spending estimate, not automatically a percentage of your salary.
Income and spending can move very differently over time.
A person earning $150,000 may spend $65,000 per year, while another person earning the same amount may spend $100,000.
Your Coast FIRE calculation cares about the amount your future retirement portfolio needs to fund.
Typical categories include:
- • Housing
- • Food
- • Transportation
- • Healthcare
- • Insurance
- • Travel
- • Taxes
- • Hobbies
- • Family support
- • Other recurring expenses
The number does not have to be perfect, but it should represent the retirement lifestyle you actually want to model.
Step 2: Calculate Your Retirement Portfolio Target
Next, convert annual retirement spending into a portfolio target.
A commonly used planning formula is:
Suppose we use a 4% withdrawal-rate assumption.
With $60,000 of annual spending:
So the modeled retirement portfolio target is:
You may also see this described as multiplying annual spending by 25:
That works because: 1 ÷ 0.04 = 25
But the 4% figure is an assumption, not a rule built into Coast FIRE.
Different withdrawal assumptions produce different retirement targets.
| Withdrawal Rate | Retirement Target for $60,000 Spending |
|---|---|
| 3% | $2,000,000 |
| 3.5% | $1,714,286 |
| 4% (Baseline) | $1,500,000 |
| 4.5% | $1,333,333 |
| 5% | $1,200,000 |
Lower withdrawal assumptions create larger portfolio targets.
William Bengen's 1994 research examined historical retirement withdrawals and highlighted the importance of actual return sequences and inflation rather than simply relying on long-term averages.
Later research by Cooley, Hubbard, and Walz tested multiple withdrawal rates, retirement periods, and portfolio mixes. These studies provide historical evidence, not a guarantee that one withdrawal rate will work for everyone.
Step 3: Calculate the Years Until Retirement
Now determine how long your existing investments have to grow.
Suppose:
Then:
Your investments therefore have 30 years to compound.
Time has a major effect on Coast FIRE.
The SEC's Investor.gov explains compound interest as earning returns not only on the original principal but also on previously accumulated returns.
Its educational example shows that $100 earning 5% annually grows to more than $162 after 10 years and almost $340 after 25 years without another contribution.
Coast FIRE applies the same principle to a much larger retirement balance.
Step 4: Choose a Return Assumption
Now choose the investment return you want to model.
For this example, we will use:
A real return means the return is adjusted for inflation.
That matters because our $60,000 retirement-spending target is being expressed in today's purchasing power.
There is no universally correct Coast FIRE return assumption.
A better approach is to compare several scenarios rather than assume one future return will definitely occur.
For example:
- 3% real
- 4% real
- 5% real
- 6% real
- 7% real
Each produces a different Coast FIRE number.
Real Return vs Nominal Return
A common shortcut is:
For small percentages, this can be a useful approximation.
The more precise formula is:
For example:
8% nominal return and 3% inflation
does not equal exactly 5% real return.
The more precise result is about:
7% nominal return and 3% inflation
produces a real return of approximately:
Small annual differences can become meaningful over 20 or 30 years of compounding.
Step 5: Calculate Your Coast FIRE Number
We now have all the inputs.
| Input | Value |
|---|---|
| Current age | 35 |
| Retirement age | 65 |
| Years until retirement | 30 |
| Retirement spending | $60,000/year |
| Withdrawal-rate assumption | 4% |
| Retirement portfolio target | $1,500,000 |
| Real-return assumption | 5% |
Now use:
The result is approximately:
Under these assumptions, about $347,000 invested at age 35 would be mathematically projected to grow to about $1.5 million by age 65 without additional retirement contributions.
Check the Math Forward
A useful way to verify the Coast FIRE calculation is to reverse it.
Take the Coast FIRE number:
Grow it for 30 years at 5%:
The forward calculation should approximately reproduce the retirement target.
If it does not, the calculation may be mixing assumptions or dollars incorrectly.
Compare Your Current Investments With the Coast FIRE Number
Suppose you currently have:
Your Coast FIRE target is:
The current modeled gap is:
That means your current portfolio is about $47,066 below today's modeled Coast target.
But this does not necessarily mean you need to contribute exactly $47,066.
If you continue investing, your portfolio grows over time.
At the same time, your Coast FIRE target also changes.
Your Coast FIRE Target Moves as You Get Older
This is one of the most important parts of Coast FIRE calculations.
Today's Coast FIRE number is not necessarily the number you need five years from now.
Using the same:
- $1.5 million retirement target
- retirement age of 65
- 5% real return
the Coast FIRE number changes with age.
| Age | Years Until 65 | Coast FIRE Target |
|---|---|---|
| 35 | 30 | $347,066 |
| 36 | 29 | $364,419 |
| 37 | 28 | $382,640 |
| 38 | 27 | $401,772 |
| 39 | 26 | $421,861 |
| 40 | 25 | $442,954 |
The required amount gets larger because there is less time remaining for compound growth.
At age 35, the money has 30 years to grow.
At age 40, it has only 25.
Therefore, more money must already be invested at age 40 to reach the same target at age 65.
A Real Coast FIRE Question From 2026
A useful example appeared in the Coast FIRE community in August 2026.
A couple, both age 30, reported approximately:
- • $473,830 invested
- • $4,672 per month of ongoing investments
- • $71,983 of modeled annual retirement spending
- • 4% withdrawal-rate assumption
- • 4% return assumption
- • target retirement age of 50
Their calculated retirement target was around $1.8 million.
Their current Coast FIRE target was approximately: $821,000
The important question was:
If they expected to reach Coast FIRE several years later, shouldn't their Coast target at that future age be higher?
Yes.
At age 30, there are 20 years until age 50.
At age 36, there are only 14 years.
Using the same target and return assumptions:
The retirement target stayed the same.
The amount required today increased because the portfolio had fewer years left to compound.
To estimate when you will actually reach Coast FIRE, you need to project two moving values:
Your projected Coast date is where those two lines meet.
How to Calculate Your Future Coast FIRE Date
If you have not reached Coast FIRE yet, calculate each future year separately.
Project your portfolio
For each year:
- Start with the current investment balance
- Add planned contributions
- Apply the selected investment return
Recalculate the Coast target
Use:
Compare the two
Your modeled Coast FIRE date occurs when:
This is more accurate than freezing today's Coast number and waiting for your portfolio to reach it.
How Contributions Affect Coast FIRE
The Coast FIRE number itself assumes:
That is what makes it a Coast calculation.
Before reaching Coast FIRE, however, you can keep contributing.
Suppose your current portfolio is $300,000 and today's Coast target is $347,066.
The date when you reach Coast FIRE will depend on:
- • Current investments
- • Monthly contributions
- • Investment return
- • Remaining years until retirement
- • The rising Coast target
Contributions affect your path toward Coast FIRE.
They do not change what Coast FIRE means.
Which Assets Should Count?
A basic Coast FIRE calculation should generally include assets you actually expect to use for retirement.
Examples can include:
- ✓ 401(k)
- ✓ 403(b)
- ✓ TSP
- ✓ Traditional IRA
- ✓ Roth IRA
- ✓ Taxable investments intended for retirement
- ✓ Other investable retirement assets
Do not automatically use your entire net worth.
For example, home equity may be valuable, but it does not directly fund retirement spending unless your plan explicitly includes:
- Selling the home
- Downsizing
- Borrowing against the property
- Otherwise converting the equity into spendable assets
Emergency savings may also have a different purpose.
The important question is:
“Will this asset actually fund retirement under the scenario I am calculating?”
If not, it should not automatically be included.
Inflation: Today's CPI Is Not a 30-Year Forecast
Inflation matters because Coast FIRE projections can last decades.
The U.S. Bureau of Labor Statistics reported that consumer prices were 3.4% higher in July 2026 than one year earlier. Prices excluding food and energy increased 2.5% over the same period.
Those numbers describe what happened over the previous year.
They are not a prediction of inflation for the next 30 years.
A long-term Coast FIRE calculation should therefore distinguish between:
Two Consistent Ways to Handle Inflation
There are two main approaches.
Use:
plus:
• Real return: 5%
This is the approach used throughout this guide.
You can instead inflate retirement spending into the future.
For example:
Then use a nominal investment return.
Both methods can describe the same economic scenario if used consistently.
Problems occur when someone mixes:
- future inflated spending + real return
- or: today's spending + nominal return without adjusting for inflation.
How Much Does the Return Assumption Matter?
Using the same:
- $1.5 million retirement target
- and: 30-year growth period
produces these Coast FIRE numbers:
| Real Return | Coast FIRE Number |
|---|---|
| 3% | $617,980 |
| 4% | $462,478 |
| 5% (Baseline) | $347,066 |
| 6% | $261,165 |
| 7% | $197,051 |
The difference between the 3% and 7% scenarios is more than:
That is why a useful Coast FIRE calculator should make assumptions editable and show sensitivity rather than hide everything behind one default percentage.
Common Coast FIRE Calculation Mistakes
Using Today's Coast Number as a Future Target
Your Coast FIRE target generally increases as retirement gets closer because fewer compounding years remain. Recalculate it at each future age.
Mixing Real and Nominal Returns
Today's-dollar spending should normally be paired with real returns. Future nominal spending should normally be paired with nominal returns.
Treating One Return Rate as Certain
A 5%, 6%, or 7% return is an assumption. Compare multiple scenarios.
Estimating Spending From Salary Alone
Retirement needs depend on spending, not automatically on a percentage of current income.
Counting Assets Twice
Do not include the same retirement account in two different fields. Likewise, Social Security and pensions are generally future income streams rather than current investment balances.
Ignoring Fees
Investment fees reduce the return that actually compounds. If your return assumption is before fees, the model may overstate future growth.
How Accurate Is a Coast FIRE Calculation?
The arithmetic can be precise.
The future cannot.
A Coast FIRE calculation cannot know in advance:
- • Future market returns
- • Future inflation
- • Your eventual retirement spending
- • Tax-law changes
- • Healthcare costs
- • Pension changes
- • Social Security changes
- • Job interruptions
- • Large portfolio withdrawals
- • Investment fees that are not entered
That is why Coast FIRE is better viewed as a scenario model rather than a permanent certification.
The SEC's compound-interest calculator itself asks users to enter an estimated return rate and provides the option to model a range of rates.
That is a useful way to think about Coast FIRE as well.
Coast FIRE Calculation Checklist
Before relying on a result, check these inputs:
Does the number reflect the retirement lifestyle you actually want to model?
Is the assumption clearly visible?
Is the starting age correct?
Is the model using your intended retirement date?
Is it real or nominal?
Are today's dollars and future dollars handled consistently?
Are investment costs included or intentionally excluded?
Are you counting only assets intended to fund retirement?
Are pensions and Social Security starting at their actual modeled ages?
Is the calculator comparing your portfolio against a moving Coast target?
If these assumptions are clear, the result is much easier to understand.
Frequently Asked Questions
Which formula steps are used to calculate Coast FIRE?↓
Coast FIRE Number = Retirement Portfolio Target ÷ (1 + Real Return)^Years Until Retirement. The retirement portfolio target is commonly calculated from annual retirement spending and a selected withdrawal rate.
How do I calculate my retirement target?↓
One common model is: Annual Retirement Spending ÷ Withdrawal Rate. For example: $60,000 ÷ 4% = $1,500,000.
Should I use 7% for Coast FIRE?↓
There is no universally correct return assumption. A more useful approach is to calculate the result at several return rates and understand how sensitive your Coast number is to that input.
Why does my Coast FIRE number increase as I get older?↓
Because fewer years remain for compounding before retirement. The same future retirement target requires more money today when less time remains.
How do I calculate when I will reach Coast FIRE?↓
Project your investment portfolio forward while separately recalculating your Coast FIRE target at each future age. Your modeled Coast date occurs when your projected portfolio first equals or exceeds the Coast target for that age.
Should I subtract inflation from investment returns?↓
Subtracting inflation is an approximation. A more precise formula is: (1 + Nominal Return) ÷ (1 + Inflation) − 1.
Should home equity count toward Coast FIRE?↓
Not automatically. Include home equity only if the retirement model explicitly assumes that value will become available to fund retirement spending.
The Bottom Line
The Coast FIRE calculation has a simple structure:
Using the example in this guide:
- Current age: 35
- Retirement age: 65
- Annual spending: $60,000
- Withdrawal rate: 4%
- Real return: 5%
produces:
and:
But that number only has meaning together with its assumptions.
Change the retirement date, spending, withdrawal rate, inflation treatment, or return assumption and the Coast FIRE number changes too.
The more useful question is therefore not:
“What is the Coast FIRE number?”
It is:
“What is my Coast FIRE number under assumptions I understand and can change?”
Calculate Your Coast FIRE Number
Use the Coast FIRE Calculator to enter your current age, retirement age, current investments, retirement spending, return assumption, inflation assumption, and withdrawal rate.
Verified Sources & Methodology
Compound Interest Calculator
Consumer Price Index (CPI)
Safe Withdrawal Rate Study
Trinity Study (AAII 1998)
Community Discussions
Retirement Claiming Ages (62–70)
Consumer Expenditures 2024
2026 Federal Brackets