Coast FIRE Assumptions: Return and Withdrawal Rates
Your Coast FIRE number can move sharply when return, inflation, fees, or withdrawal-rate assumptions change. Here’s how to choose a useful range.
A Coast FIRE formula can give you an answer to the dollar. That does not make the answer precise.
The result depends heavily on two assumptions: how fast your portfolio grows before retirement and how much of it you expect to spend each year after retirement. Small changes compound over decades, so a calculator can move from “not there yet” to “you have reached Coast FIRE” without any change to your actual finances.
The practical response is not to hunt for one perfect rate. It is to use a defensible range, keep every input in the same dollar convention, and see whether the decision still works under less favorable assumptions.
Start with the Ask Linc Coast FIRE calculator, then use this guide to choose the return and withdrawal-rate cases you want to compare.
The two assumptions doing most of the work
The classic Coast FIRE calculation has two stages:
FIRE number = annual retirement spending ÷ withdrawal rate
Coast FIRE number = FIRE number ÷ (1 + real return)years until retirement
The withdrawal rate determines the portfolio you want at retirement. The real return determines how much needs to be invested today to reach that target without future contributions.
Those inputs are not independent facts. They are planning choices shaped by your asset mix, fees, taxes, time horizon, spending flexibility, and tolerance for a plan that may require adjustment.
Use a real return when spending is in today’s dollars
Most people estimate retirement spending in today’s dollars because “$60,000 of annual spending” is easier to understand than an inflated number decades in the future. If your spending target is in today’s dollars, your growth assumption should be inflation-adjusted too.
Real return = (1 + nominal return) ÷ (1 + inflation) − 1
For example, a 7% nominal return and 3% inflation produce a real return of about 3.88%:
(1.07 ÷ 1.03) − 1 = 3.88%
Simply subtracting inflation gives a close shortcut at modest rates, but the formula above keeps the calculation internally consistent.
The avoidable mistake is mixing conventions—using a retirement spending target in today’s dollars with a nominal return, or inflating spending separately and then also using a real return. Pick one convention and use it throughout.
Why the return assumption moves the Coast FIRE number so much
Consider an illustrative case with a $1.5 million portfolio target, 30 years until retirement, and no future contributions:
| Assumed real return | Coast FIRE number today |
|---|---|
| 3% | About $618,000 |
| 4% | About $462,000 |
| 5% | About $347,000 |
The same retirement target produces Coast FIRE numbers more than $270,000 apart. That spread comes entirely from the assumed growth rate.
This is why “What return should I use?” is the wrong question if it expects one universal answer. A more useful question is: What range is plausible for the portfolio I actually plan to hold, after inflation and investment costs?
How to choose a return range
Start with the planned asset mix
A portfolio invested mostly in stocks should not use the same expected return as a portfolio holding mostly bonds and cash. If you expect to become more conservative as retirement approaches, a single rate applied for the entire period may overstate later growth.
Make the assumption consistent with the allocation you are willing to keep during bad markets—not the allocation you hope to tolerate when markets are calm.
Account for investment costs
Fund expenses, advisory fees, and other recurring costs reduce the return that reaches you. A calculator that uses a market return before fees but ignores what you pay is modeling a different portfolio.
Keep taxes separate and explicit
A pre-tax retirement account balance is not interchangeable with the same balance in a Roth or taxable account. If the calculator does not model account-level taxes, make sure your retirement spending or portfolio target leaves room for them. Do not quietly “solve” taxes by lowering the return unless you can explain what that adjustment represents.
Run more than one case
A three-case range is often more informative than one forecast:
- Lower-growth case: tests whether the plan still works if compounding is weaker than hoped.
- Planning case: reflects the allocation, costs, and inflation assumptions you consider reasonable.
- Higher-growth case: shows the upside, but should not be the only case supporting a major life decision.
The 3%, 4%, and 5% real returns in the table are an illustration, not a recommended range for every investor.
The withdrawal rate changes the target before compounding begins
A withdrawal rate turns annual retirement spending into a full FIRE number. For $60,000 of annual spending:
| Starting withdrawal rate | Full FIRE number | Coast number with 30 years and 4% real growth |
|---|---|---|
| 3.5% | About $1.71 million | About $529,000 |
| 4.0% | $1.50 million | About $462,000 |
| 4.5% | About $1.33 million | About $411,000 |
A lower withdrawal rate requires a larger portfolio because each invested dollar is expected to support less first-year spending. A higher rate lowers the target but leaves less room for poor returns, unexpected expenses, or a longer retirement.
The familiar 4% rule is a useful starting point, not a promise. It was built from historical withdrawal outcomes over defined time periods and portfolios. Your retirement length, allocation, fees, taxes, and ability to cut spending can all change what is reasonable. Ask Linc’s guide to the 4% rule for early retirement explains the tradeoff in more detail.
Do not test each assumption in isolation
A weak plan can look sturdy if you lower the expected return but pair it with an aggressive withdrawal rate, or if you raise retirement spending while assuming a later retirement date.
Test combinations that could plausibly occur together:
- Lower real returns and a lower starting withdrawal rate
- Higher retirement spending and an earlier retirement date
- Higher fees and a more conservative asset allocation
- Lower Social Security income and a longer bridge before benefits begin
The point is not to make every input pessimistic. It is to learn which assumptions the decision depends on.
Average returns are still not a stress test
Even a reasonable average return draws a smooth line that markets will not follow. The order of returns matters. A long weak stretch late in the coasting period can leave you near retirement with less time to recover, even if the full-period average eventually looks acceptable.
After calculating a range, use the Ask Linc retirement calculator with future contributions set to $0. Compare the same plan across historical market and inflation sequences. Our guide to retirement stress testing explains what to look for.
Signs your Coast FIRE assumptions may be too optimistic
- Your result works only with the highest return you tested.
- Your return does not match the asset allocation you intend to hold.
- You used a nominal return with spending in today’s dollars.
- You ignored recurring investment fees.
- Your withdrawal rate assumes a shorter retirement than you are planning.
- Your spending target excludes taxes, healthcare, or major irregular expenses.
- A one-year change in retirement timing breaks the plan.
None of these proves the plan will fail. They tell you where the forecast is fragile.
A better way to use the Coast FIRE number
Treat the result as a range and a decision boundary, not a certificate.
If your current portfolio is below every reasonable case, you probably have more saving to do. If it is above every case and the plan survives historical stress tests, you have stronger evidence that coasting is an option. If it sits between cases, the next decision may be to keep a smaller contribution, delay a change, or choose a reversible version of the plan.
That middle ground is often the most useful result. Coast FIRE does not require an all-or-nothing switch.
Coast FIRE assumptions FAQ
Should I use 7% for Coast FIRE?
Only if you are clear whether 7% is nominal or real and it is consistent with your portfolio, fees, and inflation assumption. A 7% nominal return with 3% inflation is about 3.88% real, not 7% real.
Should I use a 4% withdrawal rate?
It is a common starting point, but not a universal rule. Test rates above and below it, especially if you expect a retirement longer than 30 years or have little ability to reduce spending.
What inflation rate should I use?
Use a range rather than assuming inflation will be smooth. More importantly, make sure inflation and return are combined consistently so the entire calculation stays in today’s dollars or future dollars.
How often should I update the assumptions?
Review them after a major change in spending, retirement age, asset allocation, fees, guaranteed income, or tax situation—and before using the Coast FIRE result to change jobs or reduce saving.
Calculate a range, then test the decision
Use the Coast FIRE calculator to compare return and withdrawal-rate cases. Then move the most important scenarios into Ask Linc’s planning experience and ask the question you actually care about: not only “Did I reach the number?” but “What can I safely change now?”
This article is for educational purposes and is not individualized financial advice. Historical results do not guarantee future performance.
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