What Is the Return Rate in a Coast FIRE Calculator?
The investment return rate represents the annual growth assumed for the invested portfolio during the period between the current age and the selected retirement age.
For example, a Coast FIRE model might use:
On this site's current calculation engine, those inputs produce a modeled real return of approximately 3.79% after the calculator's fee and inflation adjustments.
That 3.79% is a calculated scenario result.
It is not a prediction that an investment portfolio will earn exactly 3.79% after inflation.
Is 7% a Realistic Coast FIRE Return?
Seven percent can be used as a calculator scenario, but the calculator cannot establish that 7% will occur in the future.
The important distinction is whether 7% means:
- nominal return;
- real return after inflation;
- return before or after fees;
- historical average;
- annualized historical result;
- or a future modeling assumption.
Those are different numbers.
This distinction is a major source of confusion in Coast FIRE discussions. A July 2026 r/coastFIRE thread asked whether the model should use 7% real growth, 5% real growth, or a range of outcomes. Other users described testing several rates instead of treating one percentage as certain.
Reddit is useful here for identifying the question people actually ask. It is not authority for selecting an investment-return assumption.
Historical Return Is Not the Same as a Future Return Assumption
The S&P 500, maintained by S&P Dow Jones Indices, is often used when discussing historical U.S. stock-market performance.
As of August 29, 2026, the S&P 500 Total Return Index showed annualized trailing returns of approximately:
| Period | S&P 500 Total Return |
|---|---|
| 1 year | 20.01% |
| 3 years | 21.87% |
| 5 years | 12.93% |
| 10 years | 15.43% |
These are historical results for specific trailing periods.
They do not mean a Coast FIRE portfolio will earn 15.43% per year for the next decade.
Historical Returns and Forward-Looking Estimates Can Be Very Different
Another way to see the distinction is to compare trailing market performance with a forward-looking model.
In its July 2026 Capital Markets Model update, Vanguard estimated a 4.2%–6.2% nominal annualized 10-year return range for U.S. equities, based on market conditions as of June 30, 2026. Vanguard explicitly states that these projections are hypothetical, can change over time, and are not guarantees of future results.
Compare that with the S&P 500's trailing 10-year total return of roughly 15.43%.
Those numbers are not contradictory.
They answer different questions:
Historical result over the previous 10 years.
One institution's forward-looking nominal model for the next 10 years.
A possible editable Coast FIRE calculator assumption.
A calculator should keep these categories separate rather than presenting historical performance as proof that one future return assumption is correct.
Nominal Return vs Real Return
One of the most important questions is whether a Coast FIRE calculator is asking for a nominal return or a real return.
Nominal return
Nominal return is investment growth before adjusting for inflation.
Real return
Real return represents growth after accounting for inflation.
Some calculators ask directly for a real return.
Others ask for nominal return and inflation separately.
For example, WalletBurst currently starts with a 7% nominal investment-return default and separately models inflation.
UngrindFi instead currently presents a 5% real-return default for its Coast FIRE calculator.
So comparing: “Calculator A uses 7%, while Calculator B uses 5%” does not tell us much until we know whether those percentages are nominal or real.
Why You Cannot Simply Compare Two Percentage Inputs
Consider these two hypothetical calculator settings:
It would be incorrect to conclude that Calculator A is automatically using the larger growth assumption simply because 7 is greater than 5.
Calculator A still has to adjust its nominal return for inflation and any modeled fees.
This is one reason Coast FIRE calculators can produce noticeably different results.
Recent r/coastFIRE discussions contain exactly this problem: people report substantially different Coast FIRE numbers from different calculators because one tool is using nominal growth plus inflation while another is effectively using a real return.
Average Return Is Not the Same as Annualized Return
Another common problem is using a simple average where compound growth is required.
Suppose an investment starts at $100.
The arithmetic average is: 0%
But the portfolio lost 25% of its original value.
That happens because investment returns compound.
FINRA gives a real calculation example where an investment's total three-year return is 25.7%. Simply dividing by three gives 8.57%, while the annualized return accounting for compounding is 7.792%.
For Coast FIRE calculations, this distinction matters because the Coast number depends on compound growth over many years.
How Much Does the Return Assumption Change the Coast FIRE Number?
Consider one consistent mathematical scenario:
- current age: 30
- retirement age: 65
- years until retirement: 35
- annual retirement spending: $50,000
- modeled retirement target: $1,250,000
- inflation assumption: 3%
- investment fees: 0.10%
Only the nominal return assumption changes.
| Nominal Return Scenario | Modeled Real Return | Modeled Coast FIRE Number |
|---|---|---|
| 5% | 1.84% | $659,281 |
| 6% | 2.82% | $472,993 |
| 7%Current Calculator Default | 3.79% | $340,403 |
| 8% | 4.76% | $245,732 |
| 9% | 5.73% | $177,925 |
Notice how large the difference becomes over 35 years.
At a 5% nominal assumption, the modeled Coast target is roughly:
$659,000
At a 9% nominal assumption:
$178,000
The difference exists because compound growth is applied over a long time horizon.
This is why the return assumption deserves to be visible and editable rather than hidden inside a calculator.
Does a Higher Return Assumption Mean a Better Coast FIRE Plan?
A higher number simply gives the mathematical model more future growth.
It does not make the scenario more accurate.
Likewise, a lower return assumption is not automatically more accurate.
The Coast FIRE calculation only reports what happens under the input entered.
This is why labels such as:
- conservative;
- aggressive;
- safe;
- realistic;
- recommended;
can be misleading when attached to return assumptions without additional context.
A percentage is first a model input.
What Happens During a Market Downturn?
A Coast FIRE calculator usually represents long-term growth with an annualized assumption.
Actual markets do not grow at one fixed percentage every year.
Returns can be:
- positive;
- negative;
- flat;
- or highly variable.
A 2025 r/coastFIRE discussion asked whether Coast FIRE math still works if markets experience a prolonged downturn and whether an 8%–10% historical average can simply be assumed going forward.
The mathematical answer is straightforward:
A fixed-rate Coast FIRE projection does not predict the sequence of actual yearly market returns.
It calculates a scenario using the annualized rate entered.
Should a Coast FIRE Calculator Use One Return or a Range?
A calculator can model either approach.
Shows the result produced by one selected assumption.
Shows how the Coast FIRE number changes across several assumptions.
For Coast FIRE, sensitivity analysis can answer:
What happens to the calculated Coast number if the return input changes?
It does not answer:
Which return will actually happen?
Why Do Coast FIRE Calculators Use Different Return Rates?
Different calculators may differ because they use different:
- default return assumptions;
- nominal versus real returns;
- inflation treatment;
- investment-fee treatment;
- compounding methods;
- asset assumptions;
- contribution timing;
- and time horizons.
For example:
WalletBurst currently uses a 7% nominal return default and separately subtracts inflation in its calculation.
UngrindFi currently uses a 5% real return default.
Neither percentage can be compared meaningfully without reading the methodology.
That is why two calculators can receive apparently similar personal inputs and still return very different Coast FIRE numbers.
Frequently Asked Questions
What return rate should I use for Coast FIRE?▼
Is 7% guaranteed for Coast FIRE?▼
Is 7% nominal or real?▼
Is 5% a better Coast FIRE return than 7%?▼
Why does a small return-rate change make such a big difference?▼
Can I use the historical S&P 500 return as my Coast FIRE return?▼
How do return assumptions cause Coast FIRE calculators to show different numbers?▼
The Bottom Line
A Coast FIRE return rate is an assumption used by a mathematical model.
It is not:
- a guaranteed market return;
- the same thing as a historical average;
- automatically the same as an S&P 500 return;
- or automatically comparable with another calculator's percentage.
The important question is therefore not simply:
“Is 5%, 7%, or 9% correct?”
It is:
“What does this percentage represent, and what result does the calculator produce when it changes?”
Research Sources
Used for compound-interest modeling, estimated interest-rate terminology, and the distinction between past performance and future results.
Used for annualized-return methodology and the distinction between a simple average and compounded annualized return.
Used for current historical S&P 500 Total Return data. As of August 29, 2026, the S&P 500 Total Return Index showed a 15.43% trailing 10-year annualized return. This is historical data, not a future forecast.
Used as an example of a forward-looking model that differs from trailing historical returns. Vanguard's June 30, 2026 model estimated 4.2%–6.2% nominal annualized returns for U.S. equities over the following decade and explicitly states that its projections are hypothetical and not guaranteed.