Why do Coast FIRE calculators disagree on my number?
Your Coast FIRE number is the amount you'd need invested today so that, without adding another dollar, growth alone carries it to your full retirement target by the age you want to stop. Calculators disagree because every input is an assumption, and many fill those in for you. Mixing up before- and after-inflation returns alone moves the answer by more than 2x.
The formula is short:
Coast number = (yearly spending ÷ withdrawal rate) ÷ (1 + return)years until retirement
Every piece of that formula is an assumption. The one people get wrong most often is whether the return is before or after inflation.
Every figure on this page is a made-up example, with the inputs stated so you can redo the math yourself. It's planning and education, not financial advice.
A worked example
Take a made-up 30-year-old with $200,000 invested. They want $80,000 a year of spending in retirement, in today's dollars, and they'd like the option to stop at 60. That's 30 years away.
At a 4% withdrawal rate, the full FIRE target is $80,000 ÷ 0.04 = $2,000,000. Assume 7% a year after inflation. The coast number is $2,000,000 ÷ 1.0730, which is $262,734.
Their $200,000 is short of that. They aren't coasting yet, at least not on these assumptions. Change the assumptions and the verdict flips, which is the whole problem.
The nominal vs real mistake
A nominal return is what your balance does in plain dollars. A real return is what's left after inflation, so it measures buying power. The long-run US stock figure people usually quote is about 10% a year nominal, which is roughly 7% real once you take out about 3% inflation. When people say "7%", they nearly always mean 7% real.
The confusion comes up constantly. People ask which return everyone actually uses, argue over whether 7% still needs inflation subtracted, or ask for help with math where the numbers don't line up.
The rule that settles it: the return and the target have to be in the same kind of dollars. Our $2,000,000 target came from today's spending, so it's in today's dollars. The return has to be real too. Or you can do everything in future dollars, as long as you inflate the target as well.
Here's what happens to the same example person (age 30, $200,000 invested, $2,000,000 target in today's dollars, retiring at 60) depending on how the return gets handled. Inflation is 3% a year throughout.
| How the return is handled | Return used | Coast number | Does $200,000 coast? |
|---|---|---|---|
| 10% nominal growth checked against the today's-dollars target (mixed up) | 10% | $114,617 | Yes, wrongly |
| 10% nominal, with the target inflated to 2056 dollars ($4,854,525) | 10% | $278,206 | No |
| 10% nominal converted to real: 1.10 ÷ 1.03 - 1 | 6.80% | $278,206 | No |
| 7% real treated as nominal, so inflation is removed a second time | 3.88% | $637,725 | No, by a long way |
The two middle rows are the correct ones, and they agree to the dollar. That's a good test of any calculator: doing it all in future dollars or all in today's dollars should give the same coast number.
The first row is the one that tells people they're done when they aren't. It grows the balance at 10% and compares the result to a target built from today's grocery bill.
Mixed up like that, $200,000 reaches $2,000,000 in about 24 years, at age 54. Handled properly at 6.80% real, it takes just over 35 years, so a little past 65. That's an 11-year error. The gap between $114,617 and $278,206 is exactly 1.0330, about 2.43, which is just 30 years of inflation compounding.
The last row is the overcorrection. Someone reads that 7% "doesn't include inflation", subtracts it again, and ends up at about 3.9% real. Every coast number then looks more than twice as far away as it really is.
Is it 7% or 6.8%?
Real return isn't quite nominal minus inflation. It's this:
real return = (1 + nominal) ÷ (1 + inflation) - 1
So 10% nominal with 3% inflation is 6.796% real, not 7%. Over 30 years that's not a rounding error. $200,000 grows to $1,522,451 at 7% and $1,437,785 at 6.796%, about $85,000 apart. Most people quoting "7% real" are using the shortcut. It's fine as a round number. Just don't treat it as exact.
The other assumptions hiding in the defaults
Return isn't the only input a calculator can quietly pick for you. Retirement age matters about as much, and some calculators pre-fill 65 where it's easy to miss. Here's the coast number for the same made-up person and the same $2,000,000 target, with the return already real:
| Retire at | 5% real | 6% real | 7% real |
|---|---|---|---|
| 55 (25 years) | $590,606 | $465,997 | $368,498 |
| 60 (30 years) | $462,755 | $348,220 | $262,734 |
| 65 (35 years) | $362,581 | $260,210 | $187,326 |
Every cell is a reasonable assumption, and the corners are more than 3x apart. Our person with $200,000 is past the line in only one of the nine: retiring at 65 at 7% real. Someone who accepted a default of 65 and someone who typed 55 are answering different questions, and neither calculator is broken.
The withdrawal rate moves things least, though it gets argued about most. Holding 6% real and retirement at 60, dropping from 4% to 3.5% raises the target to $2,285,714 and the coast number from $348,220 to $397,966. That's about 14% more, against the 2.4x swing from the nominal vs real mix-up.
How to check any Coast FIRE calculator
- Find out whether the return is before or after inflation. If the label doesn't say, check its help text, or enter the example above and see whether you get $262,734 at 7%.
- Check whether the target is in today's dollars or future dollars, and make sure it matches the return.
- Look for the retirement age field, including any default you never touched.
- Note the withdrawal rate. It matters less than the other three, but it should still be one you chose.
If two calculators still disagree after that, put the same four numbers into both. They should land within a few dollars of each other.
What this arithmetic leaves out
It uses one fixed average return, so there's no sequence-of-returns risk. Someone sitting exactly on their coast number who then takes a 30% drop in year two isn't on it any more, and this formula can't see that. A Monte Carlo simulation would, and it would give you a probability rather than a single age.
Taxes are ignored. The $80,000 is treated as spendable, but withdrawals from a Traditional 401(k) are taxed, so that money buys less than the same amount from a Roth or taxable account.
Retirement spending is held flat in real terms, which is usually where healthcare costs late in life break the plan. The 4% rule comes from one stretch of US history, not a law. And contributions are assumed to stop completely, which few people actually do (many coasters keep at least an employer match going), so that last one makes every number here a bit conservative.
Running it in Simura
Simura's retirement planner works in today's dollars. Its expected return field is labelled "after inflation", defaults to 7%, and warns that a figure like 10% is before inflation and will overstate your result. From your current balance and target, it shows the age you'd reach Coast FIRE with no further contributions. If your savings are in a Traditional account, it grosses the target up for tax on withdrawals, which the arithmetic on this page doesn't.
It has the same main limit as this page. It projects one average return, not a range of outcomes. And the withdrawal rate is fixed at 4% for now, so you can't test 3.5% in the app yet.