Coast FIRE Calculator With Inflation: The Formula and a Worked Example

October 16, 2026 · Personal money

Your Coast FIRE number is the amount you need invested today so that, with no further contributions, it grows into your full FIRE number by the age you want to stop work. The formula is the FIRE number divided by (1 + real return) raised to the years left. In the illustrative example below - a $1,200,000 FIRE number, 25 years to go and an assumed 3.88% return after inflation - the Coast FIRE number is $462,932. The part that trips up a coast FIRE calculator is inflation: discount at the return before inflation and the answer drops to $221,099, less than half.

Step one: the FIRE number

A FIRE calculator starts with one division:

FIRE number = yearly spending / withdrawal rate

The withdrawal rate is the share of the portfolio you plan to take out each year. It is your choice, and this post does not tell you which rate is safe. At 4%, the target is 25 times a year’s spending. With $48,000 a year ($4,000 a month), that is $1,200,000. Each $100 a month you plan to spend adds $30,000 to it, at the same rate.

Spending here is in today’s dollars. That choice decides everything after it.

Step two: the Coast FIRE formula

Coast FIRE number = FIRE number / (1 + real return) ^ (stop age - current age)

If you already have that much invested, you could stop adding to it and still arrive at the full number on time. You would keep working to cover your spending until then, but your savings would no longer need to grow from contributions.

Real return = (1 + return) / (1 + inflation) - 1

Worked example (illustrative)

Every input below is an assumption chosen for the example, not a forecast. The return in particular is a guess about the future, and the answer moves a lot with it.

1. FIRE number: $48,000 / 4% = $1,200,000.
2. Real return: (1 + 7%) / (1 + 3%) - 1 = 3.88%.
3. Growth over 25 years at that rate multiplies money by about 2.59.
4. Coast FIRE number: $1,200,000 / (1 + 3.88%) ^ 25 = $462,932.

The example has $150,000 invested, so it is $312,932 short of coasting. Left alone at the assumed return, that $150,000 would not reach $1,200,000 until about age 90.

The same person, still adding $30,000 a year, reaches the full number in 19.9 years - about age 55. So in this example, continuing to save gets there well before the planned stop age; coasting from today does not.

Coast FIRE calculator with inflation: the mistake that halves the answer

The target is in today’s dollars. If you then grow money toward it at the 7% return before inflation, you are comparing future dollars with today’s dollars. The math runs fine and the answer is wrong:

Using the return before inflation understates the Coast number by $241,833, or 52%. In this example, it would tell the person they are only $71,099 short, when the real gap is $312,932.

The shortcut of subtracting inflation from the return is much closer, but it still understates the number by $12,792, because the exact real return, 3.88%, is slightly below the shortcut’s 4%. The fix is to pick one unit and stay in it: today’s dollars and the real return, or future dollars for both the target and the growth. Mixing them is the error.

How the stop age changes the number

Same assumptions, different stop ages:

Every extra year of growth divides the target by another (1 + 3.88%). That is why a later stop age lowers the Coast number, and why someone who starts early can coast on a smaller balance.

How the assumed return changes it

Same 25 years and 3% inflation, different return assumptions:

Between a 5% and an 8% assumed return, the Coast number changes by a factor of about 2.0. This is the non-obvious part of any Coast FIRE result: it is an output of a guess. A single figure from a calculator hides that, so run more than one return and look at the spread before you decide to stop saving.

FIRE calculator with Social Security or a pension

Other income in retirement lowers what the portfolio has to cover:

FIRE number = (yearly spending - other income) / withdrawal rate

Suppose, for illustration only, $18,000 a year in today’s dollars from a pension or Social Security. If it starts when you stop work, the target falls to $750,000 and the Coast number to $289,333, a drop of $173,600. Is the example’s $150,000 enough to coast then? No. Use your own benefit estimate here; this post makes no claim about what any benefit will be.

If the income starts later - say 7 years after you stop - the portfolio has to cover the full spending until then. One way to price that is to add the value, at the stop date, of the missing $18,000 a year for 7 years, discounted at the real return: $112,717 (a little less than the plain $126,000 sum, because the money waiting to be spent keeps growing). The target becomes $862,717 and the Coast number $332,816, between the two cases above.

How these were calculated

All figures are in today’s dollars. The real return is (1 + return) / (1 + inflation) - 1. The Coast number is the FIRE number discounted at the real return for each year to the stop age, computed by a year-by-year loop and by the closed form; years to FI assume contributions at the end of each year, found by year-by-year simulation. Each figure is computed by the code behind the pack below and again by independent code written for this page; the page only builds if they agree to the cent. Returns, inflation, spending and income are illustrative assumptions, not forecasts, and steady returns leave out the order of good and bad years, which matters in practice. This is arithmetic, not financial, tax or investment advice.

If you want to run this on your own numbers, the pack below does the FIRE number, the real return, years to FI, Coast FIRE, withdrawal-rate sensitivity, the value of a spending cut, and a couples version. It has no separate income line, so enter spending net of any pension or benefit, as in the formula above.