Coast FIRE Formula: How to Calculate Your Number by Hand

Published October 13, 2026

A 35-year-old who wants to retire at 65 on $60,000 a year does not need $1.5 million today. She needs about $197,000, invested, never touched again. That number feels like a typo the first time you see it. It is not. It is one division problem, and this post works it by hand so you can check every step.

The coast FIRE formula is a present-value calculation wearing casual clothes. It asks: what lump sum, invested today at some real return, grows into your full retirement number by the year you retire?

The formula:
FI Number = Annual Retirement Spending x 25
Coast FIRE Number = FI Number / (1 + real return)^{years until retirement}

The FI number comes from the 4% rule of the Trinity Study (Cooley, Hubbard, and Walz, 1998): a portfolio of 25 times your annual spending has historically supported 4% yearly withdrawals over long retirements. The second line discounts that future number back to today at your assumed real return. Real, meaning after inflation. The community default is 7%, roughly the S&P 500's long-run 10% nominal return minus 3% inflation. Hold that 7% in your head with a loose grip. It will matter more than anything else on this page.

How to calculate your number, step by step

Take the 35-year-old from the top. She plans to spend $60,000 a year in retirement and retire at 65.

Step one, the FI number: $60,000 x 25 = $1,500,000. Step two, the years of compounding: 65 minus 35 = 30. Step three, the growth divisor: 1.07 raised to the 30th power, which is 7.612. Her money multiplies 7.6 times over three decades without her adding a dollar. Step four: $1,500,000 / 7.612 = $197,051. Call it $197,000.

One more, with different inputs, because the shape of the answer changes with age. A 32-year-old spending $50,000 a year, retiring at 65: FI number $1,250,000, 33 years of compounding, 1.07^33 = 9.325, and $1,250,000 / 9.325 = $134,043. Three years younger and $10,000 less spending cuts the number by a third. Time is doing nearly all the work here, which is why coast FIRE is a young person's milestone and a discouraging one at 55.

The sensitivity table nobody shows you

Now the part most calculators bury: the answer is violently sensitive to the growth rate. Same 35-year-old, same $1.5 million target, three different real returns:

Real return	30-year multiplier	Coast FIRE number
5%	4.322x	$347,066
6%	5.743x	$261,165
7%	7.612x	$197,051

Two percentage points of assumed return move the answer by $150,000. Remember the $197,000 from the top? That was the optimistic version. At 5% real, which is closer to what a bond-heavy portfolio earns, the same person needs $347,000. The formula is exact and the inputs are guesses, and the guess doing the heaviest lifting is the one people spend the least time on.

I am not going to tell you which rate to use, because anyone who does is selling certainty they do not have. The honest way to use this formula is as a range: compute it at 5, 6, and 7, and treat the spread as the truth. If you have already hit the number at 5%, you are coasting with margin. If you have only hit it at 7%, you are coasting on hope, and you know it. Both are useful things to know, and a single number from a calculator never tells you which one you have.

Calculate your coast FIRE number

Enter your spending, age, and return assumption, and get your number plus the sensitivity range.

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Frequently asked questions

What is the coast FIRE formula?

Coast FIRE Number = FI Number divided by (1 + real return) raised to the power of years until retirement. Your FI number is your annual retirement spending divided by your safe withdrawal rate, usually 4%, which is the same as spending times 25.

How do I calculate my coast FIRE number?

First compute your FI number: annual retirement spending times 25. Then divide it by (1 + your assumed real return) raised to the number of years until you retire. Example: $60,000 spending gives a $1,500,000 FI number; a 35-year-old retiring at 65 with 7% real returns needs $1,500,000 / 1.07^30, about $197,000, invested today.

What growth rate should I use for coast FIRE?

Seven percent real (inflation-adjusted) is the standard assumption, roughly the S&P 500's long-run 10% nominal return minus 3% inflation. It is a guess, and the answer is sensitive to it: at 5% real instead of 7%, a 35-year-old's number rises from about $197,000 to about $347,000.

Does the coast FIRE formula include future contributions?

No. The formula assumes zero additional contributions after today: compound growth alone carries the portfolio to the FI number. That is the whole point of the milestone. If you keep contributing, you reach full FI sooner than the target date.

Is the 4% rule part of the coast FIRE formula?

Yes, indirectly. The FI number in the formula comes from the 4% rule of the Trinity Study (Cooley, Hubbard, and Walz, 1998): multiply annual spending by 25. If you prefer a more conservative 3.5% withdrawal rate, multiply spending by about 28.6 instead.