# How to Calculate Your Coast FIRE Number > The Coast FIRE number formula, worked through step by step with real numbers, plus how to pick your inputs and set it up in a spreadsheet. Source: https://mycoastfire.com/coast-fire-number Last updated: 2026-09-13 Your Coast FIRE number is your retirement target divided by how much your money will grow before you retire. Here's that sentence turned into a formula you can use. ## The formula Your Coast FIRE number is your FIRE number divided by how much your money grows, after inflation, between now and retirement. ``` FIRE number = yearly spending ÷ withdrawal rate Real return = (1 + return) ÷ (1 + inflation) − 1 Coast FIRE number = FIRE number ÷ (1 + real return)^(years to retirement) ``` There are three pieces. Your FIRE number is what you need on the day you retire, which is your yearly spending divided by your withdrawal rate. The real return is your expected investment return with inflation taken out. And the exponent is simply the number of years you have left. ## Working through it step by step Take Jordan, who is 35, wants to retire at 65, and thinks $50,000 a year will cover their spending by then. 1. FIRE number. $50,000 ÷ 0.04 = $1,250,000. 2. Real return. Jordan assumes a 7% return and 3% inflation, so (1.07 ÷ 1.03) − 1 = 0.0388, about 3.88% a year. 3. Growth factor. Thirty years at 3.88% is 1.038830 = 3.136. Every dollar Jordan has invested today should be worth about $3.14 of today’s money at 65. 4. Divide. $1,250,000 ÷ 3.136 = $398,578. So if Jordan has about $398,578 invested at 35, they could stop contributing and still have $1.25 million in today’s dollars at 65. You’ll often see people skip step 2 and just subtract inflation from the return (7% − 3% = 4%). It’s close, and it gives $385,398 here. Over 30 years that shortcut understates the target by about $13,180, so I’d use the exact version. ## Picking your inputs The formula is simple. The inputs are where people fool themselves, usually by being a little too optimistic on all of them at once. ### Retirement spending Start with what you spend now and adjust for what will change. A paid-off mortgage lowers it. Health insurance before Medicare raises it, sometimes by a lot. Keep it in today’s dollars, because the real return already handles inflation. If Social Security or a pension will cover part of it, you can subtract that income first; [here’s how to do that without being too optimistic](https://mycoastfire.com/blog/coast-fire-with-social-security). ### Expected return US stocks have returned roughly 10% a year before inflation over the last century, which is around 7% after it. Most people don’t hold 100% stocks for 30 years, though, and nobody promises the next 30 look like the last 100. For a mixed portfolio, somewhere between 5% and 7% is a sensible range. Here’s how much that one choice moves Jordan’s number: | Expected return | Coast FIRE number | | --- | --- | | 5% | $702,017 | | 6% | $528,264 | | 7% | $398,578 | Two percentage points is the difference between needing about $400,000 and about $700,000. If you only stress-test one input, make it this one. ### Inflation The long-run US average is a little over 3%, and recent years have been bumpier than that. I’d leave it at 3% unless you have a strong reason to change it. ### Withdrawal rate The 4% figure comes from [William Bengen’s 1994 research](https://www.financialplanningassociation.org/sites/default/files/2021-04/MAR04%20Determining%20Withdrawal%20Rates%20Using%20Historical%20Data.pdf) and the [Trinity Study](https://www.aaii.com/journal/199802/feature.pdf) that followed, which looked at how much retirees could withdraw each year, adjusted for inflation, without running out of money over 30 years. If your retirement could last longer, or you’d like more margin, 3.5% is a common choice. For Jordan, going from 4% to 3.5% raises the Coast FIRE number from $398,578 to $455,518. ## Doing it in a spreadsheet If you’d rather keep this in your own sheet, put the inputs in column B and use one formula: ``` A1 Yearly spending B1 50000 A2 Withdrawal rate B2 0.04 A3 Expected return B3 0.07 A4 Inflation B4 0.03 A5 Current age B5 35 A6 Retirement age B6 65 Coast FIRE number: =(B1/B2) / ((1+B3)/(1+B4))^(B6-B5) ``` With Jordan’s numbers that returns $398,578. It works the same in Excel, Google Sheets and Numbers. ## Or skip the math The box below uses the same formula with 7% returns, 3% inflation and a 4% withdrawal rate. The formula can’t tell you when your contributions will get you to the number. The [full calculator](https://mycoastfire.com/) can: it runs your monthly contributions forward and shows the age you cross the line, along with a chart of the whole path. ## Questions people ask ### Does the formula include my contributions? No. The Coast FIRE number assumes you add nothing from here on, which is the whole point of it. If you want to know when your monthly contributions will get you there, the calculator on our homepage runs that month by month. ### Should I use 4% or 3.5%? 4% is the usual starting point and comes from research on 30-year retirements. If you expect retirement to last longer than that, or you'd sleep better with some margin, use 3.5%. It raises your target by about 14%. ### Should I use pre-tax or after-tax numbers? Enter the balance you actually have, then make sure your spending figure includes the taxes you'll owe on withdrawals. If most of your savings are in a traditional 401(k), spending $60,000 a year could mean withdrawing quite a bit more than that. ### Why use a real return instead of the normal return? Because your spending figure is in today's dollars. If you pair a 7% return that includes inflation with spending in today's money, you're mixing two different units and your number comes out too low. ## Keep reading - [What is Coast FIRE?The idea in plain terms, with an example.](https://mycoastfire.com/what-is-coast-fire) - [Coast FIRE number by ageThe formula run for every age from 20 to 60.](https://mycoastfire.com/coast-fire-number-by-age) - [MethodologyEvery assumption behind our calculator.](https://mycoastfire.com/methodology)