Short answer: your Coast FIRE number is the amount you’d need invested today so that, without adding another dollar, it grows to your full FIRE number by the age you want to retire. The formula is FIRE number ÷ (1 + real return)years to retirement. With a $1,000,000 target, a 5% return after inflation and 30 years to go, that’s $231,377. Once you have it, you only need to earn enough to cover your living costs, not to save for retirement, if the return assumption holds.
The FIRE calculator shows your Coast FIRE number automatically, assuming retirement at 65. This guide explains the math so you can check it and see how much the assumptions move it.
The formula, step by step
- Find your FIRE number: yearly spending in retirement ÷ withdrawal rate. $40,000 ÷ 4% = $1,000,000. (Why you might choose a lower rate is in the 4% rule vs 3.5%.)
- Count the years until the age you want the money to be ready: 65 − 35 = 30 years.
- Pick a real return, meaning the return after inflation, such as 5%.
- Discount the target: $1,000,000 ÷ 1.0530 = $1,000,000 ÷ 4.322 = $231,377.
If your portfolio is already at or above that amount, you’ve reached Coast FIRE on these assumptions. If it isn’t, the gap tells you how much more to invest before you can ease off.
Worked examples
The younger you are, the less you need. Same $1,000,000 target, 5% real return, coasting to 65:
| Your age today | Years to 65 | Coast FIRE number |
|---|---|---|
| 25 | 40 | $142,046 |
| 30 | 35 | $181,290 |
| 35 | 30 | $231,377 |
| 40 | 25 | $295,303 |
| 45 | 20 | $376,889 |
| 50 | 15 | $481,017 |
The return assumption matters most. Age 35, $1,000,000 target, 30 years:
| Real return assumed | Coast FIRE number |
|---|---|
| 3% | $411,987 |
| 4% | $308,319 |
| 5% | $231,377 |
| 6% | $174,110 |
A bigger spending target scales it up. At $60,000 a year, the FIRE number at 4% is $1,500,000, and the Coast FIRE number at age 35 and a 5% real return is $347,066. Someone 35 with $150,000 invested would, on those assumptions, see it grow to about $648,000 by 65: well short, so they’d keep saving for now. You can open this case in the calculator and change it to your own numbers.
Inflation: use a real return
Your FIRE number is in today’s dollars, so the return you discount by must be after inflation too. Mixing them is the most common Coast FIRE error, and it makes the number look far smaller than it is.
| Return used for a $1,000,000 target, age 35 | Coast FIRE number |
|---|---|
| 7% before inflation (wrong with a today’s-dollars target) | $131,367 |
| 7% before inflation with 3% inflation, so 3.88% real | $318,862 |
Nobody knows future returns. The calculator defaults to 5% real as a planning assumption; trying a lower figure, such as 3% or 4%, shows how much margin you have.
Limits of the Coast FIRE number
- Returns aren’t smooth. The formula assumes the same return every year. Real markets have bad decades, so reaching your number on paper isn’t the same as being safe.
- Fees come out of the return. A 1% fee turns a 5% real return into roughly 4%, which raises the number by about a third over 30 years (see fees and your FIRE date).
- You still have to cover today’s costs. Coasting means your earnings pay your current spending. If you dip into the portfolio, the math no longer works.
- The retirement age is a choice. The calculator uses 65. Coasting to an earlier date needs a larger number; coasting to a later one, a smaller number.
- Taxes and Social Security aren’t modelled. Money in pre-tax accounts will be taxed when withdrawn, and Social Security, if you’ll receive it, reduces what the portfolio must cover.
Related reading: how your savings rate changes your timeline for the years before you coast, and Lean FIRE vs Fat FIRE for setting the spending target the whole calculation starts from.
