TLDR
- Coast FIRE means you’ve invested enough that, with no further contributions, growth alone will reach your full financial independence number by an age you choose.
- Your coast number is your FI number divided by (1 + your real return), raised to the number of years until that age.
- Example: spending $4,000 a month gives an FI number of $1.2 million at a 4% withdrawal rate. With 25 years to go at 5% a year after inflation, the coast number is about $354,000.
- The return you assume moves it a lot. At 4% after inflation the same coast number is about $450,000. At 6%, about $280,000.
- Once you’ve reached it, your income only has to cover what you spend today. It no longer has to fund the future.
Most people meet coast FIRE as a relief. You don’t have to save hard forever. You save hard early, then you stop, and compounding carries the pot the rest of the way.
That’s true. But the more useful thing about coast FIRE isn’t the saving part. It’s what it does to the job you need.
The formula
Start with your FI number: what you spend in a year, divided by the withdrawal rate you trust. At 4%, that’s spending times 25.
Then work backwards. Coast number = FI number ÷ (1 + real return)years.
“Real return” means after inflation. Use it, or your number is in tomorrow’s money while your spending is in today’s, and the answer flatters you.
Worked through
Spending $4,000 a month is $48,000 a year. At 4%, the FI number is $1.2 million.
| Years to grow | Return after inflation | Coast number |
|---|---|---|
| 25 | 4% | About $450,000 |
| 25 | 5% | About $354,000 |
| 25 | 6% | About $280,000 |
| 30 | 5% | About $278,000 |
The gap between assuming 4% and assuming 6% is about $170,000 in this example. That’s the dial to argue with, not the formula.
What coast actually changes
Before you hit your coast number, your income has two jobs. Pay for this year, and fund the future.
After it, your income has one job. Pay for this year. In the example, that’s $48,000, not $48,000 plus whatever you were putting away.
Coast FIRE doesn’t free you from work. It frees you from needing the job you have.
That’s the point where selling a skill directly starts to make sense for a lot of people. Three days a week of your own clients might cover this year. The salary was only ever there to cover this year and the future, and the future is now handled.
What it assumes
Coast FIRE is a projection, so it’s only as honest as its assumptions. Four to keep in view:
- You don’t touch the pot. Any withdrawal resets the clock.
- Your spending doesn’t grow faster than inflation. Children, a bigger home, ageing parents: each one moves the number.
- The return averages out. A bad decade early on can push the date back by years.
- You keep earning enough to cover this year, for every one of those years.
If your mortgage ends before your coast age, your forever number is lower than your spending today suggests. That’s worth working out properly.
The part coast calculators leave out
The coast calculators that rank today treat your income as a salary that either continues or stops. None of them can see a business: something that pays you more than a salary while you build it, and might be worth something when you stop.
My first salary was $1,700 a month. Selling the same skill directly took it to $4,800. That’s the lever coast FIRE leaves on the table, and it’s the reason I count the date the way I do. What financial freedom actually costs is the longer version of that thinking.
Run your own
The Freedom Date calculator shows your date two ways: saving a slice of your salary, and building something that pays you. Every assumption, including the return, sits on the page where you can move it.
Illustrative numbers, not advice. I’m not a licensed adviser; I show the maths and what I did, and you run yours.