What Is Coast FIRE? The Number That Lets You Stop Saving

Most retirement advice tells you to save more, for longer, forever. Coast FIRE flips that. It says there is a specific point, often reached surprisingly early, where you can stop saving for retirement entirely and still retire on time. Not because you got rich, but because you gave compounding enough of a head start to finish the job for you.

It is one of the most useful ideas in personal finance, and one of the most misunderstood. So let us define it plainly and then do the math.

Coast FIRE, defined plainly

Coast FIRE is the moment you have invested enough that compound growth alone will carry that money to a full retirement number by traditional retirement age, without you adding another dollar.

You still work. You still pay rent, buy groceries, and cover today's life out of today's income. What changes is that you no longer have to funnel money into retirement accounts. That work is already done. Your existing investments will grow on their own to where they need to be by 65. You are, in the literal sense, coasting.

The key word is enough, and enough is far less than most people assume, because you are handing the problem to time.

Why young people love it

The magic of Coast FIRE is that the coast number is much smaller than a full FIRE number. A full FIRE number is what you need to never work again, often around 25 times your annual expenses. A coast number is only what you need today for compounding to grow it into that full number by retirement. You are giving the market decades to do the heavy lifting, so you need much less up front.

Hit Coast FIRE at 30 and the whole shape of your working life changes. You are no longer trapped by the highest-paying job you can tolerate. You can:

• Take a lower-paying job you actually enjoy, because you only need to cover current expenses.
• Go freelance or part-time without panicking about the retirement contributions you are skipping.
• Take real career risks, start something, switch fields, take a sabbatical, knowing your future retirement is already funded in the background.

That is what Coast FIRE actually buys. Not early retirement, but early freedom of choice about how you earn.

Coast FIRE does not free you from working. It frees you from working only for the money. Your retirement is already handled, so today's job just has to cover today.

The math intuition

Here is the whole idea in one line: your coast number is your future retirement number discounted back to today at an expected return, over the years until you retire.

Put simply, you are running compound growth in reverse. Instead of asking "what will this grow into," you ask "how little do I need now for it to grow into the target on its own." The formula:

• Coast number = Retirement number ÷ (1 + return)^{years to retirement}

Let us make it concrete. Say you want $1.25M by age 65, and you assume a roughly 7% annual return after inflation. You are 30, so you have 35 years to let it ride.

Over 35 years at 7%, money multiplies by about 10.7 times. So the amount you need today is $1.25M divided by 10.7, which is roughly $117,000. That is it. If a 30-year-old has about $117,000 invested and never adds another cent, it grows into $1.25M by 65 on its own. Everything they earn after that can go to living, not saving.

Now the honest part. This number lives or dies on the return you assume, and no one can promise you 7%. Drop the assumption to a more conservative 5%, and money only multiplies about 5.5 times over those 35 years. Suddenly the coast number nearly doubles to around $227,000. Same goal, same age, very different answer. A single percentage point of assumed return, compounded over decades, moves the target enormously.

So treat any coast number as a planning estimate, not a finish line you cross once and forget. Use a conservative return, and recheck as the years pass and reality comes in above or below your guess.

Notice too how much your starting age matters. That same $1.25M goal needs about $117,000 at 30, but a 25-year-old, with five more years of compounding, needs closer to $83,000 at 7%. Wait until 40 and the number climbs past $230,000. The earlier you get there, the less it takes, which is why Coast FIRE rewards starting young more than earning big.

Coast FIRE vs Barista FIRE vs full FIRE

These get mixed up constantly. One clean sentence each so you can place yourself:

• Full FIRE: you have enough invested to live off it now and never work again.
• Coast FIRE: you have enough invested that it will grow into your full number on its own, so you stop saving for retirement but keep working to cover today.
• Barista FIRE: a halfway house where you work a lighter job mainly for the paycheck and benefits, covering part of your expenses while your investments cover the rest.

Coast FIRE is the earliest and most reachable of the three, which is exactly why it is the one worth aiming for first.

The catch that makes or breaks it

Every bit of this rests on one assumption: that your money is actually growing. Coast FIRE is not a savings-account strategy. Cash sitting in checking does not compound at 5% or 7%, it compounds at roughly nothing, and inflation quietly eats it. The entire plan depends on that balance being invested and left alone to do its work.

That is the part people skip. They calculate a beautiful coast number, then let the money idle where it cannot possibly hit the return the whole plan assumes. If your future retirement is supposed to run on autopilot, the money has to be somewhere it can actually grow. OpenTrade turns "I should get this invested" into a plain-English daily habit, so the balance you are counting on to coast is actually in the market, not parked in cash pretending to work.