Coast FIRE: The Age You Can Stop Saving and Still Retire On Time

Tier 4 · Big Life Decisions · intermediate · ~6 min read · firefinancial-independenceretirementcompoundinginvestingsaving

“When can I stop saving?”

Almost every retirement calculator answers one question: when can I stop working? Coast FIRE asks a different, quieter one that arrives years earlier — when can I stop saving?

They’re not the same question, and the gap between them is the whole point. Compound growth doesn’t need your contributions forever. There’s a moment when your invested pile is already big enough that, even if you never added another dollar, growth alone would carry it to your number by the time you retire. Past that moment, your monthly savings aren’t building your retirement anymore — they’re just making it arrive sooner or land richer. The retirement itself is already paid for.

That moment is called reaching Coast FIRE. Hit it and you haven’t retired — you still have a paycheck to earn for living expenses — but you’ve bought a kind of freedom most people never name: the freedom to stop feeding the retirement account. Downshift to the job you actually like, go part time, take the risk, raise the kids — without setting your future self back a single year.

The two numbers that find it

Coast FIRE lives at the crossing of two curves.

The first is the one you already know: your pile if you keep saving — what you have today, growing at your expected return, with your monthly contribution added the whole way. It climbs steadily toward (and usually past) your number.

The second is the clever one: the coast number. At any given age, it’s the pile you’d need right then so that growth alone — with zero further contributions — reaches your target by retirement. It’s just your number run backwards through compounding. The further you are from retirement, the smaller it is (more years for growth to work), so the coast number rises with age, until it equals your full number on the day you retire.

Coast FIRE is where your line clears that bar. Before the crossing, you’re still below the coast number — you need to keep saving. After it, you’re above — growth can finish the job, and everything you keep contributing is gravy.

See it for yourself

The chart races the two curves across the years from your age to retirement. The teal line is you, if you keep saving. The amber dashed line is the coast number — the bar you have to clear. The band between them is amber while you’re still below the bar (keep saving) and turns teal once you’ve cleared it (now it’s surplus). Where they cross — the dashed marker — is the age you could stop contributing and still glide to your number.

Things worth trying

• Watch where the lines cross. With the defaults, a 30-year-old with $50k saving $800/month reaches Coast FIRE in their early fifties — over a decade before retirement. That’s ten-plus years they could stop contributing and still hit $1M, freeing up well over a hundred thousand dollars of future saving.
• Drag Invested so far up. Notice the coast number today barely moves — it only depends on your target, your return, and the years left, not on what you have. But your teal line lifts, so the crossing slides earlier. Push your savings high enough and the line starts above the bar: you’re already coasting, and could stop today.
• Cut Saving each month to zero. Now the only thing that can put you over the bar is the pile you already have. If it’s not enough, the line never climbs to meet the coast number — with nothing going in, being behind is permanent, because your pile and the bar grow at the very same rate.
• Lower Real return. The coast number rises toward your target — with less growth to lean on, you need a bigger pile sooner, so Coast FIRE arrives later. At 0% it sits flat at the full number: with no growth, there’s nothing to coast on, and “stop saving early” simply isn’t on the menu.
• Raise Your number. A bigger target lifts the whole bar and pushes the crossing out — or past retirement entirely, in which case saving the whole way still falls short and you never get to coast.

Why the crossover comes so early

The surprise in Coast FIRE is how much earlier the stop-saving line sits than the finish line — and it’s the same force behind the cost of waiting and compound interest, read from a third angle.

Your earliest dollars do the heaviest lifting because they compound the longest. A dollar invested at 30 and left until 65 grows for thirty-five years; at a 5% real return it roughly quintuples. So by the time your pile is large, the growth it throws off each year dwarfs anything you could add by hand. Once your annual growth alone exceeds the slope you need to reach your number, your contributions have become rounding error — and that’s exactly the crossover. You stop being the engine; compounding takes over.

This is why the number you have to save to is smaller than the number you’re aiming for. You’re not trying to save your way to a million dollars. You’re trying to save your way to the coast number — the pile from which a million dollars is just a matter of waiting. That bar can be less than a fifth of your target when retirement is decades away.

How to actually use this

1. Front-load while you can. The entire payoff of Coast FIRE comes from getting money in early, where it has the most years to compound. The same effort spent saving hard in your twenties and thirties buys you a far earlier coast date than the same dollars saved in your fifties — because you’re really buying time on the market, the one input you can’t get back.
2. Treat the coast date as a permission slip, not a stop sign. Reaching it doesn’t mean you should stop saving — it means you can. That’s leverage: the freedom to take the lower-paying job you love, go part time, start something, or weather a gap, knowing your retirement is already on rails.
3. Re-check it when the big inputs change. Your number (what retirement actually costs), your return assumption, and your timeline all move the coast line. A higher target or a more cautious return pushes the date out; an inheritance or a raise that boosts your pile pulls it in. Recompute after any of them.
4. Don’t confuse coasting with retiring. Coast FIRE only covers your future retirement — it says nothing about today’s rent and groceries. You still need income for living expenses until you actually retire. What you’ve freed is the saving, not the working. Pair it with your savings rate and your net worth to see the whole picture.

Key terms

• Coast FIRE — the point at which your invested pile is large enough that compound growth alone will reach your retirement number by your target date, with no further contributions required.
• Coast number — the pile you’d need at a given age so that growth alone hits your target by retirement; it’s your number discounted back through compounding, and it rises with age toward the full number.
• FI number / your number — the nest egg that funds your retirement, usually your annual spending times twenty-five (the 4% rule). The target the coast number is chasing.
• Coasting — continuing to work for living expenses while no longer contributing to retirement, because growth is already carrying the pile to the finish.

Compound interest showed you that time is the lever; the cost of waiting showed you the bill for not pulling it early. Coast FIRE is the reward for pulling it early: a day, years before you retire, when you get to set the lever down and let it keep working without you.