Coast FIRE is the point where compounding takes over as the main investor in your portfolio and your salary gets demoted to covering the bills.
Past that point, your existing balance grows into your full retirement target by the date you set, all on its own. You could contribute nothing for the next thirty years and still land where you were heading.
The catch is real, though, and it belongs at the top rather than buried in a warnings section: the maths only works if you don't touch a single pound for decades. Which is harder than it sounds. One raid on the pot is all it takes: a house deposit, a redundancy, a broken boiler with no emergency fund in front of it. After that, the compounding you were counting on quietly stops being the number you computed. Coasting is less a finish line than a promise you make to your future self, and you should know that before you fall in love with the idea.
Still worth finding, though. Most people cross this line a decade or more before they could actually afford to quit, and almost nobody checks.
The sum
Take your FIRE target and shrink it back to today at your expected real return:
Here r is the real return and n the years between now and the date.
Say you're 32 with a decent lump already invested (years of saving hard, an inheritance, share options that came good) and you want £750,000 by 65 at an assumed 5% real return. That's thirty-three years of growth:
Hold £150,000 today and add nothing, and the target takes care of itself.
Before you get excited, stress the assumption, because the exponent magnifies it mercilessly. Drop your return by just two percentage points, from 5% down to 3%, and your coast target jumps from £150,000 to £283,000. That single variable decides whether you're done saving or ten years away. (At 4% it's £206,000, still a third more.) And the return must be real, after inflation, or your target is in today's money while your growth isn't; the inflation calculator shows how far those two drift apart over thirty years. Thirty-three years of average returns is a modelling convenience anyway, not a promise. A Monte Carlo simulator will give you a more honest answer than one projected line ever can.
What makes the idea land is how steeply the number falls with age. A 25-year-old chasing the same £750,000 needs only £107,000. With 40 years of growth ahead, the divisor is a full 7.0. By 40 the price of the identical retirement has climbed to £221,000. At 50 it's £361,000, and someone starting at 58, with seven years of compounding left, needs £533,000. That's half a million pounds to buy what £107,000 bought at 25.
Five times the price, same destination. And that has nothing to do with discipline or picking winners, just the count of doublings left (by 45 the divisor has fallen to 2.65; by 55 it's 1.63). Run the compound interest calculator forwards and you'll see the same cliff from the other side.
Or stop reading about other people's numbers and put yours in here. The curves show what the last two sections said in one picture: how fast the required balance falls with age, and how far apart the honest and the optimistic return assumptions sit.
To coast to £750,000 at 65, you would need £149,904 invested today at a 5% real return. You are past it. On today's balance alone you would reach the target around age 65, 1 years early. Anything you save from here is a choice, not an obligation.
The balance needed to coast to £750k at 65, by age, at 3% (amber), 5% (green) and 7% (grey) real returns. The dot is you. Below a curve means coasting at that return; the vertical gap between the curves is the price of an optimistic assumption.
The full Coast FIRE calculator goes further if you want it: contributions, milestones, and the age your current balance starts coasting under its own steam. There's a walkthrough of the calculator if you'd rather see it worked through screen by screen.
One more thing about returns before moving on, because it bites hardest at exactly this milestone. Averages hide their sequence. Reach coast, stop saving, and then watch the market fall 30% the next year: your £150,000 is suddenly £105,000, and with no new money going in, nothing is buying at the bottom. The plan still recovers if returns revert over the following decades, but it now leans entirely on that recovery. Two things blunt it. Keeping even small contributions flowing means the worst years are the ones you buy cheapest in, and holding a separate cash buffer means a bad stretch never forces you to sell. Neither shows up in the simple formula; both decide whether the formula survives contact with a 2008.
What you'd actually do with it
Mostly, the change is in your head, and in what you can now say yes to.
Picture the version of this that actually happens. You're 36, coasting, and nursery fees are about to cost you £1,500 a month for the next four years. Because retirement is already funded, you can cut your pension contributions to zero for that stretch without wrecking your sixties. The market carries the long-term plan while your salary absorbs the expensive season, and when the fees end you decide: resume saving to bring the date forward, or don't. The same logic covers the sabbatical, the startup year, the retraining.
Or the career itself changes shape. Once you're past coast, a pay cut for work you like or a four-day week no longer threatens retirement. Follow that thread far enough and you arrive at what gets called Barista FIRE: deliberately smaller work covering part of your spending while the portfolio handles the rest, a halfway house on the road to full independence, where the portfolio covers everything and working is purely a preference. Coast is the first of those three thresholds, and nearly everyone passes through it on the way to the others, noticed or not.
One warning inside all this flexibility: the date is part of the deal. A number that coasts to 65 does not coast to 55, and it's common to reach coast, enjoy the feeling for a month, then decide you'd rather retire early, at which point you aren't coasting at all and the saving isn't finished.
And "save £150,000, then stop" is simply a target a 30-year-old can picture. "Save £750,000" isn't.
Where the money lives matters
The textbook version of coast ignores accounts, and in the UK the account is half the answer. If you're under 50 today, you're already looking at an access age of at least 57 for anything in a pension, workplace and SIPP alike, which is no problem at all if your coast date is 65, and a serious problem if you're quietly hoping to stop at 52. A stocks and shares ISA you can touch any time, tax-free. So the honest question isn't just "have I got £150,000" but "have I got it in places I can reach on the schedule I'm actually planning?" Coasting at 35 with everything in a pension and everything in an ISA are two different plans wearing the same number.
In practice the two wrappers work as a relay, and the plan should say explicitly which leg each one runs. Want the money at 52? Then the ISA has to carry you from 52 until the pension unlocks at 57, five years of spending held somewhere you can actually reach, while the pension carries everything after. Get that split wrong and you hit the pension trap: a coast number that exists on paper but sits behind a locked door during the exact years you planned to use it. The pension side usually grows faster per pound thanks to tax relief and any employer match, so the temptation is to stuff everything in there; the bridge years are what stop you.
The account question also settles the "should I stop contributing?" debate in most cases. If your employer matches pension contributions, stopping means declining free money. However tidy the arithmetic, that almost never makes sense. Coast proves stopping is possible; it doesn't make it wise. Keep saving something, even much less, and you buy back slack: an earlier date, a fatter pot, cover for a return that disappoints. The useful reading of coast has never been "stop". It's "the obligation has ended, and from here saving is a choice."
Run it twice
Three inputs: a target (your annual spending times a withdrawal multiple; what a FIRE number is covers that), a date the money must be ready, and a real return you'd defend to a sceptic.
Then run the sum at your assumption and again a point lower. If the two answers would push you into different decisions, the gap between them just taught you more than either number could.