The milestone hiding inside your balance
Somewhere between your first contribution and full retirement there is a line almost nobody marks. Cross it, and the pot you already hold will reach your target by traditional retirement age without another penny added. That line is Coast FIRE, and it is not retirement: you still work past it, you simply no longer have to save.
The maths is one line. Take your target, discount it back at your expected return over the years remaining until 67, and compare with what you have:
At 5% real, £1 today becomes about £2.65 in twenty years. So a £750,000 target twenty years out needs roughly £283,000 now. Cross that line and the contributions become optional.
Everything hangs on which return you put into that exponent, and this is the page where the choice bites hardest. Over 35 years the error compounds on itself: assume 5% when 4% was the truth and you understate the pot needed by about a third. If you are unsure, use 4% real and treat anything better as upside; the sensitivity tool will show you exactly how much that one choice moves things. The target age is doing quiet work in the same formula. Coasting to 67 is a much lower bar than coasting to 55, because the runway is what does the compounding, and this tool fixes the age at 67, close to the State Pension age most of today's savers are on course for. Anyone under 50 today also cannot normally touch a pension before 57, under the rise in the normal minimum pension age, which is worth knowing before you plan a shorter runway around money that is locked.
What crossing the line buys you
Full FIRE is a distant, binary event: you are decades away, then one day you are not. Coast FIRE arrives far earlier, and it changes your options the day it happens. Saving converts from obligation to choice, since anything you add after coasting accelerates the date rather than securing it. Career risk stops being plan risk: a pay cut, a sabbatical, a lower-paid-but-better job no longer threatens the outcome, because the outcome no longer depends on the next contribution. And although the milestone arrives roughly halfway through the journey in years, it arrives far earlier in feeling, because the hard part, accumulating the seed capital, is behind you.
The lever that moves it most is not the one people expect. It is the number of years left to compound, which is why coasting at 30 requires dramatically less capital than coasting at 45, and why early contributions are worth so much more than the same pounds added later.
Rosa, five years for thirty
Rosa is 32 with £95,000 invested, adding £900 a month, expecting 5% real and targeting £30,000 a year at 4%: a £750,000 pot.
She is not there yet. With 35 years to 67, her £95,000 alone would grow to about £524,000, short of the target. But the tool puts her Coast FIRE age at 37, when the pot passes roughly £182,000. From that birthday she could stop saving entirely, work at whatever pays enough to live on, and still hit £750,000 by 67. Keep contributing instead and full FIRE lands at 55, twelve years sooner than the target date. The tool draws both lines so the trade is explicit, and the asymmetry deserves a moment: five years of saving buys her thirty years of optionality, because the target age is fixed and distant.
Two honesty conditions sit underneath Rosa's numbers. The pot has to be genuinely left alone for decades, since dipping in resets the arithmetic. And the £30,000 spending target has to survive contact with her actual future, because the required pot scales directly with it; a target set in her early thirties will drift, and the coast number drifts with it.
Would she actually stop saving at 37? Most people do not, and that is rational: continuing to save moves full retirement much closer. The value is knowing you could, which changes how you treat work. From that point every further contribution is buying an earlier retirement rather than a secure one, which is exactly when the FIRE calculator becomes the more useful page.
For the reasoning behind the discount, a table of coast numbers by age, and why the assumed return matters more here than anywhere else, read Coast FIRE explained.
What coasting still does not pay for
The years between quitting the big salary and reaching 67 need an income, because coasting only removes the need to save, never the need to eat. Your living costs still have to be covered by work, and modelling that phase properly, part-time earnings against a growing pot, is what Barista FIRE is for.
One asset that never belongs in the coast number: your house. Coast FIRE depends on invested assets that compound and can later be spent, and the home you live in does neither. It counts towards your wealth, which is why it belongs in the net worth tracker, but it cannot carry a coast plan.