Coast FIRE: the milestone that arrives ten years early

The idea

Compound growth does most of its work at the end. Which means there is a point — long before you have enough to live on — where what you have already invested will grow into your retirement number on its own, provided you leave it alone.

Past that point, every further pound you invest buys an earlier retirement rather than retirement itself. You still need an income to cover this year, but you no longer need to save.

This is the most misread idea in FIRE. Coasting does not mean not working. It means not saving. The number is what you need invested, not what you need to live on.

The arithmetic

Take your retirement target — annual spending divided by your withdrawal rate — and discount it back by however many years of real growth remain.

At 32, targeting 60, with a million-pound target and a 4% real return: about 325,000. At 45 the same target needs about 555,000, and at 55 roughly 820,000. The same retirement gets dramatically more expensive the longer you leave it, which is the clearest argument for investing early that exists, and it is arithmetic rather than exhortation.

The coast FIRE calculator runs it for your own figures.

Why it is the more useful number

Full FIRE is often twenty years out, and a twenty-year target does not change what you do on Monday. Coast might be five, and reaching it changes real decisions: dropping to four days, taking the job you want instead of the one that pays, going back to study, starting something.

It is also psychologically different in a way that matters. Full FIRE is all-or-nothing; coast is a threshold you cross and keep, and after which the pressure comes off even if nothing else changes.

Two honest caveats

It rests entirely on the return assumption. Coasting is a bet that the market delivers roughly its historical average over your remaining decades. Over thirty years that is a reasonable bet; over eight it is much less so. The shorter your runway, the less coasting is worth relying on.

It assumes your spending target holds. A coast number calculated at 30 against the spending of a single person in a flatshare is not the coast number of the same person at 40 with a family. Recalculate when your life changes, not just when the market does.