What Coast FIRE means
Coast FIRE is the moment your invested balance becomes self-sufficient. From that point, compounding alone carries it to your full retirement number by your target age — you never have to contribute again.
Crucially, it does not mean you can stop working. You still need income for today's rent and groceries. What it removes is the obligation to save. Every dollar you earn beyond current expenses is now genuinely yours to spend.
In practice this is what makes it useful: it is the point where you can take the lower-paid job, drop to four days a week, or move to the role you actually want, without pushing retirement back a single year.
The formula
The second step is just discounting: what sum today grows into the target by then.
The return must be a real return. The FIRE number is expressed in today's money, so discounting it with a nominal rate mixes units and produces a number that is far too low. Over 30 years, discounting a today's-money target at 7% nominal instead of ~3.9% real understates the coast number by more than half. This calculator derives the real rate from your return and inflation assumptions using the Fisher relation, so the units stay consistent.
A worked example
Aged 30, retiring at 60, spending $40,000 a year, 4% withdrawal rate, 7% nominal return and 3% inflation — a real return of about 3.88%:
So roughly $319,000 invested at 30 becomes $1m of today's purchasing power by 60, untouched. Reach that and you can stop saving entirely, thirty years before you stop working.
Why waiting is expensive
The coast number rises every year you delay, because there is less time left to compound. At a ~3.9% real return it grows about 3.9% annually — and in absolute terms the increases accelerate as the horizon shortens.
The by-age table in the result panel makes this concrete. The gap between coasting at 30 and coasting at 40 is not ten years of saving; it is a substantially larger target, because a decade of compounding has been removed from the calculation. This is the strongest available argument for front-loading retirement saving in your twenties and thirties rather than spreading it evenly.
What the by-age table shows
The table in the result panel gives your coast number at every age between now and retirement. It is worth reading carefully, because it makes two things concrete that are otherwise abstract.
First, the cost of delay. On the default numbers, coasting at 30 requires about $319,000; at 40 it requires roughly $467,000; at 50, about $683,000. Waiting ten years does not add ten years of saving to the target — it adds roughly $148,000, because a decade of compounding has been removed from the calculation. Wait another ten and it adds $216,000.
Second, where you already stand. Any age where your current balance already exceeds the requirement is highlighted, so you can see at a glance whether you are coasting for an earlier retirement than you assumed. Plenty of people discover they are already coasting for 65 while planning for 60, which reframes the question from "how much more do I need" to "how much earlier could I stop".
Are you actually coasting?
One caveat about what counts. The coast number assumes the money is invested and left alone at your assumed real return. Cash sitting in a savings account is not coasting — at a real return near zero it does not compound into anything, and the whole mechanism depends on compounding.
Equally, money you might need before retirement is not part of a coast balance. A house deposit, an emergency fund, or a car replacement fund all have to come out of the number before you check it against the target. Only genuinely untouchable, invested, long-horizon money counts.
The risk nobody mentions
Coast FIRE is more exposed to the return assumption than any other strategy on this site, for a structural reason: there are no further contributions to correct a shortfall.
A saver who keeps contributing and hits a bad decade can respond by saving more. A coaster who stopped at 32 and discovers at 55 that real returns came in at 2% instead of 4% has no such option — twenty-three years of compounding cannot be recreated, and the arithmetic is unforgiving.
Sensible responses:
- Use a conservative real return. Try 3% instead of 5% and see how much the number moves. If the plan only works at optimistic assumptions, it is not a plan.
- Coast partially. Reduce contributions rather than stopping. Most of the lifestyle benefit, much less of the risk.
- Re-check every few years. Coasting is not a decision you make once. If returns disappoint, resuming contributions early is far cheaper than discovering the gap at 55.
Where it sits among the variants
Coast FIRE is usually the first milestone reached. After it, Barista FIRE is the point where the portfolio starts covering part of your expenses alongside part-time work, and full FIRE is where it covers everything.
If you plan to retire before 59½ in the US, reaching a coast number is not enough on its own — the money also has to be reachable. The bridge account calculator covers that problem, which is about where your money sits rather than how much of it there is.