Traditional early retirement asks you to save aggressively until you can stop working entirely, which is a daunting target for most people. Coast FIRE offers a middle path: you build a smaller investment balance early, then let compound growth carry it to your full retirement number while you work only to cover current living costs. It is less about quitting work and more about removing the pressure to keep saving at full speed forever.

Coast FIRE is the point where you have invested enough that, without adding another contribution, your portfolio could grow to your retirement target by your chosen retirement age. After reaching that point, you still work and earn, but your income only needs to cover today's bills, not tomorrow's retirement as well. That shift opens options that full early retirement does not: you can take a lower-paying job you enjoy, work part-time, switch careers, or simply stop worrying about the retirement savings rate. The key distinction from traditional FIRE is timing. Traditional FIRE requires the full retirement fund before you stop saving; Coast FIRE requires only a seed amount early, with time and compounding doing the rest.

The calculation works backwards from your retirement goal and needs three inputs: your target retirement number, your expected annual return after inflation, and the number of years until your target retirement age. First, estimate your retirement number, commonly calculated as twenty-five times your expected annual spending in retirement, based on the idea of withdrawing around four percent per year. Then divide that number by one plus the expected annual return, raised to the power of the years until retirement. For example, suppose your retirement target is 1,000,000 and you have thirty years until retirement. At a seven percent annual real return, your Coast FIRE number is roughly 131,000, because 131,000 growing at seven percent for thirty years reaches about 1,000,000. At a five percent real return, the same target needs roughly 231,000 today, which shows how sensitive the answer is to the return assumption.

The formula's biggest weakness is that it depends on a return you cannot control. A difference of one or two percentage points in the assumed real return moves the required balance by tens of thousands, so careful planners test several scenarios rather than trusting a single number. A common approach is to calculate the Coast FIRE number at three return assumptions, such as four, five, and seven percent, and treat the most conservative figure as the real target. Age matters just as much: the same retirement target needs far less today for a thirty-year-old than for a fifty-year-old, because the younger saver has more years of compounding ahead. That is why Coast FIRE appeals most to people in their twenties and thirties, who can build the seed balance early and then enjoy decades of lower savings pressure.

Coast FIRE is a planning concept, not a guarantee, and its risks deserve honest attention. Investment returns vary year to year, and a poor sequence of returns early on can leave the portfolio well short of the projection. Inflation may run higher than assumed, which raises the future cost of living and therefore the retirement target. Life events such as illness, a career break, or supporting family members can change both spending and income in ways no formula captures. Taxes and fees also reduce real returns. The sensible response is not to abandon the idea but to build in margin: aim for a Coast FIRE number calculated at a conservative return, keep contributing something even after reaching the number if you can, and review the calculation every year or two as circumstances change. A plan that is revisited regularly survives contact with reality far better than one set and forgotten.

Start by estimating your annual spending in retirement and multiplying it by twenty-five to get a rough retirement target. Next, decide on your target retirement age and count the years from now until then. Then calculate your Coast FIRE number at two or three different return assumptions and pick a conservative figure as your goal. Build a savings plan to reach that figure, which for many people means a period of focused, higher-rate saving early in their career, often called a burst phase. Once you reach the number, reassess: you can reduce your savings rate, redirect income toward other goals such as paying off a mortgage or funding education, or change to work you find more meaningful. Keep an annual check-in on the plan, updating the target for inflation, checking the portfolio's progress, and adjusting for life changes. Coast FIRE does not promise an easy retirement, but it does offer something valuable: the freedom to stop sprinting and start choosing, years before traditional retirement would allow.