Coast FIRE is the most liberating milestone in personal finance that most people have never heard of. It is the point where you have invested enough that, even if you never add another dollar, compound growth alone should carry your portfolio to your full retirement target by the age you plan to stop working. Reaching it does not mean you can quit tomorrow, but it does mean the pressure lifts: you only need to earn enough to cover current expenses. Calculating your number takes four steps and a calculator.

Start with your annual expenses in retirement, not your current salary. Estimate what you will spend in a typical year once you stop working, including housing, food, transport, healthcare, and leisure, and excluding costs that will disappear, such as a mortgage you will have paid off or work-related expenses. Multiply that annual figure by twenty-five, following the widely used four-percent withdrawal guideline, which suggests a portfolio can sustain annual withdrawals of about four percent of its starting value. If you expect to spend forty thousand a year, your full retirement target is roughly one million.

Be honest about this estimate, because every later step depends on it. Underestimating expenses produces a Coast FIRE number that feels achievable but fails in practice, while overestimating them pushes the milestone so far away that it stops motivating you. Review your actual spending over the past twelve months as a reality check, adjust for known future changes such as children leaving home or downsizing, and add a buffer for healthcare costs, which tend to rise faster than general inflation. A target grounded in real spending is worth more than a round number that sounds impressive.

Next, decide how many years remain until your planned retirement age. This horizon is the engine of Coast FIRE: the longer it is, the smaller your number, because compounding has more time to work. Someone with thirty years until retirement needs far less saved today than someone with ten years, even with the same retirement target. Write down your horizon explicitly, since vague timelines produce vague numbers, and revisit it if your retirement plans change.

Then choose a growth assumption and keep it conservative. Many calculators use an average annual return of around seven percent before inflation, or around five percent after adjusting for inflation, but future returns are uncertain and no assumption is guaranteed. Using an optimistic return shrinks your number artificially and risks disappointment, while a conservative assumption gives you a sturdier target. Run the calculation twice, once with a moderate assumption and once with a lower one, so you can see the range rather than a single fragile figure.

The final calculation divides your retirement target by the growth factor over your horizon. In plain terms, ask what amount invested today, growing at your assumed rate for your horizon, equals your retirement target. With a thirty-year horizon and a five-percent real return, the growth factor is roughly four and a third, so a one-million target implies a Coast FIRE number near two hundred and thirty thousand. With a twenty-year horizon at the same return, the factor is about two and two-thirds, implying roughly three hundred and seventy-five thousand. Online compound growth calculators perform this arithmetic instantly; the thinking lies in the inputs, not the formula.

Once you have your number, compare it with your current invested assets, excluding your home and short-term savings. If you are already past it, congratulations: you have reached Coast FIRE, and your future contributions are optional rather than essential. If you are short, the gap divided by your monthly savings rate shows roughly how long reaching it will take. That clarity is the real prize. Coast FIRE turns an abstract retirement dream into a concrete milestone with a date, and many people find that milestone far more motivating than the distant full-retirement target itself. This article is educational only and is not financial advice.