Coast FIRE Explained: How to Front-Load Retirement and Stop Stressing About Saving

Most retirement advice is relentlessly additive: save more, contribute more, maximize every account. Coast FIRE flips the question. Instead of asking 'how much do I need to save each year until retirement?', it asks: 'how much do I need invested right now so that compound growth alone carries me to my retirement number — without a single additional contribution?' The answer is your Coast FIRE number. Hitting it doesn't mean you stop working, but it does mean your retirement is essentially funded — and every dollar you earn after that point is yours to spend, not obligated to the future.

The math behind coasting

Coast FIRE uses the future value formula in reverse. If you know your FIRE number — the portfolio value at which you can retire and withdraw 4% annually to cover expenses — and you know how many years you have until your target retirement age, you can calculate what you need invested today for compound growth at a reasonable rate (typically 7% real, inflation-adjusted) to reach that number without further contributions.

Formula: Coast Number = FIRE Number ÷ (1 + r)^n, where r is your expected real annual return and n is years until retirement.

Example: FIRE number of $1,500,000, 30 years until retirement at 7% real return. Coast number = $1,500,000 ÷ (1.07)^30 = $1,500,000 ÷ 7.61 = approximately $197,000. If you have $197,000 invested today and never contribute another dollar, you'll have $1.5 million in real terms at retirement — assuming 7% average real returns.

Why compounding makes this possible

The reason Coast FIRE works is the same reason compounding is called the eighth wonder of the world: returns generate returns. $197,000 growing at 7% real for 30 years produces $1.5 million not because you're adding to it, but because each year's growth gets added to the base that generates the next year's growth. The doubling time at 7% is approximately 10 years (the rule of 72: 72 ÷ 7 ≈ 10.3 years). So $197,000 → ~$394,000 at year 10 → ~$788,000 at year 20 → ~$1.58 million at year 30.

This is why the FIRE number, your expected return, and especially the years until retirement are the three levers that most dramatically affect your Coast number. A 35-year-old and a 45-year-old with identical FIRE numbers and return assumptions have dramatically different Coast numbers — the 10-year difference in compounding runway changes the Coast number by roughly 2×.

How to calculate your FIRE number

Your FIRE number is the portfolio value that allows you to withdraw your annual living expenses indefinitely at a safe withdrawal rate. The research-backed standard (the 4% rule from the Trinity Study) says you can withdraw 4% of your portfolio per year with high historical confidence of the portfolio lasting 30+ years.

FIRE Number = Annual Expenses ÷ 0.04. If your annual expenses in retirement are $60,000, your FIRE number is $1,500,000. If they're $80,000, it's $2,000,000. If you're aiming for a leaner lifestyle at $40,000/year, it's $1,000,000. Note that 'annual expenses' means your expected spending in retirement — which may be lower than current spending if you'll have no mortgage, no commute costs, and no retirement contributions to make.

Barista FIRE and the Coast spectrum

Coast FIRE is part of a broader FIRE spectrum. At one end is full FIRE: you have the full FIRE number and can retire entirely. Coast FIRE sits at the other end: your retirement is funded by compounding, but you still need to cover current living expenses through work. Barista FIRE is a middle point: you've saved enough that part-time or lower-stress work covers expenses while your portfolio grows.

Most Coast FIRE practitioners don't actually stop working — they stop feeling anxious about retirement. They may switch to lower-stress careers, take sabbaticals, reduce hours, or pursue passion projects, knowing that the retirement foundation is secure regardless of their income trajectory. This psychological freedom is the primary value proposition of hitting your Coast number.

Have you already coasted without knowing it?

Many people in their late 30s and 40s who have consistently contributed to a 401(k) or IRA since their 20s are already at or past their Coast FIRE number — and don't realize it. If you've been contributing 10–15% of income for 15+ years with reasonable market returns, run the Coast FIRE calculator with your current balance, your target retirement age, and your expected annual expenses. You might discover your retirement is already funded and you have more flexibility than you thought.

Even if you're not there yet, knowing your Coast number gives you a concrete milestone — far more motivating than a vague 'save as much as possible' directive. The Coast FIRE number is the point where compounding takes over, and knowing exactly how far you are from it changes how you think about savings, career choices, and financial decisions.