Coast FIRE is the moment your retirement stops needing your paycheck. Once your invested savings reach a critical mass, compound growth alone will carry them to your full retirement number by your target age — no further contributions required. From that day on, work only has to cover your living costs. This guide explains the idea properly — the honest inflation-adjusted math most calculators skip, the milestones, the stress tests, and the traps — with a live calculator woven through it that finds your critical mass and the age you'll reach it.
Everything updates live as you type. All dollar figures — your expenses, your coast number, the chart — stay in today's dollars: growth is discounted by your inflation assumption using exact division, so the tool never flatters you with nominal-dollar illusions. Contributions are treated as constant purchasing power (you raise them with inflation) and credited at month-end.
The coast number isn't one figure — it rises every year you wait, because later starts leave compounding less runway. This table computes your personal checkpoints live from the inputs above, next to where your current path puts you at each age.
| Age | Coast number then | Your projected balance | Status on current path |
|---|
What it would take to reach your entire retirement number by your target age — no coasting, saving the whole way.
The coast number isn't a fact about money — it's a fact about runway. The same retirement target costs wildly different amounts of critical mass depending on how many compounding years remain. The figures below use your own target and assumptions from the calculator, holding everything constant except the starting age.
A coast plan is only as strong as its assumptions, and the assumptions are guesses. Before treating your number as a finish line, watch how hard it moves when a guess shifts by a single point. Every row recomputes live from your inputs above.
| Scenario | Coast number today | Coast age on your path | vs your base case |
|---|
How to read this table: if a one-point return miss or a three-year change of heart about retirement age blows your plan up, you weren't at Coast FIRE — you were at Coast FIRE if everything goes right. Robust coasting means clearing your number with a margin, or keeping a token contribution flowing as insurance.
Crossing the line changes your relationship with income more than your daily life. Paychecks now have exactly one job — funding the present — which converts career risk into something you can actually afford: the interesting lower-paying role, the sabbatical, the business idea, the four-day week. Many coasters keep investing anyway, and that's not a failure of nerve; it's the cheapest hedge available against the assumptions in the table above, and it pulls full financial independence closer as a bonus.
A sensible coast posture: verify the number with conservative assumptions, keep a small automatic contribution as insurance, revisit the calculator once a year — and then genuinely let the pressure go. The entire point of the exercise is permission.
Classic FIRE — financial independence, retire early — demands you save your entire retirement number before you stop. Coast FIRE splits the problem in two: first sprint to the critical mass, then let time finish the job. Past the coast point, every career decision changes character. A lower-paying job you love, part-time hours, a risky venture — all become affordable, because your future self is already funded. You still work; you just work for today instead of for two people.
Save hard early until compounding alone will hit your number by retirement age. Then contribute nothing more — work only covers current living costs. The earliest possible finish line for "my retirement is handled."
A cousin strategy: retire from the main career once part-time income can bridge your spending until the portfolio matures. More freedom sooner, but it leans on that income actually materializing — and often on employer benefits.
The whole number, saved and banked, work fully optional. The most secure and by far the most expensive in years of maximum saving — the contrast card above shows exactly how much heavier the monthly lift is.
FIRE number = annual spending ÷ withdrawal rate · rreal = (1 + return) ÷ (1 + inflation) − 1 · years left = retirement age − current age
Two places sloppy calculators go wrong, and this one doesn't. First, inflation: if your retirement spending is in today's dollars, growth must be discounted to a real return — otherwise the tool quietly assumes your spending never rises for decades, and your coast number comes out dangerously low. Second, the conversion itself: the real return is (1 + return) ÷ (1 + inflation) − 1, by exact division. The popular shortcut of simply subtracting inflation from returns overstates real growth every single year, and the error compounds — over a thirty-five-year runway it shifts a coast number by tens of thousands of dollars.
What the withdrawal rate encodes: dividing spending by a withdrawal rate is shorthand for decades of research on how much a portfolio can sustainably pay out — the familiar version implies saving twenty-five times your annual spending. It is a planning assumption, not a law of nature: lower it for longer retirements or extra caution, and treat the result as a target zone rather than a finish-line photo.
Coast FIRE is one waypoint on the map. These calculators cover the rest of the journey.