Coast FIRE is the point where your existing investments will grow to your full retirement number on their own, with no new contributions, by the age you plan to retire. Cross it and the retirement problem is funded; your job only has to cover life until then. For a 30 year old on the calculator’s defaults, that line sits at about 226,613 dollars.
The idea earns its popularity because it converts an intimidating seven-figure target into a nearer, checkable milestone, and because the math underneath is nothing more than compound interest run in reverse. This guide works the formula with exact numbers from the Coast FIRE calculator, shows why your age moves the answer more than any other input, and is blunt about what the model leaves out.
What is Coast FIRE in plain English?
Full FIRE means your portfolio can pay for your life now. Coast FIRE means your portfolio will be able to pay for your retirement later, without another dollar of savings, because compounding finishes the job over the years you keep working. You still work, you still pay your bills, you just stop having to save for retirement.
The distinction matters for what it buys you. Coast FIRE does not fund an early exit; it funds flexibility: a lower-paying job you like more, one parent at home, a business attempt, any life where covering expenses is enough. Whether you actually stop contributing after crossing the line is a separate choice, and many people do not, precisely because the line is built on assumptions.
How do you calculate your Coast FIRE number?
Two divisions, both in today’s dollars. First, your FIRE number: annual retirement spending divided by a withdrawal rate. Second, your coast number: that FIRE number discounted backward at a real, after-inflation return for every year between now and retirement.
| Step | Value |
|---|---|
| Annual retirement spending | 50,000 dollars |
| Withdrawal rate | 4 percent |
| FIRE number (50,000 / 0.04) | 1,250,000 dollars |
| Years to retirement (30 to 65) | 35 |
| Real return assumption | 5 percent |
| Coast FIRE number today | 226,612.86 dollars |
| Current invested (default) | 50,000 dollars |
| Gap to the coast line | 176,612.86 dollars |
Notice the language the calculator itself uses: the default saver has not reached Coast FIRE, they are 176,613 dollars short of the line. What closes that gap is the ordinary combination of contributions and growth, which is where the next question comes in.
Why does your age change the number so much?
Because the coast number is compounding run backward, and every year of runway you lose makes the starting requirement grow at the same 5 percent rate. The identical 1,250,000 dollar target, retirement at 65, costs radically different amounts of today-money depending on when you ask:
| Current age | Years to 65 | Coast FIRE number today |
|---|---|---|
| 25 | 40 | 177,557.10 dollars |
| 30 | 35 | 226,612.86 dollars |
| 35 | 30 | 289,221.81 dollars |
| 40 | 25 | 369,128.46 dollars |
| 45 | 20 | 471,111.85 dollars |
| 50 | 15 | 601,271.37 dollars |
This table is the strongest argument for early retirement saving that exists, and it does not require heroic contributions, only early ones. A 25 year old needs about 178,000 dollars to be done saving forever; a 45 year old needs nearly half a million for the same outcome. The difference is not discipline, it is arithmetic.
When could you actually stop contributing?
When your balance touches a target that is itself rising. Here is the subtlety the single coast number hides: as you age, the line moves toward you, because fewer discounting years remain. The calculator projects your balance, contributions included, against that rising target and reports the crossing age. On the defaults, 50,000 dollars invested plus 12,000 dollars a year meets the target at age 58.
Two honest readings of that chart. First, the same person who is 176,613 dollars short of coasting today still retires fully funded at 65; the coast age is a milestone inside a working plan, not a verdict. Second, the crossing age is sensitive: at these contribution levels, a saver starting at 35 with the same 50,000 dollars never crosses before 65 at all, which the calculator reports honestly as not reaching the coast line by retirement.
How much do contributions move your coast age?
Dramatically, and asymmetrically. The coast age rewards front-loaded saving because every early dollar compounds against a target that has not risen yet. Holding everything else at the defaults, here is the crossing age at four contribution levels, engine-computed:
| Annual contribution | Age you cross the coast line |
|---|---|
| 6,000 dollars | not by 65 at this contribution |
| 12,000 dollars (default) | 58 |
| 18,000 dollars | 44 |
| 24,000 dollars | 40 |
Read the middle rows again: going from 12,000 to 18,000 dollars a year, 500 dollars a month, pulls the coast age from 58 to 44, fourteen years earlier. The jump from 18,000 to 24,000 buys only four more. That is the front-loading asymmetry in one table, and it suggests a strategy the FIRE community has converged on independently: save hard and early, cross the line young, then let the job only need to cover your life. The last row’s saver at 40 has a quarter century of career left in which retirement is already funded.
The same asymmetry is the honest warning for late starters: at 6,000 dollars a year the default saver never coasts before 65 at all. That does not mean retirement fails, the balance still compounds the whole way there, but the specific luxury of stopping contributions never arrives. The fix is the same lever in reverse: contributions, earlier.
What return rate should you assume?
A real one, and the difference is the most common Coast FIRE mistake. The calculator works entirely in today’s dollars: the 5 percent default is an after-inflation assumption, and the spending target keeps today’s purchasing power. Plug in a nominal 10 percent market return and the model will cheerfully tell you a fantasy. The mechanics of real versus nominal are covered in inflation explained, and the growth math itself in how compound interest works.
The withdrawal-rate input deserves the same scrutiny, because the 4 percent rule is genuinely contested: Bengen’s original research established 4 percent, his recent work argues higher, and Morningstar’s research has argued lower in recent years. Is the 4 percent rule still safe walks that whole debate. A half-point change in the withdrawal rate moves the FIRE number, and therefore every coast number, by more than 10 percent.
What this model deliberately ignores
The calculator’s own assumptions are explicit: no taxes, no Social Security, no employer match, contributions added at year-end, and one steady real return with no sequence-of-returns risk. Every one of those simplifications is visible on the calculator page, and each is a reason to cross the coast line with margin rather than to the dollar. Stopping contributions also usually forfeits an employer match, which is the single most expensive dollar most savers can give up.
Coast FIRE vs Barista FIRE: what is the difference?
Coast FIRE keeps your full income and stops your saving; Barista FIRE reduces your income and may keep some saving. A Barista FIRE plan swaps the career job for lower-stress, often part-time work, typically for health insurance and enough income to cover expenses, while the portfolio coasts underneath. Same engine, different lifestyle wrapper: both rely on an already-funded retirement growing untouched.
The useful test is which constraint binds you. If the job itself is fine but the savings pressure is heavy, Coast FIRE answers your question. If the job is the problem, you are really pricing a Barista move, and the numbers to check are your expenses against the lower income, with the coast math confirming retirement stays funded either way. Whether you are on track at all is its own check, and are you on track for retirement pairs with the retirement on track calculator for that.
Your next step
Find your line. Open the Coast FIRE calculator and enter your age, planned retirement age, invested balance, yearly contributions, and expected retirement spending. It returns your FIRE number, your coast number in today’s dollars, your surplus or gap, and the age your current pace crosses the line, plus the balance-versus-target chart from this guide with your own numbers.
Then pressure-test it: run the return at 4 percent, the withdrawal rate at 3.5, and retirement two years earlier, and see whether the coast age survives. If it does, you have a milestone worth planning around. The FIRE calculator runs the full early-retirement version, and the compound interest calculator isolates the growth engine underneath all of it.
This is educational information, not financial advice. The figures here are the calculator’s illustrative defaults, computed by the same engine, in today’s dollars, under stated assumptions that exclude taxes, Social Security, employer matches, and return variability. Your returns and your life will differ; run your own numbers and keep margin.