Most people think financial independence is binary: either you have enough to retire, or you are still grinding. Coast FIRE breaks that into two stages -- and the first stage arrives far sooner than you would expect.
Coast FIRE is the point where you can stop saving for retirement entirely and still retire comfortably on schedule. Your existing investments are already large enough that compound growth, given enough time, will carry them all the way to your full retirement number. You still work -- but only to pay this month's bills, not to fund a retirement that is, mathematically, already handled.
It is one of the most liberating milestones in personal finance, and almost nobody realizes how early they can reach it. Let us walk through exactly what it means, the math behind it, and how to find your own number.
What Coast FIRE Actually Means
To understand Coast FIRE, start with the destination. Your FIRE number is the portfolio you need to retire -- typically 25 times your annual expenses, the figure implied by the 4% rule. If you spend $50,000 a year, your FIRE number is $1,250,000.
Full FIRE means you have that entire amount today. Coast FIRE means you have enough today that you will have the full amount by retirement age -- without adding another cent.
The difference is time. A dollar invested at 30 has 35 years to compound before a 65-year-old retirement. At a 7% real return, that single dollar becomes roughly $11. So you do not need the whole $1.25M at 30 -- you need the much smaller amount that grows into $1.25M over those 35 years.
Coast FIRE separates "funding retirement" from "funding your life." Once you hit it, your retirement is on autopilot and your job only has to cover current expenses. That single shift -- not needing to save anymore -- is what gives Coast FIRE its freedom.
The Math Behind the Number
The Coast FIRE calculation is just compound growth run backwards. To find how much you need invested today so it grows to your FIRE number by retirement, you discount the goal back to the present:
Coast FIRE number = FIRE number / (1 + r)^n
Where:
- FIRE number = your annual expenses x 25
- r = your expected real (after-inflation) annual return, as a decimal
- n = the number of years until your target retirement age
Using a real return keeps everything in today's dollars, so your FIRE number does not need its own inflation adjustment.
A Worked Example
Say you are 30 years old, plan to retire at 65, expect to spend $50,000 a year, and assume a 7% real return.
- FIRE number: $50,000 x 25 = $1,250,000
- Years to grow (n): 65 - 30 = 35
- Growth multiple: (1.07)^35 ≈ 10.68
- Coast FIRE number: $1,250,000 / 10.68 ≈ $117,000
If you have about $117,000 invested at 30, you could stop contributing to retirement forever, and -- assuming a 7% real return holds -- you would still hit $1.25M by 65. Everything you earn from that point only needs to cover your current lifestyle.
Coast FIRE by Age
The single biggest driver of your Coast number is how much time you have left to compound. The earlier you reach it, the smaller it is. Here is the Coast FIRE number for the same $1.25M goal at a 7% real return, at different ages with retirement at 65:
| Current age | Years to 65 | Coast FIRE number |
|---|---|---|
| 25 | 40 | ~$83,000 |
| 30 | 35 | ~$117,000 |
| 35 | 30 | ~$164,000 |
| 40 | 25 | ~$230,000 |
| 45 | 20 | ~$323,000 |
| 50 | 15 | ~$453,000 |
Notice the shape: hitting Coast FIRE at 25 takes less than a fifth of the full $1.25M. Wait until 50 and you need over a third of it. This is the entire argument for investing early -- the first dollars you invest are, by a wide margin, the most powerful ones you will ever contribute.
Why the Return Assumption Changes Everything
The table above assumes a 7% real return. That is roughly the long-run historical average for US stocks after inflation -- but it bakes in the Great Depression, the 1970s, and 2008 alongside the booms, and the future is not guaranteed to match the past.
Because the return is raised to the power of n, small changes compound into big swings. Here is the same 30-year-old's Coast FIRE number ($1.25M goal, 35 years) at different real-return assumptions:
| Real return assumption | Growth multiple over 35 yrs | Coast FIRE number |
|---|---|---|
| 4% real | ~3.95 | ~$317,000 |
| 5% real | ~5.52 | ~$227,000 |
| 6% real | ~7.69 | ~$163,000 |
| 7% real | ~10.68 | ~$117,000 |
A two-point difference in your assumed return nearly doubles the number. This cuts both ways: an optimistic rate makes Coast FIRE look closer than it safely is. The conservative move is to plan with a lower real return (many use 5%), treat the result as a floor, and recheck the math every few years. If markets cooperate, you will overshoot -- a good problem to have.
How to Find Your Own Coast FIRE Number
You can run this in four steps:
- Estimate your annual retirement spending in today's dollars. Be honest -- include healthcare, which is often higher in retirement.
- Multiply by 25 to get your FIRE number. (Adjust to 28-30x if you want the safety of a 3.5% withdrawal rate -- see the 4% rule guide.)
- Pick a conservative real return -- 5% is a reasonable, slightly cautious default.
- Discount it back to today over the years until your retirement age.
To pressure-test it, use our compound interest calculator: plug in your current balance as the starting amount, set the monthly contribution to $0, enter your real return and the years until retirement, and see whether it grows to your FIRE number. If the future value clears your target with no contributions, you have already hit Coast FIRE.
Coast FIRE vs. the Other FIRE Variants
Coast FIRE is one of several flavors of financial independence, and they are easy to confuse:
| Variant | What it means | Do you still save? | Do you still work? |
|---|---|---|---|
| Full FIRE | You have your entire FIRE number now | No | Optional |
| Coast FIRE | Retirement is funded by future growth alone | No | Yes -- to cover current expenses |
| Barista FIRE | Part-time work covers some expenses + benefits | Maybe a little | Yes -- part-time |
| Lean FIRE | Full FIRE on a frugal budget | No | Optional |
| Fat FIRE | Full FIRE on a generous budget | No | Optional |
Coast FIRE and Barista FIRE pair naturally: once your retirement is on autopilot, you no longer need a high-paying, high-stress job to fund it -- so many people downshift to part-time or more enjoyable work. Coast FIRE is the math milestone; Barista FIRE is one lifestyle you can choose after reaching it.
The Risks Nobody Mentions
Coast FIRE removes one big danger and quietly introduces others.
What it removes: Because you are not withdrawing from your portfolio during the coast phase, you are largely insulated from sequence of returns risk -- the early-retirement trap of selling shares into a downturn. A crash during your coast years is uncomfortable but survivable, since you are not forced to sell.
What it introduces:
- Optimistic return assumptions. If your real return comes in at 4-5% instead of 7%, you may arrive at 65 short of your goal. Plan conservatively.
- A weak first decade. Compounding is back-loaded, but a long flat or negative stretch early in the coast still drags the final number down. Revisit your math periodically; resume contributing if you have fallen behind.
- You still have to cover current expenses -- including health insurance. Coast FIRE assumes you keep earning enough to pay today's bills for years. If you lose income or face a major expense, you may have to dip into the very savings that are supposed to be coasting.
- Lifestyle creep raises the target. Your FIRE number is tied to your spending. If your expenses climb, your goal -- and your Coast number -- climbs with them.
What Coast FIRE Means for You
Coast FIRE reframes the entire savings journey. Instead of one impossibly distant finish line, you get a near-term, achievable milestone: fund the retirement portion early, then let it ride. For someone in their 20s or early 30s, that milestone can be a five- or six-figure number -- ambitious, but a fraction of a full FIRE number, and very reachable with a few years of aggressive saving.
The payoff is optionality. Hitting Coast FIRE does not mean you stop working. It means you get to choose work that pays the bills without also having to fund 30 years of retirement -- a lower-stress job, a passion project, part-time hours, or a career change you could not previously afford to make. That freedom, years or decades before traditional retirement, is the real prize.
Run your number, plan with a conservative return, and check it every few years. The earlier you front-load your investing, the sooner compounding takes over the job of funding your retirement -- and the sooner you are free to do work you actually want to do.