Coast number = (annual spending × 25) ÷ (1 + r)^years.
That one line is the whole idea, and I’d rather put it up front than bury it under a lifestyle story. Coast FIRE is the version of financial independence where you invest enough — once — and then stop adding, letting compounding carry the portfolio to a full freedom number by a target age. The formula above answers a question the regular freedom number never does: how much has to be in the account today so that, with no further contributions, it grows into independence by 60? Let me unpack the formula, table it, test it honestly, and show you where it breaks.
The disclaimer, since this is directional territory: I’m not a financial advisor, and this is a math walkthrough, not advice. The example returns are planning assumptions in the historical-style 7% real range, not predictions.
Where the Formula Comes From
Two pieces. The “×25” is the 4% rule — the planning heuristic that a portfolio can historically support annual withdrawals of about 4% of its starting value, inflation-adjusted, through a long retirement. Annual spending of $60,000 therefore needs a target portfolio of $1.5 million; that’s the 60th-birthday target in every example below. The divisor is just discounting: $1 growing at 7% a year becomes $1.07 in a year, so the money you need in 28 years is 28 years of growth smaller in today’s terms. Compound those two ideas and you get the coast number — the amount that, left alone, does the rest of the work for you.
The Coast Number by Age
Here’s the table for a $1.5M target at age 60, using 7% real growth. Read it as “if you have this much invested today and never add another dollar, the math reaches the target by 60”:
| Your age today | Years of growth to 60 | Coast number ($1.5M target) |
|---|---|---|
| 25 | 35 | ~$140,000 |
| 30 | 30 | ~$197,000 |
| 35 | 25 | ~$276,000 |
| 40 | 20 | ~$388,000 |
| 45 | 15 | ~$544,000 |
| 50 | 10 | ~$763,000 |
Two things jump out. First, the numbers are shockingly human-scale at young ages — a quarter-million saved by your mid-thirties is a serious but ordinary achievement for a disciplined dual-income household, and it does the same job as more than triple that amount saved at fifty. Second, look at the shape of the curve: each five years of age adds a lot. Coast FIRE is cheap early and expensive late, which is the entire argument for front-loading savings in your twenties and thirties. Time in the market isn’t a slogan here; it’s the divisor.
A Worked Example That Isn’t Yet Coasting
Most coast-FIRE content shows you a couple who already made it. More useful is the honest middle case. Take a hypothetical couple, both 32, spending $60,000 a year. Target portfolio: $1.5M at 60 — that’s 28 years of growth, and 1.07^28 ≈ 6.65. Their coast number is $1,500,000 ÷ 6.65 ≈ $226,000.
Now say they have $80,000 invested today. Left alone at 7% real, that $80,000 grows to roughly $532,000 by 60 — respectable, but well short of $1.5M. They are not yet coasting, and the formula just told them something precise: they need about $146,000 more invested ($226,000 − $80,000) to flip the switch. That converts a vague anxiety (“are we saving enough?”) into a concrete project with a date — roughly, a few more years of aggressive contributions at their savings rate, and then the choice: keep investing, or let the portfolio run and redirect income toward life. Both answers are legitimate. The formula’s job is to make the choice visible.
One nuance the table hides: “coasting” doesn’t mean stopping all saving. It means your retirement is funded. Most coasters keep investing anyway as life allows — kids cost what they cost — but the psychological unlock is that the retirement baseline is no longer hostage to a job you hate.
What Coast FIRE Actually Buys — and What It Doesn’t
What it buys: optionality with a deadline. You can take the lower-stress job, the sabbatical, the risky business attempt, or the reduced schedule, because the machine that funds your sixties is already running. For a lot of people that’s a bigger real-world change than full early retirement, which usually arrives too late in life to enjoy. It’s the reason the concept sits naturally next to the tiers in the three-lifestyles freedom-number walkthrough — coast FIRE is often the entry point to that slope rather than a separate philosophy.
What it doesn’t buy, plainly:
- A guaranteed return. The entire structure assumes ~7% real compounding. If the market delivers less over your specific 30 years — or if a brutal decade hits early — the plan lands short. There is no version of coast FIRE without market risk, and sequence risk is real.
- Healthcare. The $60,000 spending assumption has to include insurance premiums pre-Medicare, or the target is fiction. This is the biggest hidden cost in every FIRE calculation and it deserves its own line in your spreadsheet, not a footnote.
- Protection from your future self. The math assumes you genuinely don’t cash out the portfolio at 38 for a kitchen. Coast plans fail behaviorally more often than mathematically.
- Inflation invisibility. The target is in today’s dollars; the 7% real assumption is what keeps it honest. If you plug in nominal returns and nominal targets inconsistently, the whole number quietly breaks — pick one convention and hold it.
Why I Think It’s the Most Useful FIRE Concept Anyway
Because it converts an intimidating number into a milestone that arrives decades before retirement. Full financial independence at 55 can feel like fiction when you’re 30 with $40,000 saved. “You’re 30 and the coast number is $197,000; you’re a fifth of the way there” is a sentence you can act on this year. It also reframes the boring middle: once past the coast number, every additional dollar is upside rather than obligation, which changes how you feel about your job more than any raise does. The compounding doing all this work is the same arithmetic I walked through in compound interest, run the boring way, and the behavior that gets you there — automatic, unheroic, repeated contributions — is what I mean when I call dollar-cost averaging a behavior tool, not a math edge.
Forward step: run your own line. Take your real annual spending, multiply by 25, pick your target age, and divide by your expected growth factor — if the exponent math is annoying, even a rough “7% doubles roughly every decade” version gets you within shouting distance of your coast number. Then compare it to what’s actually invested today. The gap between those two numbers is the most honest savings goal you’ll ever set. And if the full freedom picture is what you’re after rather than the milestone, start with how I calculated — and recalculated — my freedom number.
