Coast FIRE — the math of front-loading your retirement number

Coast FIRE is the point where your invested savings, left alone, will grow into your full retirement number without another dollar added. Reach it, and the arithmetic that has been demanding a savings rate stops demanding one — what you do with your income from that point on becomes a choice about the kind of work you want, not a requirement to fund a number decades away.

The coast number is not a separate goal from your FIRE number. It is the same number, discounted backward by the years and the return you expect the portfolio to earn while you leave it alone.

The math is your FIRE number, discounted backward

How compound interest works answers a forward question: given a sum invested today, what does it grow into after some years at some rate? A coast number asks the same question in reverse — given the sum you will need at the end, and the years and rate available to get there, how little needs to be sitting in the account today?

Coast number = FIRE number ÷ (1 + r)^n

where n is the years remaining until your target retirement age and r is the annual real — after-inflation — return you expect the portfolio to earn with no further contributions. Building the FIRE number itself is a separate exercise, usually your expected annual retirement spending times 25; this formula takes that finished number and works out its present-day equivalent.

Take a household with a $1,500,000 FIRE number and a target retirement age of 65. At age 35, thirty years remain. Assuming a 6% real return, the coast number is $1,500,000 ÷ 1.06³⁰, or roughly $261,200. Invest that amount in an RRSP, a TFSA, or a taxable account and never add another dollar, and — if the 6% holds — it grows into $1,500,000 by 65 on compounding alone.

The word "real" is doing real work in that sentence. Markets quote nominal returns, but a FIRE number built from today's spending is already a today's-dollars figure, so the rate discounting it back has to be today's-dollars too — a nominal return with inflation stripped out. Use a nominal rate by mistake and the coast number comes out too small, because the formula is quietly assuming the portfolio grows faster, after inflation, than it actually will.

What a coast number looks like by age

The same $1,500,000 target and 6% real return, run across ages, shows how sharply the number shrinks the earlier you're asking:

A dollar invested at 25 is doing roughly seven times the work of a dollar invested at 55, purely because it has thirty-five more years to compound instead of ten. That is the entire case for front-loading: the number is not smaller at younger ages because retirement is further away in some vague sense — it is smaller by a precise, compounding factor tied to exactly how many years remain.

Front-loading beats a steady drip — for a while

Front-loaded (invests once at 25, then coasts)Steady contributor (invests ~$9,700/year throughout)

Balance over time: one early lump sum vs. steady annual contributions

	Front-loaded (invests once at 25, then coasts)	Steady contributor (invests ~$9,700/year throughout)
25	145800	0
35	261200	127800
45	467700	356500
55	837600	766300
65	1500000	1500000

Both reach $1,500,000 by 65 at a 6% real return. The front-loaded investor contributes $145,800 once, total; the steady contributor puts in about $9,700 a year for forty years — roughly $388,000 all in.

Read the two lines and the front-loaded investor is ahead for essentially the entire chart, despite contributing less than half as much money over their lifetime. The steady contributor is putting money in every single year and still spends four decades catching up to someone who wrote one cheque and stopped. That is what compounding rewards — not the total dollars contributed, but how long each dollar has been sitting in the market.

None of this is an argument against contributing steadily; most people can't front-load $145,800 at 25 even if they wanted to, and a steady contributor still reaches the same $1,500,000. It is an argument for treating any windfall, bonus, or high-income stretch as disproportionately valuable when it lands early, because the years it buys are the ones doing the most work. Run your own numbers, including how a lump sum compares against spreading the same total out, with the compound growth calculator.

Run your numbersCoast FIRE→

What actually changes once you've coasted

Reaching your coast number doesn't mean retirement, and it doesn't mean you stop working. It means your current income no longer has to do two jobs at once — covering today's spending and funding a portfolio decades away. Once it only has to do the first job, the range of acceptable income widens considerably.

That's the practical unlock. Someone who has coasted can move to a role that pays less but suits them better, drop from full-time to four days a week, start a business that takes a few years to become profitable, or take a sabbatical-adjacent slower stretch — all without touching the retirement math, because the retirement math was already settled the day the coast number was reached. The only requirement left is that current income covers current living costs; anything above that is a bonus, not an obligation.

This sits between two other variations covered in types of FIRE. Full FIRE requires the portfolio to fund all future spending with no income at all. Barista FIRE draws on the portfolio for part of retirement income now, topped up by part-time work, rather than waiting for the portfolio to finish growing untouched. Coast FIRE is the most forgiving of the three in one specific sense: it asks nothing more of the portfolio and nothing more of current income than to keep pace with current spending — the retirement number is already solved.

Reaching the coast number is a floor, not a ceiling, and nothing about the math requires stopping there. Someone who hits their coast number and keeps contributing anyway either retires earlier than planned or ends up with a larger cushion than the target called for — both are fine outcomes, and plenty of people who coast in principle keep saving in practice, simply with less urgency behind it. What the number changes is not what you're allowed to do, only what you're required to do.

Two ways to be wrong about the return

The coast formula is elegant and also unforgiving — a small change to the assumed real return moves the number by far more than intuition suggests, because it's an exponent, not a multiplier. At age 30, thirty-five years from the same $1,500,000 target:

Two percentage points of assumed return is the difference between needing $271,900 and needing $140,500 — nearly double, for a difference in return assumption that sounds small when you say it out loud.

That asymmetry is the real risk in coasting, more than any claim about market timing. The order good and bad years arrive in doesn't change where a single untouched lump sum ends up — only their compounded product does, and that's fixed by the assumed rate holding over the whole stretch. What changes the outcome is a genuinely below-trend run of years, and a coaster facing one has already given up the option of contributing more to offset it. Someone still saving can respond to a weak market by increasing their rate; someone coasting can only wait longer or resume contributing, which is a fine fallback but only works if the retirement date has some flexibility left in it.

Spending drift undoes the number just as fast

The coast number is derived from the FIRE number, and the FIRE number moves whenever expected retirement spending does — a larger home, a child, relocating somewhere with a higher cost of living, or simply a more expensive version of the life you'd pictured. Raise annual retirement spending from $60,000 to $70,000 — a 17% increase, not an unusual one over a decade or two — and the FIRE number rises from $1,500,000 to $1,750,000, carrying every coast number in the table above up by the same 17%. The $261,200 needed at 35 becomes roughly $304,700.

That's the second way "reached" quietly stops being true: not because the portfolio underperformed, but because the target it was measured against moved. Treat a coast number as provisional rather than permanent, and recheck it at least yearly against a current spending estimate and a current sense of how the portfolio is tracking — the FIRE calculator is the fastest way to rebuild both the target and the timeline together whenever either input changes.

Test your own numbers

The formula only takes three inputs — a FIRE number, a target age, and an assumed real return — and all three are worth pressure-testing rather than taking on faith. Build the FIRE number from an honest year of spending rather than a rough guess, lean conservative on the return, and be specific about the retirement age, since a five-year shift in either direction moves the coast number by a wider margin than it looks like it should. Once those three numbers are set, the coast calculation itself is simple enough to redo in your head — the harder work is keeping the inputs current, not the arithmetic.