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What Is a Coast Number? The Amount That Lets You Stop Saving

What it means, how it connects to your coast age, how the math works, and why it matters.

By J.J. Allan LLC · Published October 17, 2026 · About 9 min read

Your coast number is the amount you need invested for retirement today so that, if you never contributed another dollar, ordinary investment growth would carry it to your full retirement target by the age you plan to retire. If your savings are at or above that amount, you can coast. If they are below it, the difference is the gap you still have to close.

The coast number has a close partner: your coast age, the age at which you can stop contributing and still reach your retirement target through compound growth alone. The two answer the same question from different angles. The coast number tells you how much you need; the coast age tells you when your savings will get there. This guide explains how each is calculated, how the Coast Clock Calculator works them out, and why the number is worth knowing even if you never plan to stop saving.

The short definition

Every retirement plan has an end target: the portfolio you need on the day you retire to fund the rest of your life. In the FIRE community that is usually called your FIRE number, and in the Coast Clock Calculator we call it your retirement number. (For background on that target and the research behind it, see our guide, What Is FIRE?)

The coast number is that same target, pulled back to the present. It asks: "How much would I need invested right now for growth alone to turn it into my retirement number by my retirement age?" Because money invested today has years to compound, the coast number is always smaller than the retirement number, and often much smaller. The more years you have left, the smaller it is.

Reaching your coast number is what people mean when they say they've hit Coast FIRE. You still work to cover today's living costs. What changes is that saving for retirement becomes optional.

Coast number versus coast age

People often describe the coast idea in terms of age: the point in your life when you can stop contributing and let compounding do the rest. That is your coast age, and it is a perfectly good way to think about it. On our site we keep the two terms separate, because they measure different things:

The link between them is that the coast target isn't fixed. Each year you wait, you have one year less of growth ahead of you, so the amount you need invested rises. Your savings rise too, through contributions and returns. Your coast age is simply the moment your savings line catches the rising target line. If your savings already exceed today's coast number, your coast age is now.

How the math works

The coast number rests on two calculations: the size of the portfolio you'll eventually need, and how much you need today for growth to get you there. The Coast Clock Calculator uses the same method you could do with a pencil, and it shows the formula in its own FAQ.

Step 1: Your retirement number

Your full target is your expected annual retirement spending divided by a withdrawal rate:

Retirement number = annual spending ÷ withdrawal rate

At the commonly used 4% rate, that works out to 25 times annual spending. The 4% figure traces to William Bengen's 1994 research and the 1998 Trinity Study, both of which tested withdrawals against historical U.S. market returns. In the calculator, the withdrawal rate defaults to 4% and can be changed in the advanced options.

Step 2: Your real return

We keep every figure in today's dollars, so we use a real (after-inflation) return rather than the headline return you might see quoted for the stock market:

Real return = (1 + expected return) ÷ (1 + inflation) − 1

With the calculator's starting values of a 7% expected return and 3% inflation, the real return is about 3.9% a year. Working in today's dollars means your retirement number reads as purchasing power you'd recognize now, not an inflated future figure that is hard to picture.

Step 3: Your coast number

Finally, work backward from the target. If your investments grow at real return r for n years until retirement, the amount you need today is:

Coast number = retirement number ÷ (1 + r)ⁿ

That is the whole idea: divide the future target by the growth you expect between now and then. Compound interest, as the SEC's Investor.gov calculators illustrate, is what makes the divisor large when n is large.

A worked example (illustrative only)

Here are the calculator's starting values: a 30-year-old earning $70,000 take-home, spending $40,000 a year, with $100,000 already invested, planning to retire at 65, with a 7% expected return, 3% inflation and a 4% withdrawal rate.

This person has $100,000 invested, so they are about $164,000 short of coasting today. But they are also saving the difference between income and spending, about $2,500 a month. Keep that up, and their savings catch the rising coast target at about age 36. That is their coast age: after it, they could stop contributing and, if the assumptions hold, still arrive at roughly $1 million by 65.

Notice how the coast target moves with time. At a 3.9% real return, the same $1 million goal needs about $319,000 invested at 35, $386,000 at 40, $467,000 at 45 and $565,000 at 50. Waiting doesn't just delay the milestone; it raises the bar.

The assumed return matters just as much. Holding everything else the same:

These figures are illustrative, not projections or recommendations. Actual returns vary year to year, and an average return over decades can hide long stretches of poor performance.

How the Coast Clock Calculator finds it

The Coast Clock Calculator runs the steps above every time you move a slider, and the math runs in your browser, so nothing is sent to us. Here is what it does, in plain terms:

The advanced options can change the target itself. If you enter a mortgage that will be paid off, we take the freed-up payment out of your retirement spending and assume it goes to savings after payoff; if the mortgage runs past retirement, we add enough to the retirement number to cover the remaining payments. A planned downsizing adds a lump sum in the year you sell, which lowers how much you need invested before then. If at your current pace you wouldn't reach your coast number before about age 80, or before your retirement age at all, the calculator flags it so you can see which changes move the clock most.

Why the coast number matters

Most retirement advice gives you a single huge target, often seven figures, that can feel impossibly far away. The coast number breaks that target into something closer and more concrete.

Common misconceptions

"My coast number is fixed." It isn't. It rises every year you get closer to retirement, and it shifts whenever your spending, retirement age or return assumptions change. It's worth rechecking at least once a year.

"Hitting the number means I can stop working." Coasting assumes you keep earning enough to cover current living costs without touching your investments. If you start drawing on the portfolio, you're no longer coasting, and the math changes. The calculator flags it when your spending is higher than your take-home income.

"Higher return assumptions are harmless." Small changes compound into large differences. In the example above, moving from a 6% to an 8% expected return nearly halves the coast number. Optimistic assumptions make the milestone look closer than it is.

"I should lower my spending to account for Social Security." Please don't do this in the Coast Clock Calculator. Your spending figure also sets how much you save today, so the benefit would be counted twice. We don't model retirement income separately yet, so the calculator's result is the cautious version.

Risks and caveats

Smooth returns are a simplification. We assume steady growth at your expected rate minus inflation. Real markets go up and down, and a long downturn soon after you stop contributing can leave you well behind schedule.

Taxes and fees. The calculator doesn't fully model taxes or investment fees. A dollar in a traditional 401(k) or IRA will be taxed when withdrawn, so your spending figure should allow for the taxes you expect to owe, and fees reduce your net return.

Fees matter more than they look, because they come straight out of the real return the coast number depends on. John C. Bogle's The Little Book of Common Sense Investing makes the case for keeping costs low with broad index funds, and it shows how even small annual fees compound over decades.

Withdrawal-rate uncertainty. The 4% rule was developed largely for 30-year retirements using historical U.S. data. A longer retirement or lower expected returns may call for a more conservative rate, which raises both your retirement number and your coast number.

Life changes. Children, caregiving, health costs and shifting goals all affect long-range plans. The coast number works best alongside an emergency fund and a willingness to resume saving if circumstances change.

The bottom line

Your coast number is your retirement target translated into today: the amount that, left alone to compound, should grow into what you need by the time you retire. Your coast age is when your savings are on track to reach it. Together they turn a distant seven-figure goal into a nearer milestone you can actually watch approach.

To see both for your own situation, visit Coast Clock or go straight to the Coast Clock Calculator, move the sliders and watch where your savings line crosses the coast target.

Further reading

If you want one book on building the savings that get you to your coast number, J.L. Collins's The Simple Path to Wealth is a good place to start. It makes a plain-spoken case for simple, low-cost index fund investing and avoiding debt, the approach much of the FIRE community relies on.

Once you reach your coast number, the hard part is leaving it alone for years while compounding does its work. Morgan Housel's The Psychology of Money explains why patience matters more than cleverness, with a memorable chapter on how most of compounding's results arrive late.

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This article is for general education only and is not financial, tax or legal advice. Withdrawal rates, returns and tax rules vary, and past market performance doesn't guarantee future results. For decisions about your own money, consult a licensed financial professional.

Sources

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