What is a coast number?

The amount that needs to be invested today for it to grow, on its own, to a retirement target by a chosen date — with no further contributions. It is target ÷ (1 + real return)^years. For a $1,000,000 target thirty years out at a 5% real return, it is $231,377.45.

Is reaching a coast number the same as being able to retire?

No, and this is the most common misreading. Passing it means further saving becomes optional, not that working becomes optional. You still have to cover rent, food and everything else between now and retirement out of income. It is a milestone about contributions, not about employment.

Why does a coast number fall so fast the further out you are?

Because it is divided by (1 + r) raised to the number of years, so it decays geometrically. At 5% real, a $1,000,000 target needs $613,913.25 ten years out but only $142,045.68 forty years out — a quarter as much for four times the horizon. Time does most of the work, which is why the figure looks small at 25 and uncomfortable at 55.

How much does the return assumption change the answer?

Enormously, and this is the honest weakness of the whole idea. For the same $1,000,000 target thirty years out: 7% gives $131,367.12, 5% gives $231,377.45, and 3% gives $411,986.76. The 3% figure is more than three times the 7% one. A coast number is only as trustworthy as the rate you assumed.

Should the return be real or nominal?

Real — after inflation — if your target is stated in today’s money, which is how most people think about spending. Mixing a nominal return with a target in today’s prices produces a number that looks reassuring and buys less than expected. Our calculator asks for a real return and says so on the field.

Does a coast number account for sequence-of-returns risk?

No. It is a single discount at a constant rate, so it cannot express the risk that returns arrive in an unhelpful order. That risk is real and measurable — the same ten annual returns in opposite orders can end $262,120.48 apart on a drawdown portfolio. A coast number is a comparison tool, not a forecast.

Derivation

What a coast number is, and what it is not

It is one division. Passing it means further saving becomes optional — not that working does, which is the misreading that makes the idea sound better than it is.

The short version

coast number = target ÷ (1 + real return)^years. For a $1,000,000 target thirty years out at a 5% real return, that is $231,377.45. It is exquisitely sensitive to the rate you assume: at 3% the same target needs$411,986.76, more than three times the 7% figure.

The arithmetic

A coast number answers a narrow question: how much has to be invested today for compounding alone to carry it to a target by a chosen date? It is the compound interest formula rearranged — the present value of a future amount.

future value = present × (1 + r)ⁿ
so present = future ÷ (1 + r)ⁿ

That is the entire calculation. For a $1,000,000 target at a 5% real return: 1,000,000 ÷ 1.05³⁰ = $231,377.45. You can check it in a spreadsheet in ten seconds with =1000000/1.05^30, which is exactly what our verification page asks readers to do.

How fast it falls with time

The denominator is raised to the number of years, so the figure decays geometrically. This is the part worth internalising:

Years to retirement	Needed today at 5% real
40 years	$142,045.68
30 years	$231,377.45
20 years	$376,889.48
10 years	$613,913.25
5 years	$783,526.17
Today (no growth left)	$1,000,000.00

Forty years out, $142,045.68 reaches a million. Ten years out it takes $613,913.25 — more than four times as much for a quarter of the time. Compounding does the work, and it does most of it early, which is why the same target feels trivial at 25 and punishing at 55.

How much the return assumption moves it

Here is the weakness, stated plainly rather than buried. The same $1,000,000 target, the same thirty years, different assumed real returns:

Assumed real return	Coast number
7%	$131,367.12
6%	$174,110.13
5%	$231,377.45
4%	$308,318.67
3%	$411,986.76

$131,367.12 at 7%. $411,986.76 at 3%. The pessimistic assumption needs more than three times the capital of the optimistic one, for the same target and the same horizon. A coast number is not a fact about your finances; it is a fact about your assumption, and it inherits all of that assumption's uncertainty.

This is why the return field on our calculator is editable and why it asks for a real return. If you enter a nominal 8% while thinking about spending in today's prices, the number you get back is optimistic twice over — once for the rate and once for the inflation you forgot to remove. That distinction is worked through in why a 7% return is not 0.583% a month.

What passing it does not mean

It does not mean you can stop working. It means you can stop adding to the investments. Rent, food, insurance and every other cost between now and retirement still has to come from somewhere, and for almost everyone that somewhere is income. Coast FIRE is a statement about contributions, and it is routinely quoted as though it were a statement about employment.

It does not mean the target is right. The target itself usually comes from a withdrawal rate applied to expected spending, and both of those are guesses about a life you have not lived yet. Where the 4% rule comes from covers how much weight that conversion can actually bear.

It does not account for the order of returns. A coast number is a single discount at a constant rate. Real returns arrive unevenly, and the order matters: the same ten annual returns in opposite orders can leave a drawdown portfolio$262,120.48 apart. A smooth curve cannot show that, and its confidence is a property of the arithmetic rather than of markets.

It does not include tax, fees or a pension. Platform and fund charges reduce your real return, so they should be subtracted from the rate you enter. State or workplace pensions arriving later reduce what the pot must cover, which the model does not know about. Both move the honest answer, in opposite directions.

Why it is still worth computing

Because it is the rare retirement number that depends on very few guesses. A full retirement projection needs assumptions about future income, future saving rates, career length, spending changes and market returns. A coast number needs three inputs and one division.

That makes it useful for the thing it is actually good at: comparing scenarios against each other. Whether an extra five years of contributions matters, what happens if you assume 4% instead of 6%, how much the target moves if spending drops by $5,000 a year. All of those are answerable, and all of them are more honest questions than "when can I retire".

Our Coast FIRE calculator computes it year by year, shows the projection against the target, and exports the table. Every figure on this page is reproducible with a pocket calculator, which is the only reason to believe any of it.

Common questions

What is a coast number?: The amount that needs to be invested today for it to grow, on its own, to a retirement target by a chosen date — with no further contributions. It is target ÷ (1 + real return)^years. For a $1,000,000 target thirty years out at a 5% real return, it is $231,377.45.
Is reaching a coast number the same as being able to retire?: No, and this is the most common misreading. Passing it means further saving becomes optional, not that working becomes optional. You still have to cover rent, food and everything else between now and retirement out of income. It is a milestone about contributions, not about employment.
Why does a coast number fall so fast the further out you are?: Because it is divided by (1 + r) raised to the number of years, so it decays geometrically. At 5% real, a $1,000,000 target needs $613,913.25 ten years out but only $142,045.68 forty years out — a quarter as much for four times the horizon. Time does most of the work, which is why the figure looks small at 25 and uncomfortable at 55.
How much does the return assumption change the answer?: Enormously, and this is the honest weakness of the whole idea. For the same $1,000,000 target thirty years out: 7% gives $131,367.12, 5% gives $231,377.45, and 3% gives $411,986.76. The 3% figure is more than three times the 7% one. A coast number is only as trustworthy as the rate you assumed.
Should the return be real or nominal?: Real — after inflation — if your target is stated in today’s money, which is how most people think about spending. Mixing a nominal return with a target in today’s prices produces a number that looks reassuring and buys less than expected. Our calculator asks for a real return and says so on the field.
Does a coast number account for sequence-of-returns risk?: No. It is a single discount at a constant rate, so it cannot express the risk that returns arrive in an unhelpful order. That risk is real and measurable — the same ten annual returns in opposite orders can end $262,120.48 apart on a drawdown portfolio. A coast number is a comparison tool, not a forecast.

Sources

US Securities and Exchange Commission — Compound Interest Calculator — retrieved 2026-08-08
Microsoft — FV function (Excel) — retrieved 2026-08-08
Every figure here is target ÷ (1 + r)ⁿ on a $1,000,000 target. The 30-year, 5% row — $231,377.45 — is the same figure the verification page asks readers to reproduce with =1000000/1.05^30, and the same one our calculator returns.

Written and maintained by Vikash Singh. Last verified 2026-08-08.