EXPLAINER

Compound Interest, Explained

Published Oct 19, 2026 · Updated Oct 19, 2026

What does starting ten years late actually cost you?

It depends on the rate, but the shape never changes: growth is back-loaded. At 10% a year, $10,000 becomes $452,593 in 40 years, and 62.8% of the gain arrives in the final decade. Start ten years late and you do not lose the first decade. You lose the last one, where most of the money is. Fees compound too: a 1% annual fee consumes $138,498 of that ending value.

Hypothetical illustration at a constant 10% annual return, computed Oct 2026. Not a projection of any investment's return.

The full math

$10,000 growing at a constant 10% a year, compounded annually:

Years invested	Value of $10,000
10 years	$25,937
20 years	$67,275
30 years	$174,494
40 years	$452,593

Growth of $10,000 at 10% a year. The curve looks flat for decades because the early doublings are small in dollars. The final decade adds more than the first three combined.

The last decade does the heavy lifting: 62.8% of the total gain arrives between years 31 and 40. Money growing at 10% doubles every 7.27 years, and every doubling is worth twice the dollar amount of the one before it. Starting late does not remove the cheap early years from the outcome. It removes the expensive final ones.

What a 1% fee does

40 years on $10,000	Ending value
At 10% a year, no fee	$452,593
At 9% a year, after a 1% annual fee	$314,095
Consumed by the fee	$138,498

The fee is quoted as 1% per year, which sounds small. Compounded over 40 years, it consumes $138,498, which is 31.3% of the total gain. The fee compounds on the same curve as the money.

The other direction: losses

Compounding cuts both ways. A 50% loss needs a 100% gain just to get back to even. The path matters as much as the percentages:

Step	Value
Start	$10,000
After a 50% gain	$15,000
After a 50% loss	$7,500

Up 50%, then down 50%, does not break even. It ends 25% below the start, because the loss is taken on the larger balance.

What this does not prove

A constant 10% is an illustration, not a market forecast. Real returns arrive unevenly, and inflation is not deducted anywhere on this page. The math argues for starting early and for watching fees. It does not argue for any specific investment.

Sources and method

Every figure on this page is a locked value from ZeroGamma's compound interest carousel and explainer video, computed in October 2026: annual compounding at a constant rate, no taxes, no contributions or withdrawals, and the fee modeled as a reduction in the annual rate. A hypothetical illustration, not a projection of any investment's return.

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Research and education only. Not investment advice.