Compound Interest Lets Earlier Earnings Join the Work
Year one added $800 to the account. Year thirty added more than $7,400 — same deposit, same rate, and not a dime contributed in between.
That's compound interest: interest calculated on the original money plus all the interest already earned. The mechanism is simple, the effect is dramatic, and it works just as hard against you when the balance is a debt.
The snowball, by the numbers
Start with $10,000 at 8%, compounded annually. Year one earns $800; year two earns $864, because the 8% now applies to $10,800 instead of the original deposit.
Left alone, the balance reaches about $21,589 in 10 years, $46,610 in 20, and $100,627 in 30. The third decade alone adds roughly $54,000 — more than the first two combined — because growth keeps compounding on growth.
The Rule of 72 gives you the shortcut: divide 72 by the rate to estimate doubling time. At 8%, money doubles about every nine years.
What changes the outcome
Three inputs drive everything: rate, time, and compounding frequency. Time is the one beginners underrate — starting ten years earlier routinely beats picking slightly better investments later.
Frequency matters at the margins, since interest added monthly starts earning sooner than interest added yearly. That's why deposit accounts quote APY, a single figure that folds compounding into the annual number.
Regular deposits amplify all of it. Add $200 a month and every new deposit starts its own compounding clock — the earliest ones do the most work.
Contrast all this with simple interest, which pays on principal only. The same $10,000 at a flat 8% earns $800 every year — $24,000 of interest over 30 years, for a $34,000 total instead of roughly $100,627.
The same math in reverse
Carry a credit-card balance and compounding switches sides. Unpaid interest joins the balance, next month's charge lands on the larger figure, and delay gets steadily more expensive.
That's why minimum payments stretch debts across decades. The lender's compounding clock never pauses.
The SEC's saving and investing guidance covers the investor side in more depth.
If you've been waiting for the right moment to start saving, the math just voted for today.