Compound interest is interest calculated not only on your original principal but also on the interest already added to it. Because each round of interest earns interest of its own, balances grow at an accelerating pace over time, a snowball effect that rewards patience and works powerfully in your favor when saving.
How does compound interest work?
With compound interest, every interest payment gets added to your balance and becomes part of the base for the next calculation. In year one you earn interest on your deposit; in year two you earn interest on the deposit plus year one's interest; and so on. The effect is small at first and dramatic over long horizons, which is why it is often called the eighth wonder of finance.
Two levers control how fast it snowballs: the rate and the compounding frequency. More frequent compounding, daily rather than annually, squeezes out slightly more growth, which is exactly what APY measures.
The compound interest formula and a worked example
The formula is A = P(1 + r/n)^(nt), where P is the principal, r the annual rate, n the compounding periods per year, and t the number of years. Say you deposit $10,000 (P) at 5% (r = 0.05), compounded annually (n = 1), for 10 years (t = 10):
- Growth factor: (1 + 0.05)^10 = 1.6289
- Ending balance: $10,000 × 1.6289 = $16,289
- Total interest earned: about $6,289
By contrast, simple interest at the same 5% would pay a flat $500 a year, or $5,000 over the decade, nearly $1,300 less. Stretch it to 30 years and the compounded balance grows to roughly $43,000, while simple interest reaches only $25,000. (Illustrative figures, not current rates.)
Compound vs simple interest
The difference between compounding and simple interest is whether past interest earns future interest. Simple interest always calculates from the original principal alone, so it grows in a straight line. Compound interest curves upward, and the gap between the two widens the longer money stays invested.
- Simple interest: linear growth, interest only on principal, predictable and modest.
- Compound interest: exponential growth, interest on principal plus accumulated interest.
- The longer the time horizon, the more dramatically compounding pulls ahead.
Compounding is a double-edged sword. On a high-yield savings account or CD it builds your wealth; on a credit card balance it works against you, growing what you owe just as relentlessly.
Why compound interest matters, and who benefits
Compound interest rewards time more than any other factor, which is why starting early beats saving more later. A modest sum left to compound for decades can outgrow a much larger sum invested closer to when you need it. For diaspora families building wealth across two countries, the lesson is the same: money that stays invested and keeps reinvesting its returns quietly does the heavy lifting.
The catch is that inflation compounds too, eroding purchasing power in the background. To actually get ahead, your compounded return needs to outpace inflation, otherwise your balance grows in dollars but stands still in real value.
Pros | Cons |
|---|---|
Accelerating, exponential growth on money left untouched. | Works against you on debt, ballooning credit card balances. |
Rewards starting early, time is the most powerful ingredient. | Early years feel underwhelming; the payoff is back-loaded. |
Reinvested returns require no extra effort once set up. | Withdrawals interrupt the snowball and reset its momentum. |
Frequently asked questions
The rule of 72 is a quick mental shortcut for how long compounding takes to double your money: divide 72 by your annual rate. At 6%, money doubles in roughly 72 ÷ 6 = 12 years. It's an approximation, but a handy way to sense the power of a given rate without a calculator.
It varies by product. Savings accounts often compound daily or monthly, CDs monthly or at maturity, and some bonds semi-annually. More frequent compounding produces slightly more growth, which is why APY, which reflects compounding frequency, is the fairest way to compare accounts.
Because compounding is exponential, the earliest contributions have the most years to snowball. A dollar invested at 25 can grow far larger by retirement than a dollar invested at 40, even though it's the same dollar. Time in the market, not timing the market, drives compound growth.
Yes. Credit cards and some loans compound the interest you owe, so unpaid interest gets added to your balance and then accrues more interest. This is why high-interest debt grows so quickly and why paying it down early saves so much.
It can erode it. If your investment compounds at 5% but inflation runs at 5%, your real purchasing power barely grows despite the larger balance. To build genuine wealth, your compounded return needs to exceed the inflation rate over time.
Updated July 21, 2026
Disclaimer
Dara provides this glossary for general educational purposes only. It is not financial, legal, or tax advice, and availability, fees, and terms vary by country and corridor.
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