Compound Interest Explainer
Compound interest occurs when interest is calculated on both the original principal and previously accumulated interest. That makes growth depend on more than the stated rate. Time, the compounding schedule, fees, deposits, withdrawals, and taxes can all change the ending amount. The same mechanism helps savings grow and makes unpaid debt expand.
For a single deposit, the standard formula multiplies principal by one plus the periodic rate, raised to the number of compounding periods. A $1,000 balance earning 6 percent annually would become $1,060 after one year and earn the next year’s interest on that larger amount. More frequent compounding produces a slightly higher effective return when the stated annual rate is unchanged.
Time is especially important because the process accelerates. Early gains look modest, but later interest is earned on a larger base. Regular contributions add another layer: each deposit compounds for a different length of time. This is why starting earlier can sometimes matter more than making a larger contribution much later, although actual investment returns are uncertain.
Borrowers face the reverse. Credit-card balances, fees, and unpaid interest can compound, increasing the cost of carrying debt. Advertised rates should be compared using an annual percentage yield for deposits or the appropriate annual percentage rate and fee disclosures for loans. Compounding does not remove risk, inflation, taxes, or volatility; it simply describes how balances change when returns or charges are added to the base.
For one deposit, future value equals principal multiplied by one plus the periodic rate, raised to the number of periods.
The rate and period must use matching units. A monthly rate is paired with months, while an annual rate is paired with years.
Each round of interest enlarges the base for the next calculation. The effect becomes more visible over long periods.
Regular deposits can be powerful because earlier contributions compound longer. Withdrawals and fees interrupt that path and reduce the future base.
Compounding grows deposit balances when interest is credited and retained. It also grows debt when charges are added and not paid off.
Compare effective rates and fees, not just a headline percentage. Variable rates can change, and investment returns are not guaranteed.
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