Compound Interest Calculator Online
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What Is Compound Interest — and Why Does It Matter?
Compound interest is the process of earning interest on both your original principal and the interest that has already accumulated. Unlike simple interest, which grows linearly, compound interest grows exponentially — meaning the longer you leave money invested, the faster it grows. This is why financial advisors consistently call it the most powerful force in personal finance.
Worked Example
Suppose you invest $5,000 at a 7% annual interest rate, compounded monthly, for 20 years. Using the formula A = P(1 + r/n)^(nt): A = 5000 × (1 + 0.07/12)^(12×20) = $19,898.60. Your $5,000 grew by nearly $15,000 — without adding a single extra dollar. If you had used simple interest instead, you would have earned only $7,000 in interest ($12,000 total).
Compounding Frequency Comparison ($10,000 at 6% for 10 years)
| Frequency | Final Value | Interest Earned |
|---|---|---|
| Annually (1×/year) | $17,908.48 | $7,908.48 |
| Semi-annually (2×/year) | $18,061.11 | $8,061.11 |
| Quarterly (4×/year) | $18,140.18 | $8,140.18 |
| Monthly (12×/year) | $18,193.97 | $8,193.97 |
| Daily (365×/year) | $18,220.40 | $8,220.40 |