Get exact numbers for your situation in seconds. Free, no signup.
Open the Compound Interest Calculator →Albert Einstein allegedly called compound interest “the eighth wonder of the world”. Whether or not he actually said it, the sentiment is accurate. Compound interest is the single most powerful force in personal finance — and understanding it can transform the way you save and invest.
Simple interest is calculated only on your original principal. If you invest $10,000 at 7% simple interest for 10 years, you earn $700 per year — a total of $7,000 in interest.
Compound interest is calculated on your principal plus all previously earned interest. Each year, your interest earns interest. Over time, this creates an exponential growth curve rather than a straight line.
| Year | Simple Interest ($10,000 @ 7%) | Compound Interest ($10,000 @ 7%) |
|---|---|---|
| 5 | $13,500 | $14,026 |
| 10 | $17,000 | $19,672 |
| 20 | $24,000 | $38,697 |
| 30 | $31,000 | $76,123 |
| 40 | $38,000 | $149,745 |
The same $10,000 invested at 7% compound interest grows to nearly $150,000 over 40 years — without adding a single extra dollar.
A = P(1 + r/n)^(nt)
Where: A = final amount, P = principal, r = annual rate, n = compounding frequency per year, t = years
With monthly compounding (n=12), $10,000 at 7% for 30 years: A = 10,000 × (1 + 0.07/12)^(12×30) = $81,165. More frequent compounding produces slightly higher returns.
A quick mental shortcut: divide 72 by your annual interest rate to estimate how many years it takes to double your money.
Consider two investors. Alice invests $300/month from age 25 to 35 (10 years), then stops — never contributing another cent. Bob waits until 35 and invests $300/month all the way to age 65 (30 years). Both earn 7% annually. Who has more at 65?
| Investor | Total Contributed | Balance at 65 |
|---|---|---|
| Alice (invests 25–35, stops) | $36,000 | ~$338,000 |
| Bob (invests 35–65) | $108,000 | ~$303,000 |
Alice contributed three times less money but ends up with more — purely because she started 10 years earlier. This is the power of compounding time.
The more frequently interest compounds, the faster your money grows. $10,000 at 7% for 20 years:
The difference between monthly and daily is small. The difference between starting now and starting in 5 years is enormous.
Enter any amount, interest rate, and time period to see exactly how compound interest builds your wealth — with a year-by-year breakdown.
Try the Compound Interest Calculator →The same force that builds wealth through investing works against you in debt. Credit card interest compounded monthly at 20% APR on a $5,000 balance — making only minimum payments — can take over 10 years to repay and cost more than the original balance in interest. Understanding compounding is just as important for managing debt as it is for growing savings.