Compound Interest
Compound interest is interest calculated on both your original principal and the interest already earned. Because each period's earnings are added to the balance, the next period earns interest on a larger amount — so growth accelerates over time instead of staying flat, which is what makes long-term saving so powerful.
The Rule of 72 gives a quick doubling time: divide 72 by the annual rate. At 7% that is 10.3 years against a true 10.24 — close enough for mental arithmetic, though it drifts at the extremes. The acceleration is what people underestimate. A single $10,000 investment at 7% reaches about $19,672 after ten years, $38,697 after twenty, $76,123 after thirty and $149,745 after forty — and almost half of that final figure arrives in the last decade alone. Those are the years you can only obtain by having started earlier, which is why time matters more than the amount. The same mechanism runs against you on a credit card balance.
Put it to work
Compound Interest Calculator
See how your savings grow with compound interest and monthly contributions — final balance, interest earned, and a year-by-year growth table.
Savings Calculator
Watch your savings account grow: enter your balance, monthly deposit, and APY to see the future value and every dollar of interest along the way.
Investment Calculator
Project an investment portfolio's growth with monthly contributions — final value, your money vs market growth, and the year-by-year path.