Compound Interest Calculator

Compound interest has been called "the eighth wonder of the world" and "the most powerful force in the universe"—attributed (perhaps apocryphally) to Albert Einstein. Whether or not Einstein said it, the sentiment captures an important truth: compound interest can turn modest savings into substantial wealth over time, or conversely, turn small debts into overwhelming burdens.

Our Compound Interest Calculator helps you visualize the growth of your investments or savings over time. It supports various compounding frequencies, optional monthly contributions, and provides a year-by-year breakdown showing exactly how your money grows.

Understanding Compound Interest

Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. This creates exponential growth: your interest earns interest, which then earns more interest, and so on. The formula is A = P(1 + r/n)^(nt), where A is the final amount, P is principal, r is the annual rate, n is compounding frequency per year, and t is time in years.

The key difference from simple interest is that compound interest accelerates over time. In year one, you earn interest only on your principal. In year two, you earn interest on your principal plus year one's interest. This snowball effect becomes increasingly powerful over long time periods.

Compounding Frequency Matters

The more frequently interest compounds, the more you earn. The same 5% annual rate yields different results depending on compounding:

• Annually (once/year): $10,000 becomes $10,500
• Monthly (12 times/year): $10,000 becomes $10,511.62
• Daily (365 times/year): $10,000 becomes $10,512.67

The difference seems small for one year, but over decades, more frequent compounding makes a noticeable difference. This is captured by the Effective Annual Rate (EAR), which shows what annual rate would give the same result with annual compounding.

The Power of Regular Contributions

While compound interest is powerful on its own, combining it with regular contributions creates even more impressive results. Our calculator includes an optional monthly contribution field to model regular savings.

Consider the difference: investing $10,000 at 7% for 30 years grows to about $76,000. But investing $10,000 plus $500/month at the same rate grows to over $605,000. The regular contributions contribute $180,000, but the compound growth adds another $425,000.

The Rule of 72

A quick mental shortcut: divide 72 by your interest rate to estimate how many years it takes to double your money. At 6% interest, money doubles in about 12 years (72/6). At 8%, it doubles in about 9 years. This approximation is remarkably accurate for rates between 6% and 10%.

Real-World Applications

Compound interest applies to many financial situations:

Retirement Accounts: 401(k)s and IRAs grow through compound interest over decades. Starting early is crucial because time is the key ingredient—someone who starts saving at 25 will likely accumulate more than someone who starts at 35, even if the latter saves more per month.

Credit Card Debt: The dark side of compounding. Credit card interest compounds daily, turning small balances into large debts quickly if not paid off. A $5,000 balance at 20% APR grows by over $1,000 in the first year alone.

Mortgages and Loans: Understanding how amortization works helps you see how early extra payments save more than later ones—because they stop that principal from compounding.

Privacy

All calculations happen locally in your browser. Your financial projections are never sent to any server or stored anywhere. You can plan your financial future with complete privacy.