Albert Einstein reportedly called compound interest the eighth wonder of the world. Whether he actually said it or not, the concept is truly remarkable. Compound interest is the process by which your money earns interest on previously earned interest, creating exponential growth over time.
This guide explains everything you need to know about compound interest, including how to use our compound interest calculator to project your investment growth and plan for your financial future.
Compound interest is interest calculated on both your initial principal and the accumulated interest from previous periods. Unlike simple interest, which only earns interest on the principal, compound interest grows at an accelerating rate because each period's interest calculation includes the interest already earned.
For example, if you invest $10,000 at 7% annual compound interest, after one year you have $10,700. In the second year, you earn 7% on $10,700 (not just the original $10,000), giving you $11,449. Over 30 years, that same $10,000 grows to over $76,000 without adding any additional money.
The frequency with which interest compounds has a significant impact on your total returns. Common compounding frequencies include:
Our compound interest calculator online lets you choose any compounding frequency so you can see exactly how different schedules affect your investment growth.
The single most important factor in compound interest is time. Starting early gives your money more time to compound, and the difference of just a few years can mean hundreds of thousands of dollars at retirement.
Consider two investors: Alice starts investing $5,000 per year at age 25 and stops at age 35 (10 years of contributions). Bob starts at age 35 and invests $5,000 per year until age 65 (30 years of contributions). Assuming 7% annual returns:
Alice ends up with more money despite contributing one-third as much, simply because she started ten years earlier. Use our investment calculator compound interest tool to see how starting earlier affects your own numbers.
While time is the most powerful factor in compounding, adding regular contributions supercharges your growth. Even small monthly additions make a dramatic difference over long periods. Adding just $100 per month to a $10,000 investment at 7% interest adds over $100,000 to your total after 30 years compared to not contributing regularly.
Compound interest works against you when it comes to debt. Credit cards, payday loans, and some other types of debt use compound interest, which can cause balances to balloon rapidly if not paid off. This is why high-interest debt is so dangerous and why financial advisors recommend paying it off as quickly as possible. Compare your debt situation with our mortgage calculator to see how interest affects long-term loans.