Compound Interest Calculator

Compound interest is the process of earning interest on both your original principal and the interest that has already been added to your balance. Unlike simple interest, which is calculated only on the initial amount, compound interest accelerates the growth of your money over time.

Albert Einstein reportedly called compound interest the "eighth wonder of the world." Whether or not that attribution is accurate, the math behind it is powerful. A $10,000 investment earning 7% annually grows to about $19,672 after 10 years, and to $76,123 after 30 years, without any additional contributions. Add $200 per month and that 30-year figure jumps to over $317,000.

The key insight is that compound interest rewards patience. The longer your money stays invested, the more dramatic the growth becomes. This exponential curve is why financial advisors stress the importance of starting to invest early, even with small amounts.

Compounding frequency determines how often interest is calculated and added to your balance. The most common frequencies are:

• Annually (1x/year): Interest is calculated once at the end of each year.
• Quarterly (4x/year): Interest is calculated every three months.
• Monthly (12x/year): Interest is calculated every month.
• Daily (365x/year): Interest is calculated every day, common with savings accounts.

More frequent compounding means interest is added to your balance sooner, which in turn earns interest itself. However, the practical difference between monthly and daily compounding is often small. For example, $10,000 at 7% for 10 years yields $19,672 with annual compounding and $20,097 with monthly compounding, a difference of about $425.

Most bank savings accounts compound daily, while many investment returns are effectively compounded based on market performance. When comparing financial products, look at the Annual Percentage Yield (APY), which accounts for compounding frequency and provides a standardized rate for comparison.

While a lump-sum investment benefits from compound interest, adding regular monthly contributions dramatically accelerates growth. This strategy is known as dollar-cost averaging and has two major benefits:

• Consistent growth: Regular deposits ensure your investment base keeps growing regardless of market conditions.
• Reduced timing risk: By investing a fixed amount regularly, you buy more shares when prices are low and fewer when prices are high, averaging out your cost over time.

Consider this comparison over 20 years at 7% annual return: a one-time $10,000 investment grows to about $38,697. But if you also contribute $200 per month, the total reaches $142,436. Your contributions total $58,000, but compound interest adds an additional $84,436 in earnings.

The earlier you start contributing, the more time compound interest has to work. Someone who invests $200 per month starting at age 25 will accumulate significantly more by retirement than someone who starts the same contributions at age 35, even though the difference in total contributions is only $24,000.