Compound Interest
Calculator USA 2026

Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, which is only calculated on the original amount, compound interest means your money earns interest on interest. Over long time horizons this creates exponential growth, which is why Albert Einstein reportedly called it the eighth wonder of the world.

Compound interest works the same way globally. American investors encounter it in savings accounts (compounded daily by most banks), certificates of deposit (CDs), 401(k) and IRA investments (compounded through reinvested returns), and brokerage accounts. US high-yield savings accounts offer around 4.5% to 5.0% APY in 2026, while the S&P 500 has historically returned roughly 10% annually over the long term.

The Rule of 72 is a simple formula to estimate how many years it takes for an investment to double in value. Divide 72 by the annual interest rate to get the approximate doubling time. At 7% per year, your money doubles in roughly 10.3 years. At 10%, about 7.2 years. At 12%, roughly 6 years. This rule works best for rates between 2% and 15%.

APR (Annual Percentage Rate) is the stated interest rate without compounding. APY (Annual Percentage Yield) includes the effect of compounding and represents the actual return you earn in a year. A savings account with 5.0% APR compounded daily has an APY of 5.13%. When comparing savings accounts, always compare APY, not APR, as it reflects the true return.

Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus accumulated interest. For example, $10,000 at 7% simple interest earns $700 every year. With compound interest, year one earns $700, year two earns $749 (7% of $10,700), and so on. Over 20 years, simple interest gives $24,000 while compound interest gives approximately $38,697.

At 5% compounded monthly with no extra contributions: $10,000 becomes $16,470 after 10 years, $27,126 after 20 years, and $44,677 after 30 years. At 10% (approximate S&P 500 average): $10,000 becomes $27,070 after 10 years, $73,281 after 20 years, and $198,374 after 30 years. Adding $500 per month at 10% over 20 years turns your total contributions of $130,000 into approximately $416,000.

The best rate depends on your risk tolerance. High-yield savings accounts offer up to 5.0% APY (low risk). CDs offer 4.0-5.0% locked in (low risk). US Treasury bonds offer 4.0-4.5% (virtually no risk). The S&P 500 has returned roughly 10% annually long-term (higher risk). 401(k) target-date funds typically return 7-9% over long periods. Higher returns always come with higher short-term volatility.

Your 401(k) contributions are invested in funds that grow through compound returns. When dividends and capital gains are reinvested (which happens automatically in a 401(k)), those returns generate their own returns. With a $19,500 annual contribution and 8% average return, a 25-year-old would have over $1 million by age 55 and nearly $2.5 million by age 65, with compound growth responsible for the vast majority of the final balance.

Yes, compound returns apply to stocks and ETFs through capital growth (price appreciation) and reinvested dividends. When you reinvest dividends, you buy more shares which generate their own dividends and growth, creating a compounding effect. The S&P 500 total return (including reinvested dividends) has averaged roughly 10% per year since 1926.

Inflation erodes the real purchasing power of your returns. If your investment earns 7% and inflation is 3%, your real return is roughly 4%. When planning for long-term goals, subtract the expected inflation rate (historically about 3% in the US) from your nominal return to estimate your real, inflation-adjusted growth. This is especially important for retirement planning.