APY Calculator

Understanding APY and Compounding

When a bank advertises a 5% interest rate compounded monthly, you don't earn exactly 5% over the year. You earn slightly more because interest is calculated and added to your account every month. The next month, you earn interest on the new, higher balance.

APY captures this compounding effect. The formula is APY = (1 + r/n)^n - 1, where r is the interest rate and n is the number of compounding periods per year. Daily compounding (n=365) produces the highest APY from a given interest rate, followed by monthly (n=12), quarterly (n=4), and annual (n=1).

The difference between the stated rate and APY grows as the rate increases. At 1%, daily compounding adds only 0.005% to the APY. At 10%, it adds over 0.5%. The boost is proportional to the rate because higher rates generate more interest available for compounding.

Why Banks Advertise APY

Federal regulations require banks to disclose APY for all deposit accounts. This levels the playing field for consumers. Without APY, one bank might advertise 4.5% compounded daily while another shows 4.55% compounded annually. Comparing those directly is misleading because the first option actually earns more.

APY removes that ambiguity. The 4.5% compounded daily becomes 4.60% APY, clearly beating the 4.55% APY of the second account. Consumers can confidently compare numbers without doing compound interest math.

This transparency benefits savers. It also pushes banks toward more frequent compounding. Since APY must be disclosed, banks that compound daily can advertise slightly higher APY from the same base rate, giving them a competitive edge at minimal cost.

APY vs. APR: Opposite Perspectives

APY measures earnings on savings and investments. APR measures costs on loans and credit cards. Both account for compounding, but from opposite perspectives. With APY, compounding works in your favor, boosting returns. With APR, compounding works against you, increasing costs.

The formulas are similar, but APR adds complexity for fees and different payment structures. For variable-rate loans or credit cards with changing balances, APR becomes an approximation. APY is more straightforward because savings accounts typically have stable rates and predictable compounding.

When evaluating financial products, always use the right metric. Compare savings accounts by APY and loans by APR. Mixing the two creates confusion and leads to poor decisions. Some marketers exploit this by emphasizing whichever number looks more favorable.