What is APY (Annual Percentage Yield) and How is it Calculated?
APY, or Annual Percentage Yield, shows how much a deposit can grow over an annual period after compounding is included. It turns the stated interest rate into a more complete yield by counting interest earned on earlier interest, not just interest earned on the original balance. That makes APY the clearest way to compare savings accounts and certificates of deposit, often called CDs or fixed-term deposits, when the products compound interest at different intervals.
What APY measures
A stated interest rate tells you the basic rate an account uses to calculate interest. APY tells you the result after that interest has had time to compound.
Compounding means interest gets added to your balance, then that larger balance earns interest later. The more often this happens during the annual period, the more chances your interest has to earn its own return. APY captures that effect in a single percentage.
This is why APY is usually more useful than the stated rate when you compare deposit accounts. If providers advertise the same stated rate but compound at different intervals, the account with more frequent compounding can produce a higher APY. The stated rate looks the same. The growth is not.
How compounding frequency affects APY
Compounding frequency is how often interest is added to the account balance. It might happen at a regular interval such as monthly, quarterly, or daily, depending on the product terms.
The pattern is simple. When interest is credited more often, it starts earning interest sooner. That raises the APY above the stated rate, unless interest is credited only at the end of the annual period.
The effect is real, but it is not unlimited. Moving from infrequent compounding to more frequent compounding can make a noticeable difference. After compounding is already frequent, each extra interval usually adds less. APY helps you see the final effect without comparing all the mechanics yourself.
How APY is calculated
APY is calculated by taking the stated annual rate, splitting it into the rate used for each compounding interval, applying that interval rate across the full annual period, and then expressing the ending growth as a percentage of the starting balance.
In plain terms:
- start with the stated annual interest rate
- divide it across the compounding intervals used by the account
- apply each interval’s interest to the growing balance
- compare the final balance with the starting balance
- express that growth as the APY
The important detail is that each interval builds on the balance left by the previous interval. APY is not just the stated rate repeated in another format. It is the stated rate after the compounding schedule has done its work.
Why APY is the comparison measure
APY is common on savings accounts and CDs because it lets you compare true deposit growth more fairly. A savings account may offer flexible access, while a CD or fixed-term deposit may lock money away for a set term. Those product features still matter. But for the growth rate itself, APY is the measure that includes compounding.
Use APY when you want to compare like with like. Check that the assumptions match: the same currency, similar access terms, and interest left in the account to compound. Taxes, fees, withdrawals, and early-access penalties can change what you actually keep, so APY is an education and comparison tool rather than personal financial advice.