Why Does a CD's Interest Rate Differ From Its APY?
Two numbers usually appear side by side on a CD’s disclosure, and they’re rarely identical. Understanding why closes the gap between confusion and confidence when comparing offers.
The short answer
A CD’s interest rate is the base rate used to calculate earnings before compounding, while its APY, or annual percentage yield, reflects the actual return over a year after accounting for how often interest is added to the balance and starts earning its own interest. The APY is always equal to or higher than the stated rate, and it’s the more useful number for comparing CDs against each other.
What compounding actually does
When a CD credits interest, say monthly, that interest gets added to the balance, and the next round of interest is calculated on the new, slightly larger balance. Over a full year, this compounding effect means the total earned is a bit more than simply multiplying the stated rate by the deposit once. APY captures that entire effect in a single number, which is why it’s always at least as large as the base rate, and larger whenever compounding happens more than once a year.
Compounding frequency changes the gap
- Annual compounding. Interest is added once a year, so the stated rate and the APY are identical, since there’s no compounding within the year to create a gap.
- Monthly compounding. Interest is added twelve times a year, creating a modest gap between the stated rate and APY.
- Daily compounding. Interest is added every day, producing the largest gap of common compounding schedules, since even small returns start earning their own interest almost immediately.
A simple illustration
Consider a CD with a stated annual rate of 5%. If compounded once a year, the APY is also 5%, since there’s nothing to compound. But if that same 5% rate compounds monthly, the effect of interest earning interest across the year pushes the APY slightly above 5%, even though the underlying rate never changed. The exact size of that gap depends on the compounding frequency and the rate itself, but the direction is always the same: APY moves up relative to the stated rate whenever compounding happens more than once a year.
Why APY is the better comparison tool
Two CDs could advertise the same stated interest rate but compound on different schedules, resulting in genuinely different actual returns. Comparing APYs directly accounts for this automatically, since it already folds compounding frequency into a single figure. This same logic applies when weighing a promotional CD rate against a standard offer, since a flashy stated rate can look less impressive once both are converted to APY.
Where else this distinction shows up
The gap between a stated rate and its compounded yield isn’t unique to CDs. It shows up whenever compound interest is involved, including ordinary savings accounts and money market accounts, which is why reading past the headline rate is a habit worth applying broadly, not just when shopping for a single CD.
A practical habit
When comparing CD offers across different banks, it helps to line up the APY figures rather than the stated rates, since the stated rate alone doesn’t reveal how often interest compounds. A CD with a slightly lower stated rate but more frequent compounding could end up outperforming one with a higher stated rate but only annual compounding, and the APY is what reveals which is actually the better deal.