APY vs. interest rate: what is the difference?
APY includes the effect of compounding. The stated interest rate does not. A 4.50% nominal rate compounded monthly works out to a 4.594% APY.
What is the difference between APY and interest rate?
The CFPB definition is direct. Annual percentage yield reflects the interest rate and compounding frequency over a 365-day period. The interest rate is the annual rate before compounding.
Suppose a bank states a 4.50% interest rate and compounds monthly. Interest credited in month 1 can earn interest in month 2. That pushes the one-year APY above 4.50%, to 4.594%.
Why should you compare APYs?
APY puts deposit offers on one annualized basis. A bank compounding a 4.50% nominal rate daily produces a 4.602% APY. Monthly compounding produces 4.594%. The nominal rate alone hides that difference.
Regulation DD requires a deposit advertisement that states a return to identify it as an annual percentage yield. A bank may show the interest rate beside it, but the rate cannot be more prominent than the APY.
How do you convert an interest rate to APY?
For a nominal annual rate r compounded n times each year:
APY = (1 + r / n)^n - 1
Put 0.045 in for r and 12 in for n. The result is 4.594%. The extra 0.094 percentage points come from earning interest on prior interest during the same 12-month period.
The reverse calculation also matters. A 4.50% APY is equivalent to a 4.41% nominal rate when interest compounds monthly.
How much does 4.50% APY earn on a short CD?
APY is annualized. It does not mean a 6-month CD pays 4.50% of the deposit in 6 months. Under SaverGrid's month-based model, $10,000 at 4.50% APY earns about $222.52 over 6 months.
| Term | Deposit | APY | Estimated interest |
|---|---|---|---|
| 6 months | $10,000 | 4.50% | $222.52 |
| 12 months | $10,000 | 4.50% | $450.00 |
The 12-month estimate is exactly $450.00 because a full year at 4.50% APY earns 4.50% under the model. Actual bank results can differ slightly with the calendar dates and account terms.
Does compounding frequency change CD earnings?
Not when two CDs have the same APY and the same terms. A 4.50% APY already reflects the bank's compounding method. Applying daily or monthly compounding again would count the effect twice.
Frequency does matter when the starting number is the same 4.50% nominal rate:
| Compounding | Nominal rate | Calculated APY |
|---|---|---|
| Annual | 4.50% | 4.50% |
| Monthly | 4.50% | 4.594% |
| Daily | 4.50% | 4.602% |
Where should you find the APY?
Check the bank's rate sheet, account-opening disclosure, renewal notice, or online account page. The CFPB requires the term “annual percentage yield” to appear at least once when an advertisement uses the abbreviation APY.
Use the APY tied to the exact balance and term. A 12-month CD may pay 4.50% on $10,000 while a different balance tier or a 6-month term pays another rate.