When shopping for savings accounts, credit cards, or personal loans, financial institutions present interest rates in two formats: Nominal Interest Rate and Effective Interest Rate (or APY).
While they may sound similar, failing to recognize the distinction can cost you thousands of dollars over the lifetime of a loan or investment.
Table of Contents
What is a Nominal Interest Rate?
The nominal interest rate (also called the stated rate) is the annual interest rate specified in a loan contract before taking into account compounding periods during the year.
For example, if a savings account states a nominal rate of 12% per year compounded monthly, the account actually accrues 1% interest each month (12% / 12 months).
What is an Effective Interest Rate (EAR / APY)?
The effective interest rate (or Annual Percentage Yield / EAR) reflects the actual yield or true annual borrowing cost when intra-year compounding is taken into account.
Because interest earned in month 1 earns its own interest in month 2, the true annual rate earned is higher than the stated nominal rate.
The Mathematical Formula for Effective Interest Rate
EAR = (1 + r / n)^n - 1Where: r = nominal annual interest rate (as a decimal), n = number of compounding periods per year.
Compounding Frequency Impact Table
Here is how a 10% nominal interest rate changes as compounding frequency increases:
| Compounding Frequency | Periods per Year (n) | Effective Annual Rate (EAR) |
|---|---|---|
| Annual | 1 | 10.000% |
| Semi-Annual | 2 | 10.250% |
| Quarterly | 4 | 10.381% |
| Monthly | 12 | 10.471% |
| Daily | 365 | 10.516% |
APR vs. APY: What Borrowers Must Know
Lenders and banks intentionally emphasize APR when selling loans and APY when offering savings deposits:
- When borrowing money: Always compare Effective Annual Rate / APY to know your true out-of-pocket interest cost.
- When saving/investing money: Compare APY to see which account compounds fastest.
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