Rates & InterestAPYLetter A

Annual Percentage Yield (APY)

The real rate of return earned on a deposit or investment over one year, taking into account the power of compound interest.

Overview & Practical Application

Annual Percentage Yield (APY) measures the total amount of interest earned on a bank deposit account (such as high-yield savings, certificates of deposit, or money market accounts) over the course of one full year, fully factoring in compounding interest frequency.

Because interest can compound daily, monthly, quarterly, or annually, APY reflects the exponential effect of earning interest on previously accumulated interest.

Rule of Thumb: - When borrowing money, look at the APR (lower is better). - When saving or investing money, look at the APY (higher is better).

Mathematical Formula & Calculation

APY = (1 + r / n)^n - 1

Compounding formula where nominal rate is divided by compounding periods and exponentiated by periods per year.

Variable Definitions:

r=Nominal annual stated interest rate (in decimal format, e.g. 0.05 for 5%)
n=Number of compounding periods per year (e.g. 365 for daily, 12 for monthly)

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Interactive Financial Calculator

Test real-world numbers and see instant underwriting results.

Base Loan Amount ($)

Nominal Interest Rate (%)

Upfront Lender Closing Fees ($)

Nominal Note Rate6.250%

Effective True APR6.402%

ℹ️ True APR reflects the cost of borrowing including $4,500 in upfront fees over 30 years.

Real-World Underwriting Example

Scenario: You deposit $10,000 into a high-yield savings account with a stated nominal rate of 5.00% compounding daily (n = 365).

Calculation:

APY = (1 + 0.05 / 365)^365 - 1 = (1.00013698)^365 - 1 = 5.127%

Takeaway: At the end of one year, you earn $512.70 in interest rather than $500.00, demonstrating the compounding bonus.

Key Underwriting Takeaways

APY is always greater than or equal to the stated nominal interest rate when compounding occurs more than once per year.
Daily compounding produces higher yields than monthly or annual compounding on the same base rate.
Required by the federal Truth in Savings Act (TISA) on all consumer deposit disclosures.

Overview & Practical Application

Mathematical Formula & Calculation

Variable Definitions:

Interactive Financial Calculator

Real-World Underwriting Example

Key Underwriting Takeaways

Related Financial Terms

Annual Percentage Rate (APR)

Prime Rate

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