APY Formula
Annual Percentage Yield accounts for compound interest:
APY = (1 + r/n)n - 1
Where r is the nominal annual interest rate (as a decimal) and n is the number of compounding periods per year.
For continuous compounding:
APY = er - 1
APR vs. APY
| Feature | APR | APY |
|---|---|---|
| Full name | Annual Percentage Rate | Annual Percentage Yield |
| Compounding | Not included | Included |
| Value | Lower or equal | Higher or equal |
| Used for | Loans, credit cards | Savings, CDs |
| Regulation | Truth in Lending Act | Truth in Savings Act |
Effect of Compounding Frequency
Higher compounding frequency increases APY, but with diminishing returns. The table below shows APY for a 6% nominal rate:
| Frequency | Periods (n) | APY |
|---|---|---|
| Annually | 1 | 6.000% |
| Semi-annually | 2 | 6.090% |
| Quarterly | 4 | 6.136% |
| Monthly | 12 | 6.168% |
| Daily | 365 | 6.183% |
| Continuously | ∞ | 6.184% |
Future Value with Compound Interest
If you deposit a lump sum, the future value after t years is:
FV = PV × (1 + r/n)n×t
Where PV is the initial deposit, r is the nominal rate, n is compounding periods per year, and t is the number of years.