These two abbreviations look nearly identical, but the letter that differs — R for "Rate" versus Y for "Yield" — points to a real mathematical gap that can cost or earn you money depending on which side of a transaction you're on. Lenders love quoting APR because it looks smaller. Banks advertise APY on savings products because it looks bigger. Neither is lying; they're just measuring different things.
The Formula
Worked Example: $10,000 at 5% APR
Take a $10,000 deposit earning a nominal 5% APR, compounded at different frequencies:
| Compounding | Periods/Year | Resulting APY | Interest Earned |
|---|---|---|---|
| Annually | 1 | 5.00% | $500.00 |
| Quarterly | 4 | 5.09% | $509.45 |
| Monthly | 12 | 5.12% | $511.62 |
| Daily | 365 | 5.13% | $512.67 |
Notice the gap grows with compounding frequency, but it caps out — going from monthly to daily only adds about a dollar on $10,000. The big jump is from annual (no compounding advantage) to quarterly.
Why It Matters in Practice
On a mortgage, APR is federally required to include certain lender fees and discount points, which is why the APR on a mortgage disclosure is usually a bit higher than the loan's plain interest rate — it's a more honest "true cost of borrowing" number. On a savings account or CD, APY is the number that reflects what you'll actually see in your account balance a year from now, because banks nearly always compound interest more than once annually (usually daily or monthly).
If you're comparing two credit cards, comparing APRs is standard and appropriate. If you're comparing two high-yield savings accounts, comparing APYs is the correct apples-to-apples method — comparing one bank's APR to another's APY would understate or overstate your real return.
Figures above are illustrative examples only and not financial advice. Always confirm the exact APR/APY disclosed by your specific lender or bank.