Understanding how your Certificate of Deposit earns interest is the first step to making smarter savings decisions. Here's everything you need to know.
The Compound Interest Formula
Banks calculate CD interest using the compound interest formula:
A = P × (1 + r/n)^(n × t)
Where:
- A = final balance at maturity
- P = your initial deposit (principal)
- r = annual interest rate (as a decimal, so 5% = 0.05)
- n = number of times interest compounds per year
- t = number of years
Example
You deposit $10,000 in a 1-year CD at 4.5% APY, compounded monthly:
- P = $10,000
- r = 0.045
- n = 12 (monthly)
- t = 1
A = $10,000 × (1 + 0.045/12)^(12 × 1) = $10,000 × 1.04594 = $10,459.40
You earn $459.40 in interest.
Compounding Frequency Matters
The more often interest compounds, the more you earn. Here's how $10,000 at 5% APR for 1 year compares:
| Frequency | Periods/Year | End Balance | Interest |
|---|---|---|---|
| Annually | 1 | $10,500.00 | $500.00 |
| Semi-Annually | 2 | $10,506.25 | $506.25 |
| Quarterly | 4 | $10,509.45 | $509.45 |
| Monthly | 12 | $10,511.62 | $511.62 |
| Daily | 365 | $10,512.67 | $512.67 |
| Continuous | ∞ | $10,512.71 | $512.71 |
The difference between annual and monthly compounding is $11.62 on a $10,000 deposit. Not huge, but it adds up on larger deposits and longer terms.
APY vs APR: What's the Difference?
- APR (Annual Percentage Rate) is the nominal interest rate before compounding.
- APY (Annual Percentage Yield) includes the effect of compounding.
When comparing CDs, always compare APY — it's the true return you'll earn.
The conversion formula:
APY = (1 + r/n)^n − 1
For a 5% APR compounded monthly: APY = (1 + 0.05/12)^12 − 1 = 5.116%
Continuous Compounding
Some CDs offer continuous compounding, calculated using:
A = P × e^(r × t)
Where e is Euler's number (≈ 2.71828). This yields the theoretical maximum return, though the difference from daily compounding is negligible.
How to Calculate It Yourself
- Divide your APR by the number of compounding periods per year
- Add 1 to that result
- Raise to the power of (periods × years)
- Multiply by your principal
Or just use our CD Calculator — it handles all the math instantly and lets you compare up to 4 scenarios side-by-side.
Key Takeaways
- CD interest uses compound interest, not simple interest
- More frequent compounding = slightly more earnings
- Always compare CDs using APY, not APR
- Longer terms and larger deposits benefit more from compounding
Ready to see the numbers? Try our free CD Calculator to estimate your earnings instantly.