What is a CD and how does it earn interest?
A Certificate of Deposit (CD) is a savings product offered by banks — the credit union equivalent is called a share certificate. You deposit a fixed amount for a fixed term (commonly 3 months to 5 years) and the institution pays you a guaranteed fixed interest rate. At maturity, you receive your original deposit plus all interest earned.
CDs earn interest through compounding — interest earned in one period is added to the balance and then earns interest in the next period. Most CDs compound daily or monthly, which produces slightly more interest than annual compounding at the same stated rate.
Three things that determine how much interest a CD earns:
- Principal — the amount deposited
- APY (Annual Percentage Yield) — the effective annual rate after compounding
- Term — how long the CD runs before maturity
CD interest formula
CD interest is calculated using the compound interest formula:
P = Principal (initial deposit)
r = Annual interest rate as a decimal (APR ÷ 100)
n = Compounding periods per year (365 = daily, 12 = monthly)
t = Term in years
If you have the APY (simpler version)
Most banks and credit unions advertise APY — the effective annual yield that already accounts for compounding. If you use APY, the formula simplifies to:
Here is a full maturity waterfall for a $10,000 CD at 4.75% APY, 12-month term:
Compounding frequency — how much does it matter?
At the same nominal rate (APR), more frequent compounding produces more interest. The difference between daily and annual compounding is small but real — especially on large balances or long terms.
Here is a $10,000 CD at 4.75% nominal APR (not APY) over 12 months at different compounding frequencies:
The difference between daily and annual compounding on a 1-year CD is $11.22 — small on a single $10,000 CD but meaningful at larger balances or multi-year terms. This is why, when comparing two CDs with the same APR, you should prefer the one with more frequent compounding — or better, compare APY (which already accounts for compounding) rather than APR.
APY vs APR — which rate to use
Includes the effect of compounding. The true effective annual return. If two CDs have different APRs and different compounding frequencies, APY lets you compare them on equal terms. Use APY with the simplified formula: A = P × (1 + APY/100)^t
The stated rate before compounding. Use APR with the full formula A = P × (1 + r/n)^(n×t) where r = APR and n = compounding frequency. APR is always ≤ APY. For CDs, banks are required to display APY so you can compare accurately.
The practical rule: always use APY when it is available (which it nearly always is for CDs). APY tells you exactly what you will earn as a percentage of principal in one year, regardless of how often interest compounds.
Step-by-step calculation
Worked examples
6-month CD
$8,000 · 4.50% APY · 6-month term
✅ $178.98 in 6 months · $357.96 annualised
1-year CD — daily compounding
$25,000 · 4.80% APR · daily compounding
🔵 APY = 4.918% · $1,229.41 earned
5-year CD — compounding effect
$20,000 · 4.25% APY · 5-year term
✅ $4,616 earned · 23.1% total return
18-month jumbo CD
$100,000 · 5.10% APY · 18-month term
🟡 $7,717 in 18 months · $5,144/yr avg
Early withdrawal — how the penalty reduces net interest
Most CDs charge an early withdrawal penalty if you redeem before maturity. The penalty is typically expressed as a number of days of interest — commonly 60 days for short-term CDs and 150–180 days for longer terms. Here is how it affects your actual return:
Example: $10,000 · 4.75% APY · 12-month CD withdrawn at 6 months. Penalty = 90 days interest:
The 90-day penalty cuts the 6-month interest in half. On a 12-month CD with a 180-day penalty, withdrawing at month 6 would yield zero net interest — you would simply get your principal back. Before opening a CD, always check the penalty and model your break-even holding period.
CD ladder — earn more while keeping liquidity
A CD ladder splits your savings across multiple CDs with staggered maturity dates. Each rung matures at a different time, giving you regular access to funds while the longer-term rungs earn higher rates.
Example: $20,000 split equally across four CDs:
When each rung matures, you reinvest into a new 24-month CD (the longest rung). After the first two years, every 6 months a CD matures — giving you access to $5,000 + interest regularly while the rest stays earning at the higher long-term rate. This balances yield with liquidity better than a single long-term CD.
Frequently asked questions
What is the formula to calculate interest earned on a CD?
If you have APY: A = P × (1 + APY/100)^t, where t is the term in years. Interest earned = A − P. If you only have the nominal APR and compounding frequency: A = P × (1 + r/n)^(n×t), where r = APR as a decimal, n = compounding periods per year, and t = term in years.
Should I use APY or APR to calculate CD interest?
Use APY whenever it is available — which is nearly always for CDs, since banks are required to disclose it. APY already incorporates the effect of compounding, so A = P × (1 + APY/100)^t gives the correct maturity value without needing to know the compounding frequency separately.
How does compounding frequency affect CD interest?
More frequent compounding produces slightly more interest at the same nominal APR. A $10,000 CD at 4.75% APR earns $486.22 with daily compounding versus $475.00 with annual compounding — a difference of $11.22 over one year. The effect grows significantly on larger balances or longer terms.
What happens to CD interest if I withdraw early?
Early withdrawal triggers a penalty — typically 60 days of interest for short-term CDs and 90–180 days for longer terms. The penalty is subtracted from accrued interest and, if interest earned is insufficient, may reduce your principal. Always model the penalty before committing to a CD if there is any chance you will need the funds early.
Is CD interest taxable?
Yes. In the US, CD interest is taxed as ordinary income in the year it is credited (or in the year it accrues for multi-year CDs). Your bank will send a Form 1099-INT reporting the interest earned. CDs held inside a tax-advantaged account such as an IRA are not taxed until withdrawal (traditional IRA) or not taxed at all on earnings (Roth IRA).