How CD Interest Is Calculated: APY, Compounding, and Maturity Value
Understand exactly how CD interest is calculated — from APR to APY, compounding frequency effects, and how to calculate your exact maturity value using the compound interest formula.
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APR vs. APY: What Banks Advertise
Banks advertise CDs using APY (Annual Percentage Yield) — the effective annual return that accounts for compounding. APY is always ≥ APR (Annual Percentage Rate, the base rate before compounding).
Formula: APY = (1 + r/n)^n − 1
Where r = annual interest rate (APR) and n = compounding periods per year
Example: APR = 4.90%, daily compounding (n = 365): APY = (1 + 0.049/365)^365 − 1 = (1.0001342)^365 − 1 = 5.021%
The difference is small but meaningful on large balances over long terms.
Compounding Frequency Comparison
Starting with 5.00% APR:
Daily compounding offers the highest APY, though the difference from monthly compounding is minimal.
Maturity Value Formula
Maturity Value = Principal × (1 + APY)^Years
For fractional years (e.g., 9-month CD): Maturity Value = Principal × (1 + APY)^(Days/365)
Example 1: $15,000 in a 12-month CD at 5.00% APY: MV = $15,000 × (1.05)^1 = $15,750 Interest earned: $750
Example 2: $15,000 in a 6-month CD at 5.00% APY: MV = $15,000 × (1.05)^(182/365) = $15,000 × 1.02470 = $15,371 Interest earned: $371
Example 3: $50,000 in a 3-year CD at 4.75% APY: MV = $50,000 × (1.0475)^3 = $50,000 × 1.1489 = $57,447 Interest earned: $7,447
Interest Payment Timing
CDs handle interest in two main ways:
Monthly/quarterly interest payments: Interest is distributed to a linked account. You receive smaller, regular payments. This reduces compounding effectiveness slightly but provides cash flow.
At-maturity payment: All interest compounds until maturity, then principal + total interest is paid. This maximizes compounding but no interim cash flow.
For maximum growth: choose at-maturity interest payment and reinvest into another CD at rollover.
Rolling Over a CD
When a CD matures, you typically have a 7–10 day "grace period" to: 1. Withdraw without penalty 2. Reinvest in a new CD (at current rates) 3. Allow auto-renewal (usually at current rate for same term)
Important: Auto-renewal at a lower rate is the most common mistake. Always check current rates and compare before allowing auto-renewal.
Tax Implications
CD interest is ordinary income, taxable in the year credited (even if the CD doesn't mature until a future year for multi-year CDs). Banks issue Form 1099-INT. Keep records of interest accrued each tax year for multi-year CDs.
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Frequently Asked Questions
- How is CD interest calculated?
- CD interest uses compound interest: Maturity Value = Principal × (1 + APY)^(Term in Years). For a $10,000 CD at 5.00% APY for 2 years: $10,000 × (1.05)^2 = $11,025 — earning $1,025 total. Banks advertise APY (includes compounding effect), not APR.
- What is the difference between APR and APY on a CD?
- APR is the base interest rate; APY is the effective annual yield after accounting for compounding frequency. A 5.00% APR compounded daily becomes a 5.127% APY. Banks advertise APY on CDs because it's the higher, more attractive number. APY is what you actually earn.
- How often is CD interest compounded?
- Most online banks and credit unions compound CD interest daily. Traditional banks may compound monthly or quarterly. Daily compounding provides a slightly higher effective yield than monthly. The difference on a $10,000 CD over 1 year between daily and monthly compounding is approximately $1–2.
- How is CD interest taxed?
- CD interest is taxed as ordinary income (not capital gains) in the year it's credited, regardless of when the CD matures. For multi-year CDs, you may owe taxes on accrued interest each year even if you can't access the funds yet. Banks issue Form 1099-INT for interest of $10 or more.
Last updated 7/28/2026