Yes, CDs do compound interest. The interest your deposit earns is added back to the balance on a schedule the bank sets, and from that point forward the bigger balance earns interest too. How much that compounding adds depends on how often the bank runs the calculation and whether you leave the earned interest inside the account.
How a CD Builds Interest on Interest
When you open a CD, the bank starts calculating interest on your deposit. At the end of each compounding period, the interest earned is folded into your balance. The next period, the bank calculates interest on that larger figure rather than on your original principal. The rate stays the same, but the base keeps growing, so the dollar amount of interest grows with it.
Simple interest works differently. On a $10,000 deposit at 5% simple interest, you earn exactly $500 every year, forever, because every calculation uses the original $10,000. With compounding, you earn $500 the first year, roughly $525 the second year because that first $500 is now part of the earning balance, and the gap keeps widening as the term runs on.
How Often CDs Compound
Banks set a specific compounding schedule for each CD and must disclose it in your account agreement. Common schedules are daily, monthly, quarterly, semi-annual, and annual. The more frequently interest compounds, the slightly higher the end balance, because earned interest joins the principal sooner and starts producing its own return earlier.
Take a $10,000 CD at a 5% nominal rate held for one year:
- Annual compounding runs the calculation once and produces exactly $500, ending at $10,500.
- Quarterly compounding runs four times and produces roughly $509.45.
- Monthly compounding runs twelve times and produces about $511.62.
- Daily compounding runs 365 times and produces approximately $512.67.
On a one-year, $10,000 deposit the differences are modest. On a five-year CD with $50,000, the gap between annual and daily compounding can reach several hundred dollars. Compounding frequency is worth checking when you compare products, though as the next section explains, the headline comparison number already includes it.
Why APY Is the Number to Compare
The annual percentage yield is the single most useful figure for comparing CDs. It states what you would earn over one year with the compounding schedule factored in. A 5.00% nominal rate compounded daily shows an APY of roughly 5.13%, while the same 5.00% rate compounded annually stays at 5.00% APY. The formula is set out in Appendix A of Regulation DD and standardizes the figure across institutions.1Consumer Financial Protection Bureau. Appendix A to Part 1030 — Annual Percentage Yield
Federal law requires banks to disclose the APY, using that exact term, in account agreements and advertisements. The rule comes from the Truth in Savings Act and its implementing regulation, Regulation DD.2eCFR. 12 CFR Part 1030 — Truth in Savings (Regulation DD) If two CDs both advertise a 4.50% APY, they will produce the same effective return over a year, even if one compounds daily and the other monthly. The difference is already priced in.
Letting It Compound vs. Taking Payouts
When you open a CD, you usually pick what happens to the interest as it accrues. The choice changes how much compounding you actually get.
Leaving interest inside the CD is the arrangement that produces the full advertised APY. Every payment rolls into the balance and earns additional interest for the rest of the term, and your balance at maturity reflects the full compounded amount.
Many banks also let you have interest paid out to a linked checking or savings account on a monthly, annual, or at-maturity schedule.3Capital One. CD Interest Accrual and Disbursements This gives you a steady income stream, which some savers use in retirement, but it also pulls money out of the CD before it can compound. The remaining balance stays close to your original principal, and your total earnings over the term will fall short of what the APY would otherwise suggest. The APY calculation assumes interest stays on deposit until maturity, and Regulation DD requires banks to disclose that withdrawing interest early will reduce your earnings.2eCFR. 12 CFR Part 1030 — Truth in Savings (Regulation DD)
Compounding Is Not the Same as Crediting
Two terms appear in CD disclosures and they are easy to confuse. Compounding is the mathematical event: the bank recalculating interest on an updated balance. Crediting is when the bank actually posts that interest to your account so it shows up as part of your balance. Regulation DD requires banks to disclose both the compounding frequency and the crediting frequency for every account.2eCFR. 12 CFR Part 1030 — Truth in Savings (Regulation DD)
The two schedules often match, but not always. A bank might compound daily and credit monthly. The practical consequence shows up if you close a CD between crediting dates: the bank may delay paying accrued interest that has not yet been posted until the next scheduled crediting date, though it cannot refuse to pay it entirely.2eCFR. 12 CFR Part 1030 — Truth in Savings (Regulation DD)
You Owe Tax on Compounded Interest in the Year It Is Earned
Interest earned on a CD is taxable as ordinary income at the federal level, and in most states as well. You owe tax in the year the interest is earned, not the year you withdraw the money. If the bank credits interest to the CD during the year, that amount counts as income for that tax year, even though you cannot touch it without paying an early withdrawal penalty.4Internal Revenue Service. Topic No. 403, Interest Received
For CDs longer than one year that pay all interest at maturity, the IRS treats the accrued but unpaid interest as original issue discount. You must report a portion each year even though no payment has reached you.5Internal Revenue Service. Publication 550, Investment Income and Expenses The bank typically sends a Form 1099-OID, or includes the amount on a 1099-INT, to tell you what to report.
If your CD earns $10 or more in interest during the year, the bank must send you a Form 1099-INT.6Internal Revenue Service. About Form 1099-INT, Interest Income Interest below that threshold is still taxable and should still appear on your return.