Yes, CDs do compound interest. The bank calculates interest on your deposit, adds it to the balance, and then calculates the next round of interest on that larger balance, so your earnings grow on themselves for the length of the term. How much extra you end up with depends on two things: how often the bank compounds, and whether you leave the interest in the account or take it out as it accrues.
How the Compounding Actually Builds Your Balance
When you open a CD, the bank starts calculating interest on your principal. At the end of each compounding period, the interest you earned gets folded into the balance. The next calculation runs against that new, larger number. The rate stays the same, but the base grows, so each period’s dollar figure is a little bigger than the last.
Simple interest works differently. On a $10,000 deposit at 5% simple interest, you earn exactly $500 every year for as long as the account runs, because the bank always calculates against the original principal. With compounding, you earn $500 the first year, then roughly $525 the second year (the first $500 is now earning too), and the gap between the two methods keeps widening the longer you hold the account.
How Often Banks Compound
Banks pick a schedule for each CD product and have to disclose it in your account agreement. Daily, monthly, quarterly, semi-annual, and annual are all common. More frequent compounding pushes your final balance up a bit, because each round of 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 it four times and ends around $509.45.
- Monthly compounding runs it twelve times and ends around $511.62.
- Daily compounding runs it 365 times and ends around $512.67.
On a one-year deposit of this size, the spread is small. It grows with the balance and the term. On a five-year CD holding $50,000, the difference between annual and daily compounding can reach several hundred dollars. Still, you don’t have to run these numbers yourself when comparing offers, because the APY already does that work.
Why APY Is the Number to Compare
The annual percentage yield reflects the total interest you’d earn over a year with the bank’s compounding schedule baked in. A 5.00% nominal rate compounded daily shows up as roughly 5.13% APY. That same 5.00% compounded annually stays at 5.00% APY. The formula, laid out in Appendix A of Regulation DD, factors in both the interest earned and the number of days in the term to produce a standardized figure.1Consumer Financial Protection Bureau. Appendix A to Part 1030 — Annual Percentage Yield
Federal law under the Truth in Savings Act and Regulation DD requires banks to disclose the APY using that exact term in account agreements and advertisements.2eCFR. 12 CFR Part 1030 — Truth in Savings (Regulation DD) That’s what lets you compare products directly. If two CDs both advertise a 4.50% APY, they’ll produce the same effective return over a year, even if one compounds daily and the other monthly. Compare the APYs, not the nominal rates.
Compounding vs. Crediting
CD disclosures list two schedules that sound similar and aren’t. Compounding is when the bank recalculates interest on an updated balance. Crediting is when the bank actually posts that interest to your account so it becomes part of your available balance. Regulation DD requires banks to disclose both.2eCFR. 12 CFR Part 1030 — Truth in Savings (Regulation DD)
Often they match. Sometimes they don’t. A bank might compound daily and credit monthly. The practical catch: if you close a CD between crediting dates, accrued interest that hasn’t been posted yet can be delayed until the next scheduled crediting date. The bank can’t refuse to pay it, but you may not see it right away.2eCFR. 12 CFR Part 1030 — Truth in Savings (Regulation DD)
Whether You Actually Get the Full Compounding Benefit
Compounding only does its full work if the interest stays in the account. When you open a CD, you usually pick what happens to interest as it accrues, and the choice matters.
Leaving interest inside the CD is the default that produces the advertised APY. Each payment rolls into the balance and earns for the rest of the term. Your maturity balance ends up meaningfully higher than your original deposit.
Many banks also let you have interest paid out to a linked checking or savings account on a monthly or annual schedule, or at maturity.3Capital One. CD Interest Accrual and Disbursements That gives you a steady income stream, which some retirees find useful, but each payout removes money from the CD before it can compound. The APY calculation assumes interest stays on deposit until maturity, so your actual total earnings over the term will be lower than the APY implies. Regulation DD requires banks to disclose that withdrawing interest early will reduce earnings.2eCFR. 12 CFR Part 1030 — Truth in Savings (Regulation DD)
Taxes on the Interest You Compound
Interest earned on a CD is taxable as ordinary income at the federal level and, in most cases, at the state level as well. You owe the tax in the year the interest is earned, not the year you withdraw the money. If your bank credits interest to your CD balance during the year, that amount counts as income for that tax year even though you can’t touch it without paying an early withdrawal penalty.4Internal Revenue Service. Topic No. 403, Interest Received
Longer CDs work the same way even when the bank pays all interest at maturity. For CDs with a term longer than one year that hold interest until the end, the IRS treats the accrued but unpaid interest as original issue discount. You have to report a portion each year despite receiving no payment yet.5Internal Revenue Service. Publication 550, Investment Income and Expenses Your bank will typically send a Form 1099-OID, or include the amount on a 1099-INT, so you know the figure 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 Earnings below that threshold are still taxable and still belong on your return, form or no form.