Interest on a certificate of deposit accrues by applying a fixed rate to your balance at regular intervals, and each round of interest folds back into the balance so the next round earns slightly more. A CD advertised at a 5.00% rate compounded daily actually returns about 5.13% over a full year because of that snowball effect. What you ultimately walk away with depends on the rate, how often the bank compounds, how long the money stays locked up, and whether you let the interest ride or take it as payouts along the way.
The Interest Rate and the APY Are Not the Same Number
Every CD disclosure shows two percentages. The interest rate is the base figure the bank uses to calculate what you earn on your balance. The annual percentage yield (APY) is what you actually receive once compounding has run for a full year. On short terms or low rates the two numbers sit close together, but the gap widens as rates and compounding frequency climb.
The math behind APY is not a mystery. Take the nominal rate, divide by the number of compounding periods in a year, add one, raise the result to the power of the number of periods, and subtract one. A 5.00% rate compounded daily works out to (1 + 0.05/365)^365 − 1, or roughly 5.13%. That extra 0.13 percentage points is money generated purely by compounding, and on a large deposit it is not trivial.
Federal regulations require every bank and credit union to display the APY alongside the interest rate when advertising or opening a deposit account.1eCFR. 12 CFR 1030.4 – Account Disclosures The rule exists because the nominal rate alone understates what you earn. When you are comparing CDs across institutions, APY is the only figure that gives you a clean comparison.
How Often the Bank Compounds Changes the Total
Compounding is the engine that makes a CD earn more than simple interest would. Each time the bank calculates interest and credits it to your balance, the next calculation runs on a slightly larger number. Whether that happens once a day or once a quarter makes a real difference over the life of the deposit.
Daily compounding is the most common schedule on high-yield CDs. The bank divides the annual rate by 365, applies the tiny daily rate to your current balance, and rolls the result into tomorrow’s balance. Monthly compounding divides by 12. Quarterly divides by 4. Annual compounding applies the rate just once at year-end.
The practical effect is modest on short terms and real on long ones. A $10,000 CD at 4.00% compounded daily earns about $408 in its first year. The same CD compounded quarterly earns about $406. On a five-year, $50,000 deposit, the gap between daily and quarterly compounding can run into the hundreds of dollars. That is also why APY matters more than memorizing the schedule. Two CDs with the same APY will produce the same return no matter how often they compound.
Variable-Rate CDs
Most CDs lock in a fixed rate for the entire term. Some banks offer variable-rate CDs tied to a benchmark such as the prime rate, a Treasury bill index, or the Consumer Price Index. The APY on those products moves with the underlying benchmark, so you could earn more if rates rise and less if they fall. The account disclosure spells out which index controls the rate and how often it resets.
What Happens to the Interest You Earn
How the interest reaches you is a separate choice from how it accrues, and it affects both your return and your taxes. Most banks give you two or three options when you open the account.
Reinvestment into the CD is the default on most accounts. Accrued interest rolls back into the principal, so each later calculation runs on a growing balance. This produces the highest total return. The interest stays locked up with the principal until maturity.
A periodic payout sends the earned interest to a linked checking or savings account on a regular schedule, usually monthly or quarterly. You get cash flow you can spend or move elsewhere, but the CD balance stays flat because nothing is being added back. Total interest over the full term will be lower than with reinvestment.
Payment at maturity holds all the interest inside the CD and pays it out in a single lump when the term ends. For compounding purposes during the term it works the same as reinvestment, and it is the standard approach for CDs with terms under two years.
Brokered CDs Work Differently
CDs purchased through a brokerage account rather than directly from a bank typically pay simple interest, not compound interest. The rate applies only to the original deposit, and interest does not fold back in to generate more interest. The trade-off is liquidity. Instead of paying an early withdrawal penalty, you can sell a brokered CD on the secondary market before maturity. What a buyer pays depends on market conditions. If rates have risen since you bought in, your older lower-rate CD is worth less and you may sell at a loss; if rates have dropped, you could sell at a premium.
Interest Is Taxable in the Year It Is Credited
CD interest is taxable as ordinary income in the year it becomes available to you, even if you do not withdraw it.2Internal Revenue Service. Topic No. 403, Interest Received For a one-year CD that pays at maturity, that is simple: you report the interest in the year the CD matures. For a multi-year CD that credits interest to your account annually, you owe tax each year on the amount credited, not just in the year you finally cash out. The IRS treats interest credited to your account the same as interest handed to you.
If a bank pays or credits you $10 or more in interest during the year, it will send you Form 1099-INT documenting the amount.3Internal Revenue Service. About Form 1099-INT, Interest Income You owe tax on the interest regardless of whether that form arrives. Someone holding several small CDs across different banks may slip below $10 at each institution and never receive a 1099-INT, but the interest is still reportable.
One offset works in your favor. If you pay an early withdrawal penalty, that penalty is deductible as an adjustment to gross income, and you can claim the deduction even if it exceeds the interest earned on the CD that year.4Internal Revenue Service. Penalties for Early Withdrawal
Accrued Interest at Maturity and the Auto-Renewal Trap
All that accrued interest finally becomes freely available when the CD matures. Banks are required to notify you before maturity. For CDs longer than one month that renew automatically, the notice must arrive at least 30 calendar days before the maturity date, or at least 20 days before the end of a grace period of at least five days.5eCFR. 12 CFR 1030.5 – Subsequent Disclosures For CDs longer than one year, the notice must include the full account disclosures for the new term, including the rate and APY if known.
If you do nothing on the maturity date, most banks roll the balance, including all the accrued interest, into a new CD of the same term length at whatever rate the bank is currently offering. The grace period after maturity, typically five to ten days, is your window to withdraw funds or change your instructions without triggering an early withdrawal penalty on the renewed CD. Miss the window and you are locked in for another full term at the new rate.
This is where accrued interest quietly loses ground. A CD that matured at 5.00% might auto-renew at 3.50% if rates have fallen in the meantime, and the depositor who ignores the maturity notice is stuck for another year or more at the lower rate. Set a calendar reminder a few weeks before maturity so you can shop around and decide on purpose.