Interest on a certificate of deposit accrues when the bank applies a fixed rate to your balance at set intervals and folds each round of interest back into the balance, so the next round is calculated on a slightly larger number. That is why a CD advertised at a 5.00% rate with daily compounding actually returns about 5.13% over a full year. What you ultimately walk away with depends on the rate, how often the bank compounds, how long the money stays locked up, and what you tell the bank to do with the interest along the way.
The Rate, the APY, and the Gap Between Them
Every CD listing shows two percentages. The interest rate is the base figure the bank uses in its calculations. The annual percentage yield is what you actually receive after a year of compounding. On short terms or low rates the two numbers sit close together. On longer terms or higher rates the APY pulls noticeably ahead.
The math behind APY is simple enough to run yourself. Take the nominal rate, divide by the number of compounding periods per year, add one, raise the result to the number of periods, and subtract one. A 5.00% rate compounded daily works out to (1 + 0.05/365)^365 − 1, or about 5.13%. That extra 0.13 percentage point is generated purely by compounding, and on a large deposit it adds up.
Federal regulations require every bank and credit union to display the APY alongside the 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 compare CDs from different banks, the APY is the only number that gives you an apples-to-apples read.
How Often the Bank Compounds
Compounding is the engine. Each time the bank calculates interest and adds it to your balance, the next calculation runs on a slightly larger number. Whether that happens once a day or once a quarter changes what you end up with.
Daily compounding is the most common schedule for high-yield CDs. The bank divides the annual rate by 365 to get a tiny daily rate, applies it to your balance that day, and rolls the result into the balance for tomorrow. Monthly compounding divides the rate by 12. Quarterly divides by 4. Annual compounding applies the rate once, at year-end.
The dollar effect is smaller than most people expect on short terms and larger than most people expect 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. That gap looks trivial, but it grows with balance and time. On a five-year, $50,000 deposit, the difference between daily and quarterly compounding can reach several hundred dollars.
This is exactly why the APY matters more than the compounding schedule. Two CDs with the same APY produce the same return regardless of how often they compound. The APY has already done the comparison for you.
What Happens to the Interest You Earn
How interest accrues is one question. What the bank does with it once it accrues is another, and you usually choose at account opening. The choice affects your total return.
- Reinvestment back into the CD is the default at most banks. Accrued interest rolls into the principal, and future calculations run on the growing balance. This produces the highest total return. The interest stays locked up until maturity.
- Periodic payout sends earned interest to a linked checking or savings account on a set schedule, typically 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 to it. Total interest over the full term is lower than with reinvestment.
- Payment at maturity holds all interest inside the CD and pays it out in a single lump sum at the end. This is standard for CDs with terms under two years and behaves like reinvestment during the term.
When You Owe Tax on the Interest
CD interest is taxable as ordinary income in the year it becomes available to you, even if you don’t take it out.2Internal Revenue Service. Topic No. 403, Interest Received For a one-year CD paying at maturity, that’s straightforward: 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 cash out. The IRS treats interest credited to your account the same as interest handed to you in cash.
If a bank pays or credits you $10 or more in interest during the year, it sends you Form 1099-INT.3Internal Revenue Service. About Form 1099-INT, Interest Income You owe tax on the interest whether or not that form arrives. Someone holding several small CDs at different banks can slip below the $10 threshold at each institution and never see a 1099-INT, but the interest is still reportable.
One Note on Brokered CDs
CDs bought through a brokerage account rather than directly from a bank generally do not compound. They typically pay simple interest, which means the rate applies only to the original deposit and interest never folds back in to generate more interest. The upside is liquidity: instead of paying an early withdrawal penalty, you can sell a brokered CD on the secondary market. What a buyer pays depends on where rates have gone since you bought it. If rates have risen, your older CD is worth less and you may sell at a loss. If rates have fallen, you could sell at a premium. The framing in the rest of this article assumes a traditional bank CD that compounds.
If You Take the Money Out Early
Pulling money out before maturity forfeits some of the interest that has accrued. The penalty is not a flat fee; it is a portion of the interest earned or that would have been earned. Federal rules require at least seven days’ simple interest for withdrawals made within the first six days after deposit.4eCFR. 12 CFR 204.2 – Definitions Beyond that floor, banks set their own schedules, and the penalty typically scales with the CD’s term.
A common structure runs 90 days of simple interest for terms of 12 months or less, 180 days for terms between one and four years, and a full year of interest for terms of four years or longer. There is wide variation among banks. The exact schedule sits in the account disclosure you receive at opening.
If you withdraw early enough in the term, the penalty can exceed the interest that has actually accrued so far. When that happens, the bank pulls the shortfall from your original principal, so you get back less than you put in. If you pay a penalty, it 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.5Internal Revenue Service. Penalties for Early Withdrawal
A handful of banks offer no-penalty CDs that let you withdraw the full balance without forfeiting interest, usually starting seven days after deposit. The APY is noticeably lower than a traditional CD of similar length, and most require you to withdraw the entire balance and close the account rather than take out part of it.