The advertised rate is the annual rate; the APY is what you actually earn once compounding is counted. On CDs the two are close but not identical, and the difference compounds in your favor.
Rate, APY and where the difference comes from
The advertised rate is nominal: 4.5% paid as 4.5% ÷ 12 each month. But each month's interest then earns interest itself, so a year ends slightly above 4.5% — that figure is the APY, and every CD advertisement is required to show it next to the rate. The calculator derives it rather than asking for it, so you can check the bank's number.
The formula
Value at maturity = deposit × (1 + rate ÷ periods)^(periods × years), where periods is 12 for monthly, 4 for quarterly, 365 for daily compounding. The APY is (1 + rate ÷ periods)^periods − 1. This is exact — the same compounding the bank's disclosure performs.
What a CD is for
CDs sit between savings accounts and bonds: a fixed rate for a fixed term, insured up to $250,000 by the FDIC, with the penalty for early withdrawal as the price of the guarantee. They suit money with a known deadline — a house down payment two years out, tuition next fall — where a market dip before the deadline is unacceptable but a savings account rate feels too low.
The classic strategy is the ladder: split the money into 6-, 12- and 24-month CDs. Each maturity either gets spent or rolls into the longest term, so part of the money is always close while the rest earns the longer-term rate.
One caution the advertised rate never mentions: rates change between openings. A CD bought today at 4.5% locks that figure, which is an advantage if rates fall and a real cost if they rise. Nobody knows which comes, which is precisely the argument for splitting the money across terms instead of guessing right with all of it at once.
Frequently asked questions
Is the interest taxable?
Yes, in the year it is credited — even on a CD that has not matured. The bank sends a 1099-INT each January, and the interest is taxed as ordinary income at your marginal rate.
What happens if I withdraw early?
The bank charges a penalty, usually a number of months of interest (often 3–6 on short terms, more on long ones). On a short-term CD the penalty can exceed everything earned so far. Check the penalty schedule before opening — it is the one term that matters more than the rate.
CD or high-yield savings?
The CD pays more because you give up flexibility. If the money might be needed at any moment, the savings account wins despite the lower rate — one early-withdrawal penalty erases months of the gap. If the date is known, the CD usually wins.