If a bank is advertising a CD or savings rate and you want to know exactly what you'll actually earn — not the marketing number, the real one — the whole answer hinges on two letters most people gloss over: APY versus APR.
Banks are legally required to advertise the Annual Percentage Yield (APY) on deposit accounts, but the underlying math — the stated rate, the compounding schedule, and the term — is where the real number actually comes from. Understanding the mechanics means you're not just trusting a number on a rate-comparison page; you can check it yourself.
What's the actual difference between APR and APY?
Quick answerAPR is the stated, nominal rate before compounding. APY is the effective rate you actually earn once compounding is factored in, and APY is always equal to or higher than APR.
The formula that connects the two is straightforward: APY = (1 + APR/n)^n − 1, where n is the number of times interest compounds per year. If a bank compounds annually, APR and APY are identical. The moment compounding happens more than once a year — quarterly, monthly, or daily — APY pulls ahead of APR, because you start earning interest on interest within the same year instead of waiting until the end of it.
Why this matters: under the federal Truth in Savings Act, US banks must disclose APY specifically so you can compare accounts on equal footing, regardless of how often each one compounds. Two CDs advertising slightly different APR numbers can end up paying out the same real return once you convert both to APY.
Does compounding frequency actually move the needle?
Quick answerYes, but with diminishing returns — going from annual to monthly compounding matters a lot more than going from monthly to daily.
For the same stated APR, more frequent compounding always produces an equal or higher APY, but each additional step up in frequency adds a smaller improvement than the last. Here's an illustrative comparison of a 5.00% APR converted to APY at different compounding schedules:
Illustrative: 5.00% APR converted to APY by compounding frequency
| Compounding | Periods/yr | Resulting APY |
|---|
| Annually | 1 | 5.00% |
| Quarterly | 4 | ~5.09% |
| Monthly | 12 | ~5.12% |
| Daily | 365 | ~5.13% |
Notice how the gap between annual and quarterly (about 0.09 points) is roughly double the gap between quarterly and daily (about 0.04 points) — that pattern holds generally, which is why chasing "daily compounding" as a selling point matters far less than comparing the actual APY two banks are quoting.
A worked example
Say, purely as an illustration, someone deposits $10,000 into a 1-year CD at a 5.00% APY. Because one-year interest at a stated APY is simply principal multiplied by the APY, the CD earns $500.00 in interest, maturing at $10,500.00 before tax. If that same $10,000 instead went into a savings account earning a 4.50% APY with $100 added monthly over 10 years, the combination of a lower rate but ongoing monthly contributions and compounding produces a materially different balance than the one-time CD deposit — which is exactly the kind of side-by-side a calculator handles faster than doing it by hand.
Key fact: Over long horizons, the gap between simple interest and compound interest becomes large. As an example only: $10,000 at 5% for 20 years grows to $20,000 under simple interest, but to roughly $26,500 under annual compounding — a difference of over $6,000 purely from interest earning interest.
These figures are hypothetical examples for illustration only, not a projection for any specific bank, product, or account.
What about CDs vs. savings accounts, and CD ladders?
Quick answerA CD locks your rate and your money for a fixed term; a savings account keeps your money accessible but its rate can change anytime. A CD ladder is a way to get some of both.
A CD typically suits money you won't need until a known date, since it charges an early withdrawal penalty — often a forfeiture equal to some number of months' interest — if you break it early. A savings account suits money you might need on short notice, like an emergency fund, but its rate floats with the market and can drop without notice. A CD ladder splits a lump sum across several CDs with staggered terms (say, 1 through 5 years) so that a rung matures periodically, giving you recurring access to part of your money while the rest keeps earning CD-level rates.
CDs and savings accounts at FDIC-member banks are typically insured up to $250,000 per depositor, per bank, per ownership category, which is part of why they're considered a low-risk place to park savings you're not ready to invest elsewhere.
Frequently asked questions
What is the difference between APR and APY on a CD?
APR (Annual Percentage Rate) is the stated, nominal rate before compounding is factored in. APY (Annual Percentage Yield) is the effective annual rate you actually earn once compounding is applied, and it is always equal to or higher than the APR. The formula is APY = (1 + APR/n)^n - 1, where n is the number of compounding periods per year. US banks are required to disclose APY under the Truth in Savings Act so you can compare accounts fairly.
How much does a $10,000 CD earn in a year?
At a 5.00% APY, a $10,000 CD earns $500.00 in interest over one year, for a total of $10,500.00 at maturity. The amount scales directly with the APY over exactly one year: a 4.50% APY earns $450.00 and a 5.50% APY earns $550.00, since one-year interest equals principal multiplied by the APY.
Does compounding frequency really make a noticeable difference?
For the same stated APR, more frequent compounding produces a higher APY, but the gains shrink quickly as frequency increases. For example, a 5.00% APR compounds to a 5.00% APY when compounded annually but to roughly 5.13% APY when compounded daily. The jump from annual to monthly compounding matters more than the jump from monthly to daily.
Is CD and savings interest taxable?
Yes. In the US, interest earned on CDs and savings accounts is taxable as ordinary income in the year it's earned or credited, even if you don't withdraw it, and your bank reports it on Form 1099-INT if it's $10 or more. Banks generally don't withhold tax automatically, so you're responsible for accounting for it on your return.
What happens if I need to withdraw from a CD before it matures?
Most CDs charge an early withdrawal penalty, commonly a forfeiture of a set number of days' or months' interest, such as roughly 90 days' interest on a 1-year CD, with steeper penalties on longer terms. If you withdraw very early, the penalty can exceed the interest earned so far, meaning you could get back slightly less than your original principal.
What is a CD ladder and why would I build one?
A CD ladder splits a lump sum across several CDs with staggered terms, such as 1, 2, 3, 4, and 5 years, instead of locking it all into a single CD. Each rung matures at a different time, giving you periodic access to part of your money while the rest keeps earning CD-level rates, and as each rung matures you can withdraw it or roll it into a new long-term CD.