Two banks advertise a savings account “at 4 percent.” You put $10,000 in each. A year later, one account has earned $400 and the other has earned $407.42. Nobody lied to you — one bank was quoting an interest rate and the other was quoting an APY, and those are two different numbers.

If you only remember one thing from this piece, make it this: when you compare savings accounts or CDs, compare the APY — the annual percentage yield — and ignore everything else. Federal rules actually require banks to hand you that number, precisely so you can comparison-shop without doing algebra. Here’s the two-minute version of why.

The interest rate is the ingredient; the APY is the meal

The interest rate (sometimes called the nominal or stated rate) is the raw percentage a bank uses to calculate what it owes you. The APY is what you actually earn over a full year once compounding is included — that is, once the interest you’ve already been paid starts earning interest of its own.

Say your bank pays a 4.00 percent interest rate, compounded monthly. Each month you earn one-twelfth of 4 percent — about 0.333 percent — not on your original deposit, but on your current balance, which includes every previous month’s interest. Twelve rounds of that snowballing turns 4.00 percent into 4.07 percent by year’s end. That 4.07 percent is the APY. Compound daily instead of monthly and the APY creeps a bit higher still, to about 4.08 percent.

Small difference? On $10,000 for one year, yes — a few dollars. But the gap grows with bigger balances, higher rates, and more years, because compounding feeds on itself. It’s also exactly the kind of fine print that makes two “4 percent” accounts pay different amounts.

Why you can trust the APY on the disclosure

This isn’t a courtesy banks extend when they feel like it. The Truth in Savings Act, implemented by Regulation DD, requires depository institutions to disclose the APY on consumer deposit accounts, and its account-disclosure rules spell out that you must be told the rate, the APY, and how often interest compounds and is credited. The regulation even prescribes the exact formula banks must use, in Appendix A, based on a 365-day year — so an APY at one bank means the same thing as an APY at another.

That standardization is the entire point. The interest rate alone can’t be compared across banks, because one may compound daily, another monthly, another quarterly. The APY bakes all of that in and gives you a single apples-to-apples number.

A worked example you can steal

Here’s the arithmetic once, so you never have to do it again. The formula: take the interest rate, divide by the number of compounding periods per year, add 1, raise it to the power of that same number of periods, subtract 1.

For 4.00 percent compounded monthly: 0.04 ÷ 12 = 0.00333. Add 1, raise to the 12th power, subtract 1, and you get 0.0407 — a 4.07 percent APY. On $10,000, that’s $407.42 in year one. Leave it alone and year two pays interest on $10,407.42, so the dollars grow a little faster every year you don’t touch the account.

Don’t want to do exponents at the kitchen table? Fair. The SEC’s free compound interest calculator at Investor.gov does it in ten seconds, and it’s a genuinely useful way to see what a decade of compounding does to a balance.

Why this matters more than it used to

Because the spread between banks right now is enormous. The FDIC’s national deposit rates put the average savings account at just 0.38 percent as of April 2026 — while plenty of online banks and credit unions pay roughly ten times that. On a $10,000 emergency fund, the difference between the national average and a high-yield account is the difference between about $38 a year and several hundred dollars a year, for identical FDIC-insured safety.

In other words: the compounding math above decides the last few dollars, but which account you’re in decides the first few hundred. Check the APY on your current savings account — it’s on your statement or in your banking app. If it starts with a zero, you’re being paid the average, and the average is terrible.

Three quick things to remember at the bank

One: on savings products, the APY is always equal to or higher than the interest rate, so if a bank leads with the lower number, it’s quoting the rate — ask for the APY. Two: APY assumes the money and the interest stay put for a full year; pull cash out monthly and you’ll earn a bit less than the APY implies. Three: this logic runs in reverse for debt. On loans and credit cards, compounding works against you, which is why the equivalent disclosure there (the APR) exists — and why the balance on a card you ignore grows faster than you expect.

None of this requires you to love math. It just requires you to compare one standardized number, which federal rules already force every bank to print. Find the APY, compare the APY, and let the compounding work for you instead of the bank.

This article was produced with AI assistance and reviewed by a human editor. Figures are linked to their primary sources; where a claim could not be verified from the public record, we say so.