How APY Is Actually Calculated (And Why Two 5% Accounts Can Pay Differently)

Two banks can both advertise an APY of 5.00% and still pay you different amounts on the same deposit — not because either one is lying, but because APY already bakes in an assumption about how often interest compounds, and small print about compounding frequency changes what that number actually means. The formula behind APY isn't a marketing construct. It's defined by federal regulation, and the Consumer Financial Protection Bureau publishes the exact math.

What APY actually is

Under Regulation DD — the Truth in Savings rule — the Annual Percentage Yield is defined as "a percentage rate reflecting the total amount of interest paid on an account, based on the interest rate and the frequency of compounding for a 365-day period." That's the important part most explanations skip: APY isn't just the interest rate. It's the interest rate combined with how often that interest compounds, expressed as a single number so you can compare accounts apples-to-apples. That's also why banks are required to disclose APY, not just a nominal rate, on savings, money market, and CD accounts — a bare interest rate alone doesn't tell you what an account will actually pay over a year.

The formula the CFPB actually publishes

Regulation DD, Appendix A sets out the calculation banks are required to use:

APY = 100 × [(1 + Interest ÷ Principal)^(365 ÷ Days in term) − 1]

• Principal is what you had on deposit at the start.
• Interest is the total dollar interest actually earned over the term.
• Days in term is the actual number of days that term covers.

For an account with no stated maturity — a savings account, for instance — or one with an exact 365-day term, that formula simplifies to:

APY = 100 × (Interest ÷ Principal)

The regulation includes its own worked example: a $1,000 deposit that earns $61.68 in interest over a year. Plugging that into the formula — 100 × (61.68 ÷ 1,000) — gives an APY of 6.17%. That's not a hypothetical from a bank's marketing page; it's the example the CFPB itself uses to illustrate the rule.

Why compounding frequency changes the number

Here's the part the formula doesn't spell out on its own: two accounts can advertise the exact same nominal interest rate — the rate before compounding is factored in — and still produce different APYs, because compounding more frequently means you start earning interest on your interest sooner.

Take a hypothetical (not a live product rate) nominal annual rate of 5.00% on a $10,000 deposit:

• Compounded daily, that 5.00% nominal rate works out to an APY of roughly 5.13% — about $512.67 in a year.
• Compounded monthly, the same 5.00% nominal rate works out to an APY of roughly 5.12% — about $511.62 in a year.

The gap is small in this example — about a dollar on $10,000 over a year — but it's real, it compounds (literally) with larger balances and longer horizons, and it's entirely a function of frequency, not of either bank offering a "better deal" on the stated rate. This is exactly why Regulation DD requires disclosure of the APY itself rather than letting institutions advertise a bare nominal rate: the nominal rate alone doesn't tell you what you'll actually earn, and APY is the number that's supposed to make different accounts comparable without you doing this math yourself.

What this means when you're comparing accounts

A few practical takeaways follow directly from how the formula works:

• Always compare APY, not the nominal or "stated" rate. If a bank's disclosure shows both, the APY is the number that already accounts for compounding — use that one for comparison shopping.
• A higher compounding frequency at the same nominal rate always produces a higher (or equal) APY. Daily compounding will never produce a lower APY than monthly compounding on the same nominal rate — it's a mathematical property of the formula, not a bank-specific promotion.
• The gap matters more with bigger balances and longer time horizons. On a small balance over a single year, compounding-frequency differences are often just a few dollars. On a larger emergency fund or over several years, the effect adds up.
• APY assumes the rate holds for a full year and the balance stays untouched. Most high-yield savings and money-market rates are variable — the bank can change them — so a disclosed APY is a snapshot, not a guarantee for the year ahead. See how a Fed rate move flows into your savings account's APY for how that variability actually works.

The bottom line

APY isn't a marketing number — it's a federally defined calculation, spelled out in Regulation DD Appendix A, that folds compounding frequency into a single comparable rate. Two accounts advertising the same nominal rate can still pay you different amounts depending on how often interest compounds, and the APY figure is specifically designed to surface that difference so you don't have to compute it yourself. When you're comparing a high-yield savings account against a money market account or a CD against other places to park cash, the APY on the disclosure is the number that already does this math — read it, not the nominal rate, and remember it's a snapshot of a variable rate, not a fixed promise.

ClearValue Banking is a publisher and comparison resource, not a bank — we don't hold deposits or set any institution's rates. The APY figures banks disclose are set and published by each institution; this guide explains the federal formula behind that number, not a specific product's current rate.