How Regulation DD Requires Banks to Calculate and Disclose a Savings Account's Annual Percentage Yield

When you open a savings account, the bank has to tell you the Annual Percentage Yield, or APY, and that number is not something the bank gets to invent on its own. A federal rule called Regulation DD spells out exactly how APY must be calculated and disclosed, so you can compare one savings account against another using a single, apples-to-apples number. Here's how that math actually works, using a $1,000 deposit earning a 5.00% interest rate compounded daily as our example.

What Regulation DD Is and Why It Exists

Regulation DD's stated purpose is to let consumers make informed decisions and meaningful comparisons among deposit accounts and institutions. To do that, it requires depository institutions to give you disclosures built on a standardized formula, rather than letting each bank define 'yield' however it likes. The rule defines APY as a percentage rate reflecting the total interest paid on an account, based on the interest rate and how often it compounds, annualized over a 365-day period (366 days is permitted in leap years). That calculation has to follow the specific rules laid out in Appendix A to Part 1030.

The Appendix A Formula, Step by Step

The general formula in Appendix A is APY = 100 × [(1 + Interest ÷ Principal)^(365 ÷ Days in term) − 1]. Principal is the amount assumed to be deposited at the start of the account, Interest is the total dollar amount earned over the term, and Days in term is the actual number of days in that term. The formula assumes the principal and all interest stay in the account for the full term, with no other deposits or withdrawals, unless the account specifically requires interest to be withdrawn, in which case that rule has to be reflected instead. For a typical savings account, though, there is no stated maturity date, so the rule requires the calculation to assume a 365-day term. When days in term equals 365, in that way, the formula simplifies to APY = 100 × (Interest ÷ Principal), which is much easier to work through by hand.

The CFPB's Own Worked Example

The Consumer Financial Protection Bureau's own Appendix A illustration uses a $1,000 deposit that earns $61.68 in interest over a 365-day year in a NOW account, an interest-bearing checking account with no stated maturity. Plugging those numbers into the general formula produces an APY of 6.17%. That example matters for savings accounts too, because a NOW account without a stated maturity is treated the same way under Appendix A as an ordinary savings account.

• Interest: 61.68
• Principal: 1000
• DaysInTerm: 365
• Formula: 100*((1+Interest/Principal)^(365/DaysInTerm)-1)
• Result: 6.168

A $1,000 Savings Deposit at a 5% Rate Compounded Daily

Now imagine you open a savings account with a $1,000 deposit that earns a 5.00% stated interest rate, compounded daily, and you leave the money and all interest untouched for a full 365-day year, with no fees or balance tiers involved. In this hypothetical illustration, that deposit would earn $51.27 in interest over the year. Because days in term equals 365 for an account with no stated maturity, the simplified formula applies, and $51.27 divided by $1,000 works out to an APY of 5.13%. Notice that the disclosed APY, 5.13%, is higher than the plain 5.00% interest rate the bank quotes, because the APY calculation captures the effect of daily compounding while the stated interest rate alone does not.

• NominalRate: 0.05
• CompoundingPeriods: 365
• Formula: 100*(((1+NominalRate/CompoundingPeriods)^CompoundingPeriods)-1)
• Result: 5.1267

Rounding, Accuracy, and the Statement You Actually Get

Once the APY is calculated, it has to be rounded to the nearest 0.01 percentage point and shown to two decimal places, so a raw result of 5.644% would be disclosed as 5.64%, while 5.645% would round up to 5.65%. The underlying interest rate shown on account disclosures, however, is allowed to run to more decimal places even though the APY itself is capped at two. A disclosed APY is treated as accurate as long as it falls within 0.05 percentage points, one-twentieth of one percent, of the figure the Appendix A formula actually produces, though that tolerance exists to cover inadvertent errors rather than to let institutions deliberately round in their own favor. On your periodic statement, banks use a related but distinct figure called APY Earned, calculated as 100 × [(1 + Interest earned ÷ average daily Balance)^(365 ÷ Days in period) − 1], though a special version of that formula applies instead when statements are issued more often than interest is compounded. That formula is based on the interest you actually earned and your average daily balance for that specific statement period, rather than a projected full-year amount, and it applies to accounts that receive periodic statements at least four times a year.

APY vs. APY Earned

Feature	APY (account disclosure)	APY Earned (periodic statement)
Formula	100 × [(1 + Interest ÷ Principal)^(365 ÷ Days in term) − 1]	100 × [(1 + Interest earned ÷ Balance)^(365 ÷ Days in period) − 1]
What 'Interest' represents	Projected total dollar interest over the assumed term	Actual dollar interest earned during the statement period
Time base for savings accounts	Assumed 365-day term since there is no stated maturity	Actual number of days in the statement period
Accuracy tolerance for inadvertent errors	Within 0.05 percentage points of the Appendix A result; may not be deliberately built into the calculation	Within 0.05 percentage points of the Appendix A result; may not be deliberately built into the calculation

Comparing the two APY-related figures Regulation DD requires.

Key Takeaways

• Regulation DD requires a fixed formula so you can compare savings accounts on equal footing, not on marketing language.
• APY reflects both the interest rate and how often it compounds, annualized over 365 days, under the rules in Appendix A.
• For a savings account with no stated maturity, the days-in-term is assumed to be 365, simplifying the formula to 100 × (Interest ÷ Principal).
• In this article's own hypothetical illustration, a $1,000 deposit at a 5.00% rate compounded daily for 365 days, with no fees, balance tiers or withdrawals, earns $51.27 in interest and converts to a 5.13% APY, higher than the 5.00% stated rate because compounding is baked into APY but not into the plain interest rate.
• Disclosed APY figures are rounded to two decimal places, and are considered accurate within a tolerance of 0.05 percentage points of the formula result, though that tolerance is meant only to cover inadvertent errors and may not be deliberately built into a bank's calculation.
• Your periodic statement shows a different figure, APY Earned, based on your actual interest and average daily balance for that period rather than a projected year.

Frequently Asked Questions

Why is my savings account's APY higher than its stated interest rate?

Because APY reflects the total interest paid on the account, based on both the interest rate and how frequently it compounds, annualized over a 365-day period. In the worked example above, a 5.00% rate compounded daily produces $51.27 in interest on $1,000, which converts to a 5.13% APY, higher than the plain rate because daily compounding adds extra interest that the stated rate alone doesn't capture.

Does Regulation DD let banks calculate APY differently from each other?

No. The general formula, APY = 100 × [(1 + Interest ÷ Principal)^(365 ÷ Days in term) − 1], is set out in Appendix A to Part 1030 and applies across covered institutions.

How much can a bank round the APY it discloses to me?

The APY has to be rounded to the nearest 0.01 percentage point and shown to two decimal places, so a raw calculation of 5.644% is disclosed as 5.64% while 5.645% becomes 5.65%. Separately, the disclosed figure is considered accurate as long as it lands within 0.05 percentage points of the number the Appendix A formula actually produces, a tolerance meant to cover honest errors rather than deliberate rounding in the bank's favor.

What's the difference between the APY on my account disclosure and the APY Earned on my statement?

The APY on your account-opening disclosure is a projection using an assumed term and a projected interest amount under the general Appendix A formula. APY Earned, shown on periodic statements for accounts that get them four or more times a year, instead uses your actual interest earned and your average daily balance for that specific period, plugged into the formula 100 × [(1 + Interest earned ÷ Balance)^(365 ÷ Days in period) − 1]; a different formula applies if statements are issued more often than interest is compounded.

Why does Regulation DD assume a 365-day term for savings accounts?

Savings accounts typically don't have a stated maturity date, and the rule specifically requires that when there's no stated maturity, the calculation assumes a 365-day term. That assumption is what allows the formula to simplify from the general version to the easier 100 × (Interest ÷ Principal) form.

Sources

• www.consumerfinance.gov
• www.consumerfinance.gov
• www.ecfr.gov
• www.consumerfinance.gov