The difference in one sentence
APY expresses the one-year effect of a rate plus compounding, assuming the rate and balance conditions hold.
The nominal interest rate describes the rate applied. APY describes the effective annual yield after compounding, under stated assumptions.
The relationship
For a nominal annual rate r compounded n times per year: APY = (1 + r/n)n − 1. Daily compounding can make APY slightly higher than the nominal rate.
What APY does not guarantee
A variable-rate account can change after opening. APY also does not account for fees, taxes, withdrawals, tier thresholds, or conditions you fail to meet.
Compare equivalent figures
Do not compare one account’s nominal rate with another account’s APY. Use APY for the same horizon and balance assumptions, then separately subtract fees. If a product compounds for less than a year or has a promotional period, calculate that actual period instead of applying a full-year APY blindly.
Use APY as an input, not a promise
The disclosure standardizes a yield assumption; it does not fix a variable rate or guarantee your balance. Contributions, withdrawals, tier thresholds, and taxes can all make the realized return differ from the quoted annual figure.
Work through a compounding example
Suppose a nominal annual rate is 4.00% and compounds monthly. The monthly rate is 0.04 divided by 12. Applying that rate for twelve periods gives an effective annual yield of about 4.07% before fees and taxes, assuming the balance stays in place. The APY is higher because interest begins earning interest.
This example is not a product quote. Actual disclosures can use different rules, and variable rates can change. Use the institution's current APY for like-for-like comparison instead of rebuilding a disclosed value from a marketing rate.
Separate yield from realized account return
| Item | Included in APY? | Handle separately |
|---|---|---|
| Compounding | Yes, under disclosure assumptions | Confirm quoted APY and period |
| Monthly contributions | Not represented by one starting balance | Project each deposit for its time in the account |
| Account fees | May not be reflected in the headline comparison | Subtract unavoidable fees |
| Rate changes | No future guarantee for a variable account | Run a lower-rate scenario |
| Taxes | No | Consider the rules that apply to the saver |
Use APY to compare yield assumptions, then compare net outcomes using the same balance and cash-flow scenario.
Apply APY without overreading it
After standardizing the yield, check whether tiers or activity rules change what the balance earns with How Savings Rate Tiers and Conditions Work.
Then compare the complete account outcome, including fees and access, through How to Compare Savings Accounts.
Convert the quoted yield into a like-for-like projection in the APY projection calculator.
APY and rate at a glance
The nominal interest rate is applied to the balance; APY expresses the effective one-year yield after the stated compounding frequency.
Figures that must use the same basis
APY compared with APY rather than a competitor's nominal rate; The same balance and time horizon; Fees, tiers, and promotional periods shown separately.
Decision the quoted APY supports
Use APY as one standardized input in a net-value comparison. Do not treat a variable APY as a guaranteed future return.
Primary reference for this method
For the US regulatory definition that connects APY with interest and compounding over a 365-day period, see the CFPB Regulation DD definition of annual percentage yield. The source is jurisdiction-specific; verify the rules that apply where you live.