An interest rate states the annual rate applied to a balance. Annual percentage yield (APY) reflects the amount earned over one year after accounting for compounding. When interest compounds more than once a year, APY is normally higher than the nominal annual interest rate.
APY is useful when comparing deposit accounts because it places different compounding schedules on a common annual basis. It does not guarantee that a variable rate will remain unchanged.
Model a balance: use the Savings Interest Calculator when an account quotes APY. Use the Simple vs Compound Interest Calculator to isolate the effect of compounding frequency.
Interest rate and APY answer different questions
- Nominal annual interest rate: the annual rate before the effect of within-year compounding.
- APY: the effective one-year yield after the stated compounding frequency is included.
The US Consumer Financial Protection Bureau defines APY as a percentage reflecting the total amount of interest paid on an account based on both the interest rate and the frequency of compounding.
How to calculate APY
For a nominal annual rate that compounds at equal intervals:
APY = (1 + r ÷ n)n − 1
Here, r is the nominal annual rate as a decimal and n is the number of compounding periods per year.
A 4% nominal annual rate compounded monthly produces an APY of approximately 4.074%. On an unchanged €10,000 balance, that is about €407.42 of interest over one year before tax or fees.
| Nominal rate | Compounding | Modeled APY | One-year interest on €10,000 |
|---|---|---|---|
| 4.00% | Annual | 4.000% | €400.00 |
| 4.00% | Quarterly | 4.060% | €406.04 |
| 4.00% | Monthly | 4.074% | €407.42 |
| 4.00% | Daily (365) | 4.081% | €408.08 |
The difference is modest at this rate and over one year. It becomes larger as the rate or time period increases.
Use APY only once
APY already incorporates compounding. A common error is to take an APY, divide it by 12 and then compound that monthly. That effectively adds another compounding assumption.
To create a monthly rate consistent with a quoted APY, use:
Equivalent monthly rate = (1 + APY)1/12 − 1
The Finance Charts savings calculator uses this conversion. It does not need the account’s nominal rate or original compounding frequency when APY is already known.
APY does not capture every account condition
A displayed APY may depend on product terms that change the return actually received. Check:
- whether the rate is fixed or variable;
- minimum or maximum balances;
- balance tiers;
- promotional periods and expiry dates;
- monthly fees or conditions required to avoid them;
- withdrawal restrictions; and
- when interest is credited and whether the account remains open long enough to receive it.
A higher APY can produce a lower net outcome if fees or eligibility conditions offset the extra interest.
APY is not an inflation-adjusted return
APY measures nominal account growth. It does not show whether the balance gains purchasing power after inflation. Tax on interest can reduce the amount retained further, depending on the account and jurisdiction.
Keep the questions separate: APY compares the account’s modeled annual yield, while inflation-adjusted return compares growth with changes in prices.
Methodology and sources
The formulas and comparison table are original Finance Charts calculations. They assume a constant nominal rate, equal compounding intervals and no deposits, withdrawals, tax or fees during the one-year example.
- Consumer Financial Protection Bureau — Annual Percentage Yield calculation
- Federal Deposit Insurance Corporation — Truth in Savings
- Finance Charts — Methodology
Important: use the current disclosure for the actual account. A calculator cannot confirm rate eligibility, fees, deposit protection or future rate changes.