To calculate monthly APY, use the formula APY = (1 + r/12)^12 − 1, where r is the nominal annual interest rate expressed as a decimal. A 5% rate compounded monthly, for example, works out to (1 + 0.05/12)^12 − 1 = 0.05116, or about 5.116%. That figure is what your money actually earns over a full year once each month’s interest starts earning interest of its own.
What You Need Before You Start
Two numbers do all the work: the nominal interest rate and the compounding frequency. The nominal rate is the annual percentage the bank quotes before compounding is applied. Look for it on your account disclosure, the bank’s website, or your monthly statement. Federal rules under Regulation DD require banks to make this information available before you open an account and again whenever you ask.1eCFR. 12 CFR Part 1030 – Truth in Savings (Regulation DD)
The compounding frequency tells you how many times per year the bank credits interest to your balance. For monthly compounding, that number is 12. Your disclosure should say something like “interest credited monthly.” If it does, you’re set.
Convert the nominal rate from a percentage to a decimal before you calculate. Divide by 100: 5.25% becomes 0.0525, and 3.8% becomes 0.038. A slip here throws off every step that follows, so verify it before moving on.
The Formula, Piece by Piece
APY = (1 + r/n)^n − 1. For monthly compounding, n = 12. Each part of the expression does something specific.
The fraction r/n splits the annual rate into the rate that applies to a single month. At 5%, that’s 0.05/12, or roughly 0.004167 per month.
Adding 1 folds in your principal. The value 1.004167 represents your balance after one month for every dollar you started with. Without the 1, you’d be tracking only the interest sliver, not the growing total.
Raising that sum to the 12th power repeats the compounding across every month of the year. Each month’s interest is calculated on a slightly larger balance than the month before, and the exponent captures that stacking.
Subtracting 1 at the end strips the original principal back out, leaving only the growth as a decimal. Multiply by 100 to get the percentage.
Step-by-Step Example at 5%
Take an account paying 5% annually, compounded monthly.
- Convert the rate: 5% ÷ 100 = 0.05
- Divide by 12: 0.05 ÷ 12 = 0.004167
- Add 1: 1 + 0.004167 = 1.004167
- Raise to the 12th power: 1.004167^12 = 1.051162
- Subtract 1: 1.051162 − 1 = 0.051162
- Convert to a percentage: 0.051162 × 100 = 5.116%
The APY is about 5.116%. On a $10,000 deposit left alone for a year, that’s roughly $511.62 in interest, compared to the $500.00 the flat 5% rate suggests. The extra $11.62 is the compounding effect — twelve months of interest earning interest.
Doing It in a Spreadsheet
If you’d rather skip the arithmetic, Excel and Google Sheets both have a built-in function. The syntax is =EFFECT(nominal_rate, periods_per_year). For 5% compounded monthly, enter =EFFECT(0.05, 12). The cell returns 0.05116, which you can format as a percentage to display 5.116%.
To see the formula work through the steps in a cell instead, type =((1 + 0.05/12)^12) – 1. Both approaches give the same answer. EFFECT is faster when you’re comparing several accounts at once.
How the Answer Shifts With Other Compounding Schedules
The APY formula works the same way at any frequency. Only n changes. Using a 5% nominal rate:
- Annually (n = 1): (1 + 0.05/1)^1 − 1 = 5.000%
- Quarterly (n = 4): (1 + 0.05/4)^4 − 1 = 5.095%
- Monthly (n = 12): (1 + 0.05/12)^12 − 1 = 5.116%
- Daily (n = 365): (1 + 0.05/365)^365 − 1 = 5.127%
The step from annual to monthly is worth real money. The step from monthly to daily is worth about a hundredth of a percent, or roughly a dollar a year on a $10,000 balance at 5%. If you’re comparing two accounts with the same nominal rate but different compounding schedules, the APY tells you which one actually pays more.
There’s also a theoretical case called continuous compounding, where the number of periods approaches infinity. The formula simplifies to APY = e^r − 1, with e roughly 2.71828. At 5%, that produces about 5.127%, essentially the same as daily. Banks don’t really compound continuously, but it shows that piling on more periods eventually hits a ceiling of diminishing returns.
Why Your Actual Return May Differ
The APY you just calculated assumes a static deposit sitting for exactly one year with no fees. Two things commonly move the real number.
First, fees. Under Regulation DD, the APY a bank discloses reflects interest only. It doesn’t subtract monthly maintenance charges or other account fees.1eCFR. 12 CFR Part 1030 – Truth in Savings (Regulation DD) To see your net return, subtract total annual fees from total annual interest, then divide by your principal. A 4% APY on $5,000 produces $200 of interest, but a $5 monthly maintenance charge takes $60 back, leaving an effective yield of about 2.8%.
Second, deposits and withdrawals during the year. Banks report a backward-looking figure called APY Earned on periodic statements to show what your account actually produced. The regulatory formula is APY Earned = 100 × [(1 + Interest Earned / Balance)^(365 / Days in Period) − 1], where “Balance” is the average daily balance during the statement period.2Cornell Law School. 12 CFR Appendix A to Part 1030 – Annual Percentage Yield Calculation If a 30-day statement shows $4.10 of interest on an average daily balance of $1,000, the APY Earned is 100 × [(1 + 4.10/1000)^(365/30) − 1], or roughly 5.10%. Compare that to the account’s stated APY if you want to check whether the account is performing as advertised.