APY vs interest rate: which number goes into a compound calculator?
A worked conversion that prevents counting compounding twice when you model a savings account.
The short answer
APY already includes the effect of compounding over a year. A nominal annual interest rate does not. If a calculator expects a nominal rate, convert APY to the matching periodic rate first. Entering a 5% APY as a 5% nominal rate compounded monthly overstates the result.
Use a one-year check before a long projection
For a hypothetical 10,000 balance earning a constant 5% APY, with no deposits, withdrawals, fees or taxes, the one-year balance is 10,500. The 500 gain is already the annual yield including compounding. This is an assumed example, not a current bank rate or a return forecast.
If you instead enter a 5% nominal annual rate compounded monthly, the calculation is 10,000 × (1 + 0.05/12)^12, giving about 10,511.62. That is a different assumption. The extra 11.62 comes from treating the yield as a nominal rate, not from finding a better savings product.
Convert APY to a periodic rate
Write APY as a decimal y. For a model with 12 equal monthly periods, the equivalent monthly rate is (1 + y)^(1/12) − 1. Multiply that periodic rate by 12 to obtain the equivalent nominal annual rate for monthly compounding.
At a 5% APY, the equivalent monthly rate is about 0.407412%, and the nominal annual rate is about 4.888949%. Preserve precision through the calculation; round only the displayed figures. Substituting the periodic rate back into 12 monthly periods recovers exactly the assumed 5% annual yield, before floating-point rounding.
Monthly rate = (1 + APY/100)^(1/12) − 1
Nominal annual rate (%) = 12 × monthly rate × 100| Rate entered | Basis | Ending balance |
|---|---|---|
| 5% | APY | 10,500.00 |
| 4.888949…% | Nominal, compounded monthly | 10,500.00 |
| 5% | Nominal, compounded monthly | 10,511.62 |
Deposits do not earn a full year immediately
A monthly deposit is in the account for less time than an opening balance. You cannot multiply the total of a year’s deposits by APY and assume every deposit earned the full year’s yield. A projection needs both the cash-flow dates and an equivalent periodic rate.
Beginning-of-month and end-of-month deposits also differ by one period of growth in a constant monthly model. Keep the deposit timing explicit when comparing calculators. If your inputs are an advertised APY and daily account accrual rules, an equal-month educational model may differ from an actual bank statement.
How to use the rate basis in CalcMyCompound
The homepage frequency comparison can compare an APY basis with a nominal-rate basis. At equal APY and without intermediate cash flows, changing the nominal compounding frequency should preserve the same annual yield after conversion. At equal nominal annual rates, the effective yields can differ.
For a real account, check the account disclosure for variable rates, balance tiers, crediting rules and fees. APY is not a guarantee that the rate remains unchanged for a multi-year plan. Taxes and inflation are separate assumptions. This calculator illustrates a scenario; it does not reproduce every account’s daily ledger.
Sources and scope
Educational constant-rate calculations, not a bank offer or financial advice. Examples exclude fees, taxes and inflation, and assume reinvested interest. No current product yield is asserted.
Sources checked October 30, 2026. Worked examples are our educational calculations, not observations about people or product offers.
- CFPB Regulation DD, Appendix A: Annual Percentage Yield Calculation
Official US deposit-yield definition and compounding framework. Actual account disclosures control account-specific terms.
Publisher responsible for this guide: Kymata Labs LLC. To report a correction: Contact us.