Periodic Interest Rate Calculator

A periodic interest rate is the interest rate applied over a specific compounding period — such as a month, quarter, or day. It is derived from the annual nominal rate divided by the number of compounding periods. For example, a 12% annual rate compounded monthly gives a periodic rate of 1% per month.

The basic formula is: Periodic Rate = Annual Nominal Rate / Compounding Periods per Year (R/m). When payments and compounding frequencies differ, the more precise formula is: r = (1 + i/m)^(m/n) − 1, where i is the nominal annual rate, m is compounding periods per year, and n is payment periods per year.

The nominal rate (APR) is the stated annual interest rate without accounting for compounding within the year. The periodic rate is the share of that nominal rate applied each compounding period. They are equal only when compounding occurs once per year; otherwise the periodic rate is a fraction of the nominal rate.

The periodic rate is what actually determines how much interest accrues each period on a loan or investment. Lenders and banks use it to calculate monthly mortgage payments, credit card interest charges, and savings account growth. Understanding it helps you compare financial products on an apples-to-apples basis.

Yes. Once you know the periodic rate (r) and compounding periods per year (m), the EAR is: EAR = (1 + r)^m − 1. The EAR reflects the true annual cost of borrowing or the true annual yield on an investment after accounting for compounding.

Banks apply the periodic rate to your outstanding balance each compounding period. For credit cards, this is typically daily; for mortgages, it is monthly. Even a small periodic rate compounds over time, which is why the effective annual rate is always higher than the nominal rate when compounding occurs more than once a year.

When payments occur at a different frequency than compounding (e.g., monthly payments on a loan that compounds daily), you must use the adjusted formula r = (1 + i/m)^(m/n) − 1 rather than simply dividing the annual rate by the number of payments. This calculator handles both cases automatically.

Suppose your credit card charges 18% annually and compounds daily (365 times per year). The daily periodic rate is 18% / 365 ≈ 0.04932% per day. If you want the monthly periodic rate with monthly payments, you use (1 + 0.18/365)^(365/12) − 1 ≈ 1.4907% per month — slightly higher than the simple 1.5% due to daily compounding.