How to determine the monthly interest rate from an annual interest rate I have a calculation which gives me the annual interest rate if I already know the monthly interest rate as follows:

(Monthly interest rate + 1)^12

In this case I have 0.32% as the monthly rate (stated as a number: 0.0032 plus 1 is 1.0032 ) to the power 12 (the number of repayments per year) to give me 3.908% as the annual rate.

However let's say I have the annual rate of 2.549% but I do not know the monthly rate. I would like to find the reverse of this calculation to give me the monthly interest rate.
If the stated annual rate is $2.549\%$, you would divide by $12$ to get the monthly rate.

However, if the effective annual rate is $2.549\%$, then letting the monthly rate be $i$, we have $(1+i)^{12}=1.02549$.  So $i=\sqrt[12]{1.02549}-1\approx .0021$, or $0.21\%$
Simple:

Divide the annual interest rate expressed as a percentage by 12 to calculate the monthly interest rate expressed as a percentage. For example, if you have an annual interest rate of 7.8 percent, divide 7.8 by 12 to find the monthly interest rate is .65 percent.