How to calculate the quarterly increase given a yearly percentage increase

If I know headcount will increase by x% a year, how can I calculate the value at each quarter?

I have found that a 15% annual increase equates to a 3.55% quarterly increase.

I can say =D2*1.03555 and get the increase, but I found the 3.55 number by trial and error, and I'm hoping to find a formulaic way to do it.

Thank you for reading!

If cell A1 contains an annual percentage rate, then the rate applied each quarter that will be equivalent to that annual rate is

=(1+A1)^(1/4)-1