Both Rz and Ra express how smooth the finish of a surface is. However, the two do not measure precisely the same things. Rz measures the average height distance between a set number of the highest peaks and valleys on the surface. Ra imagines that you could level all the peaks to fill in all the valleys to come up with a theoretical mean surface line. Ra is the average deviation from the theoretical mean of the peaks and valleys. Even though Rz and Ra measure different things, it is possible to do a rough conversion between the two with simple math.
Enter the Rz value in your calculator. For example, press the "9" key.
Divide the Rz number by 7.2. For example, press the "/" button, then enter "7.2." 7.2 is the factor needed to get an approximation from Ra to Rz. So, if you already have the Rz, you divide by 7.2 to get the estimated Ra.
Press the "=" button to get a rough estimate of the Ra. For example, 9 Rz roughly converts to 1.24 Ra.