How to add columns with coordinates for centroids

[Cathina_Gunn_de_Rosas]

I have a fishnet and I need to add a column for the x and y coordinates of the centroid of each grid square. I assume I can do this somehow with the field calculator, but am not sure exactly how to do it.

What is the simplest way?

(I need to do it for about 20 fishnets, so I don't want to make things any harder than necessary)

Thank you!

[WesMiller]

Try using Add Geometry Attributes—Help | ArcGIS for Desktop