[amsat-bb] grid locator program

I'm planning a trip this spring and plan on working some of the hard grids west of me. I'm in EM59. I'm looking for a program that I can figure the grids both ways. Type in my longitude and latitude and get the grid. Type in the grid and know the longitude and latitude of that grid. If anyone knows of a program it sure would help me. I have a Mac or PC would prefer using the Mac. george wi9i