Choosing the correct size screw for a Kreg pocket hole jig is fairly simple if you know what to look for. There are some handy charts, guides, and selectors that I will highlight here!
The first step in determining the correct Kreg screw size is to determine the thickness of the wood you are working with. This is the actual thickness, not the nominal thickness
Note: if you are working with two different thicknesses, use the smallest of the two.
Determine whether the piece you are making is indoor or outdoor. If it is indoors, use zinc-coated screws. If it is outdoors, use Blue-Kote Kreg screws.
Determine the type of wood. – For softwoods like pine, plywood, melamine, particle board, and MDF, use course thread screws. For hardwoods like maple or oak, use fine thread screws
As a recap - first determine the thickness of the piece. Use a depth guide chart to find the corresponding screw. Then, determine whether your piece will be indoors or outdoors, then if it is hardwood or softwood,