How do I calculate the dpi or ppi on a photo if I do not know

Question

say i have a pic that is 4x6 in size how do I calculate the pixels

Answer 1

on a photographic print?  - thats what you mean by photo? thats what i mean by photo a printed image, ok the photo, put it under a microscope then count the number or dots in an inch..... DPI

if you have an electronic file then open it in a program like photoshop and goto image/image size, the dialog box will tell you

a