This post is the third in a series on NEMA enclosure types and the IP code. For more information, read post 1 and post 2.
As stated in earlier posts, there is no clean way to convert from IP to NEMA or vise versa. Their vocabulary is just different. Each is more specific in different areas, making exact conversion impossible.
However, with a solid understanding of your application, you can successfully make a conversion without adversely affecting enclosure performance. As with anything, it depends on the application, and the reasons behind the rating specification.