Your question made me curious enough to look on Google for a way to do this. As far as I can tell the best way is to compare directly a Pantone color with the RAL color patches. I didn't find a formulaic way or a converter online to do directly Pantone to RAL, although people sell kits to do the direct color comparisons. Good luck!
You can find a lot of tables of color conversion between PANTONE and RAL. There is a new software for Mac what enables all conversions between PANTONE, RAL and HKS - ColorLinker. Check it: http://colorlinker.com
Hey there MPW, I couldn't figure out how to send you a PM on here. I saw a post in a thread on here where you posted an idea for a short film plot. I love the concept, and just wanted to get your permission to base a re-write of my own on your plot concept idea. Could you please message me with an ok....if it's cool that I borrow your brilliance? : ) Thanks!!
What do you want with RAL?
I know there is a bunch of painters asking for RAL numbers, but the RAL system is only suitable for metal powder coating. For other uses it is way too limited. So ask again if it is for say painting cabinets or walls. You are way better off with say a Sikkens color system swatch book.
The only good way to compare is with 2 swatch books.
And do it either outside under a cloudy sky, or with indoor light at 5000K with a good color reproduction index (you need special tubes for that).