The bump value that you dial up in the material room is displayed in your preferred units, as specified in your General Preferences. You don't need to convert this, just be consistent in what units you work with if you are designing or editing materials in Poser.

In material definition files, however, the bump (and displacement) is always stored in inches. Unless you edit these by hand, I don't think you need to worry about this.

The only unit that does not have a 'real world' conversion factor is the Poser Native Unit, for which markschum gave the conversion factor above.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)