Minimum internal door width
How to Measure a Door? What is standard door width? : Minimum Internal Door Width
Quote from Article entitled How to Measure a Door? What is standard door width?:
On the market today, the standard room door width also happens to be the minimum internal door width. Therefore unless you get one custom made, you are not likely to find one that is less than 32 inches in width.
The full version of the article can be found here.