What is the percentage of new content required to make a book a second edition?

Question

I know that the industry standard used to be 20% but someone recently told me it was 30%. Does anyone know where I could find this information?

Answer 1

0%.

Hmm - I amended this answer, but it seems it did not take.  

There are different definitions of second edition.  Often, though perhaps this was more true in the old days, a second edition was one run from newly set type (these days, typesetting has mostly given way to computers.)  It could have new material, or not.    There arose a question of how much of the type needed to be changed (if the publisher was the same) in order to be considered newly-set.  A standard rule of thumb for a second or revised edition in these circumstances was, I believe, 10 %.  But there weren't hard and fast rules, and typ could be entirely reset, if publishers had changed or time had passed, without any changes to the text at all.

It may be that informal belief regarding what percentage needed to change differed with different parts of the publishing industry:  for instance, what constituted a new edition of a novel was probably different than what would constitute a new edition of a textbook.

Answer 2

Zero.