In book publishing, casting off is the process of estimating the number of signatures required to typeset a manuscript.
This word count is then divided by an estimate of the number of words or characters per full page of a model book, i.e., a previously published book with approximately the same trim size and type specifications to be applied to the manuscript.
In addition, the editor must anticipate any elements that will be added later in proofs, such as a preface or index.
Editors tend to round up rather than down, because if the book requires more pages than anticipated, it will cost more to produce, but fewer pages and therefore lower production costs are less problematic.
While some software exists to help editors in accurately casting off manuscripts, even an experienced editor often can't foresee the many variables that will affect a book's length, some of which may not arise until the book is well into production.