Newspapers publish a "per column inch" rate based on their circulation and demographic figures.
To determine the cost of the advertisement, multiply the number of column inches by the newspaper's rate.
Nowadays, most newspapers and magazines have converted to a "modular" system that simplifies ad size and eliminates the need to figure out column inches.
This harks back to the days of the late 19th century linotype machine and its relatively uniform newspaper column-widths, echoed in the phototypesetting and paste up days of the late 20th Century, when typeset newspaper stories were still printed on long strips of paper one-column wide, then pasted into page layouts.
Correspondents, for example, might be paid "by the inch" for their stories, and some organizations explained their nickname Stringer (journalism) as originating when regular-width columns of set type were measured with lengths of string.
The software used in most present-day newsrooms still measures column inches to give reporters and editors an estimate on how much space a story will take up on a page.
Newsroom staffers also measure items such as photographs and infographics using column inches.