A book’s format refers to the shape and dimensions of the physical book.  When browsing bookseller catalogs and listings on the Internet, the terms and abbreviations used to describe book sizes (8vo, 12mo, folio, quarto, etc.) can seem confusing at first, but this page will help define a super octavo from a royal octavo.

Essentially these terms are simply an explanation of the book’s evolution from single sheets of paper into a completed book with printed pages. Each term defines how many times the initial piece of paper was folded to become pages.  Using this terminology, the term ‘Folio’ would tell us that the original sheet of paper was folded once, resulting in two leaves.  Therefore a book described as ‘Quarto’ would have its original sheet folded twice and would have four leaves - each leaf being one-quarter or one fourth the original size.

Armed with this knowledge, you can determine the approximate size of any book based on the publishing format.  Please note - the size is only an approximation because not every sheet of paper is exactly the same size. However these terms provide an essential guideline.

To make life even simpler, remember ‘Folio’ is a large upright-shaped book and an ‘Octavo’ is a small upright-shaped book, while a ‘Quarto’ is in-between the two and mostly square-shaped.  

Fold Symbol Name Outside height (inches)
- miniature Less than 2" x 1.5"
64mo sexagesimo-quarto 2 x 3
48mo quadragesimo-octavo 2.5 x 4
32mo tricesimo-secondo 3.5 x 5.5
18mo octodecimo 4 x 6.5
16mo sextodecimo 5 x 7.5
12mo duodecimo 5 x 7.375
12mo (large) duodecimo 5.5 x 7.5
8vo crown octavo 6 x 9
8vo octavo 6 x 9
8vo medium octavo 6.125 x 9.25
8vo royal octavo 6.5 x 10
8vo super octavo 7 x 11
8vo imperial octavo 8.25 x 11.5
4to quarto 9.5 x 12
fo. folio 12 x 19
fo. elephant folio 23" to 25"
fo. atlas folio 25" to 50"
fo. double elephant folio 50" +
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