Book Sizes and Formats — Folio, Quarto, Octavo, and Beyond

The terminology of book sizes — folio, quarto, octavo, duodecimo, and their variations — descends from the early days of printing, when the size of a book was determined by how many times a standard printed sheet was folded before being bound. These format designations remain fundamental to bibliographic description, rare book cataloging, and the language of the book trade.

The Principle: Sheet Folding

How It Works

A printer begins with a large sheet of paper (the “broadsheet”). The number of times this sheet is folded determines the format:

Folio (2°) — the sheet is folded once, producing 2 leaves (4 pages). Each leaf is half the size of the original sheet.

Quarto (4to) — the sheet is folded twice, producing 4 leaves (8 pages). Each leaf is one-quarter of the original sheet.

Octavo (8vo) — the sheet is folded three times, producing 8 leaves (16 pages). Each leaf is one-eighth of the original sheet.

Duodecimo (12mo) — a more complex folding pattern produces 12 leaves (24 pages) per sheet.

Sextodecimo (16mo) — the sheet is folded four times, producing 16 leaves (32 pages).

The Resulting Sizes

Because the format depends on the number of folds of the original sheet, the actual physical dimensions of the book depend on the size of the original sheet. A folio printed on a large sheet (a “royal” sheet) will be much larger than a folio printed on a small sheet (a “crown” sheet). This is why format designations (folio, quarto, etc.) are not precise measurements — they are descriptions of the manufacturing process.

Standard Formats

Broadside (1°)

A broadside is a sheet printed on one side only and not folded at all. Broadsides were used for proclamations, advertisements, ballads, news sheets, and other single-sheet publications. They are technically not “books” but are an important category of printed material collected alongside books.

Folio (2° or fo.)

Fold: 1 Leaves per sheet: 2 Pages per sheet: 4 Approximate height: 30–38 cm (12–15 inches), though this varies widely

Folios are large, imposing books. The format was standard for important reference works, atlases, Bibles, legal texts, and other substantial publications from the fifteenth through eighteenth centuries. Shakespeare’s First Folio (1623) is the most famous example.

Elephant folio — an oversized folio, typically over 58 cm (23 inches) tall. Audubon’s The Birds of America is the most celebrated elephant folio.

Atlas folio — approximately 63 cm (25 inches) or taller. The term comes from the large format used for atlases and maps.

Quarto (4to or 4°)

Fold: 2 Leaves per sheet: 4 Pages per sheet: 8 Approximate height: 25–30 cm (10–12 inches)

Quartos are a versatile middle format — large enough for illustrations and maps but not as unwieldy as folios. Shakespeare’s plays were first published individually in quarto format before being collected in the First Folio.

Octavo (8vo or 8°)

Fold: 3 Leaves per sheet: 8 Pages per sheet: 16 Approximate height: 19–25 cm (7.5–10 inches)

The octavo is the most common format for books from the eighteenth century to the present. It corresponds roughly to what we think of as a standard hardcover book. The octavo became dominant because it provides a comfortable reading size while using paper efficiently.

Crown octavo — approximately 19 cm (7.5 inches) Demy octavo — approximately 22 cm (8.75 inches) Royal octavo — approximately 25 cm (10 inches)

Duodecimo (12mo or 12°)

Fold: complex (typically involving a combination of folds) Leaves per sheet: 12 Pages per sheet: 24 Approximate height: 15–19 cm (6–7.5 inches)

Duodecimos are compact, portable books — common for novels, devotional works, and pocket-sized reference books. The format was particularly popular in the eighteenth and nineteenth centuries.

Sextodecimo (16mo or 16°)

Fold: 4 Leaves per sheet: 16 Pages per sheet: 32 Approximate height: 10–15 cm (4–6 inches)

A small format used for miniature books, prayer books, almanacs, and other works where portability was paramount.

Smaller Formats

Even smaller formats exist — 32mo (tricesimo-secundo), 64mo (sexagesimo-quarto), and beyond — but these are rare and mostly of interest to collectors of miniature books.

Identifying Format

Chain Lines and Watermarks

The most reliable way to determine a book’s true format is by examining the chain lines and watermarks of the paper:

Chain lines are the horizontal or vertical lines visible in handmade paper, created by the wires of the papermaking mold. Their orientation relative to the spine indicates the format:

• Folio — chain lines run vertically (parallel to the spine)
• Quarto — chain lines run horizontally (perpendicular to the spine)
• Octavo — chain lines run vertically

Watermarks — when present, the position of the watermark within the leaf helps confirm the format. In a folio, the watermark appears in the center of the leaf; in a quarto, it appears at the spine edge; in an octavo, it appears in the upper inner corner.

Measurement

When chain lines and watermarks are not available (as with most machine-made paper from the nineteenth century onward), format is estimated from the book’s height:

These measurements are approximations — actual dimensions vary with the sheet size used.

Format in Bibliographic Description

In bibliographic and catalog descriptions, format designations serve several purposes:

Physical description — the format gives a quick sense of the book’s physical characteristics and visual impression.

Manufacturing information — the format records how the book was manufactured, which is important for bibliographic analysis and edition identification.

Standard terminology — format designations provide a shared vocabulary that allows dealers, collectors, librarians, and bibliographers to communicate efficiently about a book’s physical form.

The traditional format designations remain in use because they encode information that a simple height measurement does not — they describe the relationship between the printed sheet and the finished book, connecting the physical object in your hands to the manufacturing process that created it.