How to Change the Font Size in the LaTeX book Class

To change the main font size in a LaTeX book, put a size option in the \documentclass declaration. The standard book class supports 10pt, 11pt, and 12pt; if you omit the option, it uses 10pt.

Set the size in the class declaration

Choose the size you want in square brackets:

\documentclass[12pt]{book}

\begin{document}
\chapter{A Sample Chapter}
This text uses the 12-point document size.
\end{document}

Replace 12pt with 10pt or 11pt to use one of the other standard sizes. This option sets the class’s base text size and its associated size commands, such as \small and \large. It also lets the class use its normal settings for elements such as headings and footnotes. You do not need to load a separate font-size package for these options.

Use a size beyond 10pt, 11pt, or 12pt

The standard book class does not provide options such as 14pt or 9pt. For more standard sizes, use the extended extbook class from the extsizes package collection. For example:

\documentclass[14pt]{extbook}

extbook provides additional class sizes, including 8pt, 9pt, 14pt, 17pt, and 20pt, alongside 10pt, 11pt, and 12pt. Your LaTeX installation must include the extsizes classes. If you are working with a publisher’s template, check its requirements before switching classes; the template may depend on the standard book class.

For a particular paragraph or other passage, use a scoped size command rather than changing the class-wide size:

{\Large
This passage is larger than the surrounding text.
}

The braces limit the change to the enclosed text. Commands such as \small, \large, and \Large select named sizes relative to the document’s base size; they do not offer an exact point size.

For an exact size in a local passage, use \fontsize{size}{baseline}\selectfont. For example, \fontsize{14pt}{17pt}\selectfont requests 14-point text with a 17-point baseline distance. The second value controls the distance between lines. Scope this command with braces, as above. It is a local font selection, not a replacement for the class option: class commands for headings or other elements may still select their own sizes.

Common mistakes

• Putting the option in the wrong place: Write \documentclass[12pt]{book} near the start of the file. A size option belongs to the class declaration, not an ordinary \usepackage line.
• Using an unsupported size with book: \documentclass[14pt]{book} does not make the standard class 14-point. LaTeX may warn that the option was unused and continue with the default size. Use extbook if you need one of its extra sizes.
• Expecting \large to resize the whole book: It changes the size of text from that point until the current group ends or another size command is used. For a document-wide change, use the class option.
• Forgetting \selectfont: After \fontsize{14pt}{17pt}, LaTeX needs \selectfont to apply the requested size and line spacing.

If the text still appears unchanged after setting a supported class option, check that the declaration you edited is the one actually used to compile the document—for example, a master file may load the chapter file you are editing.