Notes about body font size and leading

Line spacing (known in typography as leading) is actually just as important as the font size itself for keeping a reader's eyes from getting tired.

If your lines are too tight, the page looks like a dense, intimidating wall of gray. If they are too loose, the text feels disjointed, and the reader struggles to group the words into sentences.

The Golden Rule of Leading

The standard rule in book design is to set your leading 2 to 4 points larger than your font size (roughly 120% to 145% of the font size).

Typesetters express this as a fraction: [Font Size] / [Leading].

Here are the most common sweet spots for standard trade books:

10 / 13 (10-point font with 13-point leading)
11 / 14 (11-point font with 14-point leading) — This is the most common default.
11.5 / 15 (11.5-point font with 15-point leading)
12 / 16 (12-point font with 16-point leading)

Three Things That Change the Rules

While those formulas are a great starting point, you’ll need to adjust your leading based on a few specific variables in your book:

1. The Font's "X-Height"

Just like font size, line spacing is highly dependent on the specific typeface you use. Fonts with a large x-height (tall lowercase letters, like Palatino or Georgia) take up more vertical space and feel heavier on the page. They need more leading to breathe. Fonts with a small x-height (like Garamond) naturally leave more white space between lines, so they need less leading.

2. Line Length (The "Measure")

The wider your physical page, the longer your lines of text will be. When a reader gets to the end of a long line, their eye has to travel a long way back to the left side of the page to find the next line. If the lines are too close together, their eye will easily skip or repeat a line.

Narrow pages (like mass-market paperbacks) can handle tighter line spacing.
Wide pages (like large non-fiction or textbooks) require more generous line spacing to guide the reader's eye safely back to the left margin.

3. Word Processors vs. Layout Software

If you are formatting your book in standard word processors like Microsoft Word or Google Docs, you don't usually set line spacing in points.

"Single spacing" in Word is typically around 115% to 120% of the font size, which is often slightly too tight for a printed book.
1.15 to 1.25 spacing is usually a better approximation for standard book leading.
If you are using professional software like Adobe InDesign or Affinity Publisher, you will set the exact point size (e.g., 14 pt) directly.