Optimal Font Size and Line Height for Digital Reading (2026)
· Alex
The three numbers that make digital text readable: 16 px minimum body size, 1.4-1.6 line height, 45-75 characters per line — with WCAG, Nielsen Norman, and Bringhurst citations.
Three numbers decide whether a paragraph is easy or painful to read: font size, line height, and line length. Get them right and your text becomes invisible — the reader absorbs meaning without noticing the mechanics. Get them wrong and even a brilliant sentence feels heavy.
Font Size: 16 px Is the Floor, Not the Ceiling
Every modern design system — from Apple's Human Interface Guidelines to Google Material Design 3 — converges on the same recommendation: 16 px (1 rem) is the minimum for body text on screen. This number is not arbitrary. It is calibrated to the average reading distance of a laptop (50-70 cm) and a phone (30-35 cm) so that the resulting visual angle falls within the range the human eye processes without strain.
The Nielsen Norman Group recommends going further on informational pages: 18-20 px for body text improves scan speed by roughly 10% in their eye-tracking studies, at almost no design cost. Pages that feel "premium" or "editorial" — The New York Times, Medium, Stripe's documentation — all typeset body text between 18 and 21 px. If your current site is stuck at 14 px "because it fits more content," you are optimising for the wrong metric: density is not retention.
Line Height: Aim for 1.4 to 1.6
Line height (in CSS, `line-height`) is the vertical space between successive lines. Too tight and the eye loses track of where the next line begins. Too loose and the paragraph fragments into disconnected strips.
The consensus from Robert Bringhurst's Elements of Typographic Style, WCAG 2.2 success criterion 1.4.12, and contemporary web-typography research is a ratio between 1.4 and 1.6 for body text. The W3C specifically mandates a minimum of 1.5 for paragraph text when users activate the enhanced-spacing user preference. For large headings, you can safely tighten line-height to 1.1-1.2 because the characters themselves are bigger, and for small caption text you may need to loosen it to 1.6-1.7 to maintain scannability.
Line Length: 45 to 75 Characters Per Line
The most overlooked of the three levers is line length — technically, the measure. Bringhurst's rule, repeated by the Baymard Institute and the Nielsen Norman Group, is that body text should run 45 to 75 characters per line, with 66 considered the ideal for printed prose. Under 45 characters, the eye jumps lines too often and loses continuity. Over 75 characters, the return sweep — the move from the end of one line to the start of the next — becomes error-prone and fatiguing.
On the web this translates to a `max-width` between 32rem and 40rem (roughly 500-640 px) for pure reading content. You will notice every well-designed long-form site enforces this constraint: Medium, Stripe's engineering blog, Anthropic's research pages and The New Yorker all cap paragraph width in the same band, even when the surrounding layout is full-bleed.
How Agentys Applies the Three Rules
Agentys is built on these three numbers. Body text in the interface is 18 px with a line-height of 1.6 and a maximum reading measure of 40rem — you can count the characters on any paragraph and you will land between 60 and 72. The long-form article layout you are reading right now is the same system, rendered through Tailwind's `text-lg leading-relaxed max-w-2xl`.
When Agentys drafts an email on your behalf, it does not inject CSS: it writes short, well-structured paragraphs so the default line-height of the recipient's client produces readable output no matter where it lands. Gmail on mobile, Outlook on a widescreen monitor, Apple Mail on an iPad — same message, always readable. Start your free trial and see what calibrated typography feels like in your own inbox.
Readable typography is not magic — it is three numbers applied with discipline: 16 px or more, line-height 1.4 to 1.6, 45 to 75 characters per line. Agentys enforces those numbers so your readers stay focused on your message rather than fighting your layout.