If Your Users Need a Magnifying Glass, Your Font Is Too Small
Base font size on mobile should be at least 16px. Ideally larger.
Below 16px, most mobile browsers zoom in on form inputs (which is jarring). Text becomes hard to read. Google flags it as a usability issue. This is part of the page experience signals that Google evaluates alongside Core Web Vitals.
The Rule of Thumb
Body text: 16px minimum. Headings: proportionally larger. Line height: 1.5 for body text. Maximum line length: about 70-80 characters.
Why It Matters for SEO
Google's mobile-friendliness assessment includes text readability. If your content is illegible without zooming, you fail the test. Failed tests mean lower rankings. Make sure your tap targets are properly sized too.
Simple fix. Big impact. Part of our 113-task checklist. Free. No credit card.