The Right Font Size for Subtitles: A Readability Guide

Font size has a dramatic effect on subtitle readability — here's how to choose the right size for different platforms, screen sizes, and audience accessibility needs.

Subtitle font size seems like a simple technical setting, but it involves several interacting factors that make it surprisingly nuanced. The right size depends on the resolution of the video, the platform it will be displayed on, the likely viewing distance of the audience, and the accessibility needs of your viewers. A font size that looks perfect on a 4K television viewed from three metres will appear tiny on a smartphone held at arm's length. A font size optimised for mobile will appear comically large on a cinema screen. The key concept is relative size — what matters is not the absolute pixel count of your text but its size relative to the overall frame. Industry standards typically specify subtitle font size as a percentage of frame height rather than an absolute pixel value, because this scales automatically with video resolution. The BBC's subtitle guidelines, which are widely referenced as a quality standard in the UK, recommend that subtitle text occupies no more than 8% of the screen height for individual characters, and no less than 5%. This relative sizing ensures legibility at standard viewing distances across different display technologies.

Different platforms have different viewing contexts that should inform your font size decisions. For YouTube, where videos are watched on everything from small phones to large smart TVs, YouTube's auto-sizing captions system adapts to the viewer's device. If you're burning subtitles into your video rather than using the platform's caption system, aim for a minimum font size of 28-32px in a 1080p export — this ensures legibility on mobile screens without appearing overly large on desktop. For TikTok and Instagram Reels, which are watched almost exclusively on mobile in portrait orientation, subtitle font size can be slightly larger — 32-38px in a 1080x1920 (9:16) export is appropriate. The vertical format and mobile-first viewing context means viewers are closer to the screen and can comfortably read slightly larger text. For broadcast television standards, Ofcom guidelines in the UK specify that subtitle text should be legible to a viewer with normal vision at standard viewing distances — typically interpreted as a minimum of 5% frame height for the character body. For cinema and large-screen projection, the viewing distance is greater and font size calculations change accordingly.

Minimum legibility for viewers with average vision is the floor requirement, not the goal. Many subtitle viewers have low vision, are elderly, or have other factors that make them more dependent on clear, large subtitle text. The best practice is to design subtitles that are as large as practical given the viewing context, rather than as small as you can get away with. Avoid the temptation to use small subtitle text to make your video look 'cleaner' — smaller subtitles might look more elegant but they directly harm accessibility. The text shadow or background band behind your subtitles is as important as font size for legibility. Without sufficient contrast between your text and the video background, even large text becomes unreadable. Use a minimum 2-3 pixel text shadow, or a semi-transparent background band of at least 70-80% opacity, to ensure text remains legible against varied video backgrounds. Line spacing matters too — cramped line spacing makes subtitle text harder to parse quickly. Allow at least 120% line height relative to your font size for comfortable reading. If you're adding subtitles for viewers who might need additional accommodation — such as educational content for older adults or health information for diverse audiences — err on the side of larger, well-spaced text that takes up more screen space but serves everyone better.