Caption & Subtitle Specs Cheatsheet for Social Video Ads
Exact font sizes per aspect ratio, words-per-card limits, reading-speed budgets, contrast rules and burn-in vs SRT guidance for captioning social video ads that get watched on mute.
Most social video is watched with the sound off, which means your captions aren't a nice-to-have accessibility layer — they're the primary script delivery mechanism. This sheet covers exact font sizes per ratio, how many words belong on a card, the reading-speed budget viewers can actually process, and when to burn captions in versus ship them as a closed-caption file.
Captions are the ad, not a subtitle track
Treat your caption as the primary read for a viewer scrolling with sound off in a public place, on a bus, in a waiting room — which is most viewers, most of the time. If the caption alone can't carry the hook, the offer and the CTA, the ad is relying on audio that a large share of the audience will never hear. Script captions first, then treat the voiceover as reinforcement, not the other way around.
Size and pacing are a math problem, not a design opinion
Two numbers do most of the work: font size relative to frame height, and characters-per-second relative to how fast a scrolling viewer can actually read. Undersized text gets skipped even when it's technically visible; oversized text with too many words per card creates flicker and forces a re-read that breaks the scroll rhythm. Both failures look fine in an editor's static preview and fail on an actual phone at actual scroll speed — this is why the pixel and CPS numbers below matter more than they look like they should.
Burn-in and SRT solve different problems
Burned-in captions are rendered permanently into the video frame — they show up whether or not the viewer toggles anything, which is why they're the default for TikTok, Reels and Stories. Closed-caption files (SRT/VTT) are a separate accessibility track that a viewer or platform can toggle on, required in some regions and expected on YouTube. Most winning creative uses both: burn the captions in for the scroll, and still attach an SRT file so the ad satisfies accessibility requirements and native CC toggles.
How to use the table below
The locked sheet covers font size by aspect ratio, words-per-card and reading-speed limits, contrast and legibility rules, a burn-in vs SRT decision table, and karaoke-style word-highlight timing for hook sections. Klip Kanvas auto-generates burned-in captions sized to each export ratio from your script, so these numbers are mainly useful for manual QA or when you're editing outside the platform.
5 tables: font size by aspect ratio, words-per-card and reading-speed (CPS) limits, contrast and legibility rules, burn-in vs SRT decision table, and karaoke word-highlight timing.
Unlock the full cheatsheet
Enter your email and we'll instantly unlock the complete cheatsheet — plus send you fresh ad playbooks every month.
No spam. Unsubscribe anytime.
Ready to put this into practice?
Create your first AI UGC video ad in minutes — no filming, no actors, no editing.
Try Klip Kanvas freeMore in this section
Win/Loss Creative Readout Sheet
A scoring sheet for creative tests: sample-size floors, win/loss/inconclusive bands by metric, what to scale vs kill vs iterate, and the one-page readout the next sprint actually uses.
TikTok Spark Ads Cheatsheet
Spark Ads as a lookup sheet: authorization codes, post-first rules, inherited social proof, CTA overlay, expiry bands and QA versus cold in-feed uploads.
Ready to make ads like these?
Paste a product link and Klip Kanvas writes the script, casts the creator and renders the ad — no filming, no actors, no editing.