One video, three publishing limits
Short vertical video is the one format that genuinely travels: film once, publish to Instagram Reels, TikTok and YouTube Shorts. What breaks the trip is rarely the footage, it's the publishing limits, and each platform documents its own set differently. Instagram's own API specification caps a Reel upload at 15 minutes and 300MB. YouTube draws a hard line at 3 minutes, past which a video simply stops being classified as a Short. TikTok publishes no single number at all, its API hands back a per-account limit instead. The table below is sourced to each platform's own documentation, then unpacked underneath.
The specs that matter
| Instagram Reels | TikTok | YouTube Shorts | |
|---|---|---|---|
| Max length | 15 minutes, 3 seconds minimum | Account-dependent, no fixed number published | 3 minutes |
| Max file size | 300MB | 4GB | Not published for Shorts |
| Resolution / aspect ratio | Up to 1920 horizontal pixels; ratio 0.01:1 to 10:1, 9:16 recommended | 9:16 recommended in creator guidance; no resolution ceiling published in the posting API docs | Square or vertical, up to 1080p (1080×1920 at 9:16) |
| Caption / title limit | 2,200 characters, 30 hashtags accepted by the API (Instagram itself now recommends up to 5) | 2,200 UTF-16 runes through the posting API | 100 characters (title) |
| Format & codec | MOV or MP4, HEVC or H264, 23 to 60 FPS, VBR up to 25Mbps | MP4 recommended, H.264 recommended | Not published for Shorts |
Sources: Instagram Graph API media reference, Instagram, @creators on hashtags, TikTok Content Posting API media transfer guide, TikTok direct post reference, TikTok creator info reference, YouTube Help: get started with Shorts, YouTube Help: three-minute Shorts.
Length: two fixed numbers, and one that isn't
Instagram's limit is the most generous and the most exact. The Graph API's Reel Specifications state a "15 mins maximum, 3 seconds minimum" duration for any Reel published through the API, the same route a scheduler uses. YouTube is the strictest and the most literal: since 15 October 2024, a video with a "square or vertical aspect ratio" and "up to three minutes in length" is classified as a Short, and anything longer or landscape simply is not one, whatever the upload intent (YouTube Help).
TikTok is the one that resists a single answer. Its Content Posting API does not return a platform-wide maximum at all, it returns `max_video_post_duration_sec`, "the longest video duration in seconds that the TikTok creator can post," from the `creator_info` endpoint, because, in TikTok's own words, "different users have different maximum video-duration privileges" (TikTok for Developers). The example response TikTok documents for its own official account shows a value of 300 seconds, five minutes, not the round 3 or 10 minutes quoted everywhere else. Anyone asking "how long can a TikTok video be" is asking a question TikTok answers per account, not once.
File size and caption limits: where an upload silently fails
A caption or file that clears one platform's ceiling can still bounce off another's. TikTok's own posting API caps a video at 4GB and recommends MP4 with H.264 (TikTok for Developers); Instagram's Reel Specifications cap the same field at 300MB, with MOV or MP4, HEVC or H264, and a bitrate ceiling of 25Mbps VBR. A master exported for TikTok's headroom can be too heavy for an Instagram container before a single frame is watched.
Captions carry the same trap. TikTok's Content Posting API caps the title field at "2200 in UTF-16 runes" (TikTok for Developers), Instagram's caption field takes the same 2,200 characters and its API will technically accept up to 30 hashtags and 20 @ tags, and YouTube gives a Short's title just 100 characters before it's cut (YouTube Help). The API ceiling isn't a target: Instagram's own creator guidance now points to "up to 5 hashtags in a reel or post" for reach, the same number our Instagram guide uses. One caption written for Instagram's character ceiling will already overflow YouTube's title field twenty-two times over; it has to be written once, then cut per platform, not pasted three times.
What each platform says moves a video, and what it never said
Instagram states on the record that Reels ranking predicts "how likely you are to watch a reel all the way through, like it, say it was entertaining or funny, and go to the audio page" (Instagram, on how Instagram works). TikTok's own explanation of the For You feed says a completed watch, "whether a user finishes watching a longer video from beginning to end, would receive greater weight than a weak indicator" (TikTok Newsroom).
YouTube has not published an equivalent, Shorts-specific weighting. Its recommendations help page lists watch history, search history, subscriptions, likes and dislikes, "not interested" feedback and satisfaction surveys as signals, and says only that "the Shorts Feed is personalized to reflect what we think you want to see next," with no breakdown of how those signals apply to Shorts specifically (YouTube Help). That gap is worth knowing rather than guessing past.
The one signal every platform agrees on is quality of the file itself. Instagram states plainly that it works to "make certain reels less visible for other reasons, such as low-resolution or watermarked reels, reels that are muted or contain borders, reels that are majority text, or reels that have already been posted on Instagram" (Instagram Ranking Explained), and its later originality guidance repeats "no visible watermarks" as a condition of eligibility for recommendation at all (Instagram, recommendations and originality). Always export the clean master from your editor, never re-download from a platform.
Publishing the trio without the tab dance
The workflow that holds up: export one clean 9:16 master under 300MB, write the caption once at Instagram's 2,200-character ceiling, then cut it down for TikTok's title field and further still for YouTube's 100 characters. A scheduler that publishes through each platform's official API, like ad2app, turns the size and caption checks into one composer pass instead of three separate uploads, and reports back which platform the video actually worked on.
The per-platform detail, with sources, lives in our free field guides: Instagram, TikTok and YouTube.
Go deeper: how to post on Instagram · how to post on TikTok · how to post on YouTube.
Every claim above links its primary source. The per-platform specifics live in the free platform playbooks, sources included.