Instagram Stories ad at 16 seconds: split into cards or trim?

Instagram says a Stories video ad under 16 seconds plays in full, and an ad of 16 seconds or longer may be split into separate Stories cards. If you want the clip to stay one card, cut it to less than 16 seconds; with Sume that is one video trim job at $0.02.

This matters for AI video because most generators hand you clips of a fixed length, and a clip that lands at exactly 16 seconds crosses the line. The numbers below come from Instagram's own Stories page, read 2026-10-10, and the Sume figures come from the docs pages named in the sources.

What Instagram says about the 16-second line

The Stories video ad specifications page lists a video duration of 1 second to 60 minutes. It then adds a rule for long videos: videos under 16 seconds play in full, and a video ad that is 16 seconds or longer may be split into separate Stories cards. Instagram will display either one, two or three cards automatically, tailored to each viewer. For qualifying longer videos, viewers may see a Keep watching option.

Two details are easy to miss. First, the page says may be split, not will be split, so the split is not something you control. Second, the number of cards is chosen per viewer, so you cannot design a single fixed cut point. If your hook, offer and call to action all live in one continuous 20-second clip, some viewers can see them across several cards.

Which side of the line your clip lands on

The rule only has two states for planning purposes. Under 16 seconds is one card. 16 seconds or more can be split. The table shows how that maps to common AI clip lengths and to the Sume step that gets you to the safe side.

Trim under the line with Sume

Video trim takes one clip that already lives on media.sume.com, a start in seconds, and exactly one of end or duration. The duration field accepts 0.2 to 900 seconds, so a value such as 15.5 sits under the Stories line with room to spare. The default precision is exact, a frame-accurate re-encode, and the result returns duration_seconds so you can confirm the length before you upload. The source is not changed; you get a new MP4.

If you assemble several clips, Timeline 1.0 takes an audio length from 1 to 1800 seconds, so set the audio duration_seconds below 16 as well. The plan endpoint is unbilled and returns the duration it computed, which makes it a free check before you pay for the render at $0.10 per output minute, rounded up.

• Import the clip first with the media import call; trim does not fetch from the open internet.
• Send start plus one of end or duration, and keep the Idempotency-Key header.
• Read duration_seconds in the result and confirm it is under 16 before you upload to Instagram.

Frame and safe-zone details for the same clip

The same Stories page gives a ratio of 9:16, a resolution of 1440 x 2560 pixels, an aspect ratio tolerance of 1%, MP4, MOV or GIF files, a 4 GB maximum and H.264 with stereo AAC audio at 128 kbps or higher. It also asks you to leave about 14% of the top, 35% of the bottom and 6% of each side free of text and logos. On a 1440 x 2560 frame that is roughly 358 pixels at the top, 896 at the bottom and 86 on each side, which leaves about 1,306 pixels of height for key creative.

Sume Timeline renders 1080 x 1920 by default, which is exactly 9:16. Output width and height can be any even value from 256 to 2160 per edge, so a 1440 x 2560 render is not available there. If the 1440 minimum matters to your account, check the page for the placement you buy and keep that limit in mind.

What Sume does not decide

Trim only changes length. It does not pick where the story beats fall, and it does not tune the bitrate: codec and crf fields are refused because the server compiles the ffmpeg command. Whether Instagram splits a longer clip is Instagram's call, so if you do ship 20 seconds, write it so each card makes sense alone.