Auto captions for Instagram Reels

Two different things called Reels captions

Search for a Reels caption generator and you get two kinds of tool. One writes the text under your post: a hook line, a description, hashtags. The other puts spoken words on the screen, timed to the audio. This page is about the second kind, the subtitles you read on the video itself.

If you wanted the first kind, the limit to know is from Instagram's developer documentation, which lists a caption parameter maximum of 2,200 characters, 30 hashtags and 20 @ tags. That figure is stated for publishing through the API, not on a Reels help page, so check the app if it matters. The caption character counter can help with the post text.

Instagram's built-in captions versus burned-in

Instagram's Help Center has an article on captions, but the version we could read in this check rendered without its body text, so this page does not describe any in-app caption feature, which languages it covers or what styles it offers. If you rely on one, test a Reel in the app and read what it produced.

What we can say is how burned-in captions behave. They are part of the video file, so they appear as soon as the Reel plays, look identical wherever you upload the same file, and carry the style you picked. A caption that the app draws for you depends on the platform to show it. The W3C notes that styling in players is inconsistent and that most web video falls back to the player default.

• If you post only to Instagram, check what the app offers for captions and whether its look suits you.
• Use burned-in captions if the same clip also goes to Shorts or TikTok, or if you want a specific style.
• Either way, read the words before posting. Automatic captions from any source can get names and numbers wrong.

Where captions can sit on a Reel

Instagram's Reels interface covers part of the frame. For organic Reels, the official pages we read give no safe-zone measurements. The only figures we found are Meta's for Reels ads, which suggest leaving at least 14% of the top, 35% of the bottom and 6% on each side free of text, logos and other important elements, so they are not cropped, covered by the profile icon or call-to-action, or placed too close to the edges of taller screens. Meta also says that on screens taller than 9:16 the picture may be zoomed to fill the screen.

Those are ad rules, but they make a cautious starting point for captions. On a 1080x1920 frame the percentages work out as follows.

KeyMo is set up to place captions at about chest height, below the speaker's mouth, with the exact height following where the face sits in the frame. Open one exported clip and check it against the numbers above. The safe zone checker is built for this, and the term is explained in the safe zone glossary entry.

Upload basics for a captioned Reel

• Aspect ratio: Instagram Help says Reels can be anywhere from 1.91:1 to 9:16, so full vertical 9:16 is the far end of the range.
• Frame rate and size: the same page states a minimum of 30 FPS and a minimum resolution of 720 pixels.
• KeyMo's export is vertical 9:16, 1080x1920, 30 fps, H.264 video with AAC audio, so it meets both stated minimums.
• Boosting: Instagram's boost help page says a Reel must be 90 seconds or less and in full-screen 9:16 vertical format to be boosted.

Captioning a Reel with KeyMo

KeyMo starts from a longer video. Upload an MP4 or MOV (5 minutes to 3 hours, up to 10 GB), or paste a YouTube link on a paid plan. It scores moments out of 10, keeps those at 7 or higher, and renders up to 15 vertical clips with word-by-word captions in the style you choose from 12. No watermark appears on any plan.

It is not an editor for a single clip you already shot, and there is no free plan or trial. Starter is $12 a month for 300 source minutes. For a podcast source, podcast to Reels shows the whole flow, and the caption styles gallery shows what you can choose.