How to Repurpose YouTube Videos to Shorts, Reels, and TikTok

The two halves of repurposing

The pipeline is two steps:

1. Transform — long 16:9 → short 9:16. This is video work: crop, pad, trim. ffmpeg is the standard tool.
2. Distribute — push the resulting MP4 to Shorts, Reels, and TikTok. This is API work, and it’s where Postproxy fits.

We’ll cover both, and how they connect.

Step 1 — crop and trim with ffmpeg

A 16:9 source at 1920×1080 needs to become 1080×1920. The simplest options:

Center crop (loses left/right edges):

ffmpeg -ss 00:01:30 -t 00:00:55 -i source.mp4 \
  -vf "crop=608:1080:656:0,scale=1080:1920" \
  -c:v libx264 -preset fast -crf 22 \
  -c:a aac -b:a 128k \
  -movflags +faststart \
  short.mp4

-ss 00:01:30 -t 00:00:55 extracts a 55-second clip starting at 1:30. The crop=608:1080:656:0 takes a vertical slice from the center.

Blurred background (keeps full frame):

ffmpeg -ss 00:01:30 -t 00:00:55 -i source.mp4 \
  -vf "split[a][b]; \
       [a]scale=1080:1920,boxblur=20:5[bg]; \
       [b]scale=1080:-1[fg]; \
       [bg][fg]overlay=(W-w)/2:(H-h)/2" \
  -c:v libx264 -preset fast -crf 22 \
  -c:a aac -b:a 128k \
  -movflags +faststart \
  short.mp4

This produces a 1080×1920 video with the original 16:9 video centered over a blurred background — the look that’s become standard on Reels and Shorts.

The output short.mp4 satisfies the spec intersection in our cross-post guide. Upload it to your CDN.

Step 2 — publish to all three in one call

curl -X POST "https://api.postproxy.dev/api/posts" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "post": {
      "body": "55 seconds: how we cut our publishing latency in half.\n\nFull video on YouTube ↓"
    },
    "profiles": ["youtube", "instagram", "tiktok"],
    "media": ["https://yourcdn.com/short.mp4"],
    "platforms": {
      "youtube": {
        "title": "How we cut publishing latency in half (55s)",
        "privacy_status": "public"
      },
      "instagram": { "format": "reel" },
      "tiktok": { "privacy_status": "PUBLIC_TO_EVERYONE" }
    }
  }'

Postproxy handles each platform’s upload flow:

• YouTube → resumable upload, sets title and description (the post body becomes the YouTube description). YouTube auto-classifies the upload as a Short when the file is 9:16 and within the Shorts length ceiling, which has been 3 minutes since October 2024 — the old 60-second rule is long gone, though clips over 60 seconds using licensed music risk a Content ID block.
• Instagram Reels → container creation, processing wait, publish. The format: "reel" flag is what routes the video to Reels instead of feed video.
• TikTok → direct post upload, processing poll, publish. privacy_status is required.

A scripted end-to-end pipeline

pip install postproxy-sdk

import asyncio
import os
import subprocess
from postproxy import (
    PostProxy, PlatformParams, YouTubeParams, InstagramParams, TikTokParams,
)

SOURCE = "long-form.mp4"
CLIP_START = "00:01:30"
CLIP_LEN = "00:00:55"
OUTPUT = "short.mp4"
CDN_URL = "https://yourcdn.com/short.mp4"

async def main():
    # 1. Cut + reformat
    subprocess.run([
        "ffmpeg", "-y",
        "-ss", CLIP_START, "-t", CLIP_LEN,
        "-i", SOURCE,
        "-vf", "split[a][b];[a]scale=1080:1920,boxblur=20:5[bg];[b]scale=1080:-1[fg];[bg][fg]overlay=(W-w)/2:(H-h)/2",
        "-c:v", "libx264", "-preset", "fast", "-crf", "22",
        "-c:a", "aac", "-b:a", "128k",
        "-movflags", "+faststart",
        OUTPUT,
    ], check=True)

    # 2. Upload to your CDN (S3, R2, etc.)
    upload_to_cdn(OUTPUT, CDN_URL)

    # 3. Cross-post via Postproxy
    async with PostProxy(os.environ["POSTPROXY_API_KEY"], profile_group_id="pg-abc") as client:
        post = await client.posts.create(
            "55 seconds on the new release. Full video on YouTube ↓",
            profiles=["youtube", "instagram", "tiktok"],
            media=[CDN_URL],
            platforms=PlatformParams(
                youtube=YouTubeParams(
                    title="Release deep-dive (55s)",
                    privacy_status="public",
                ),
                instagram=InstagramParams(format="reel"),
                tiktok=TikTokParams(privacy_status="PUBLIC_TO_EVERYONE"),
            ),
        )
        print(post.id, post.status)

asyncio.run(main())

Schedule this with cron, n8n, or a queue worker — see scheduling with cron and API.

Repurposing TikTok videos the other way

Going TikTok → Shorts and Reels is the easier direction: the clip is already 9:16, so there is no crop step. Two things change.

Start from your own export, not the posted video. A TikTok downloaded back from the app carries a burned-in watermark, and both Instagram and YouTube say recycled, visibly watermarked uploads get less reach. Keep the original render from your editor and publish that file to all three platforms.

Trim to each ceiling. A TikTok can run longer than a Short, so the only ffmpeg you need is a cut and a clean re-encode:

ffmpeg -ss 00:00:00 -t 00:03:00 -i tiktok-original.mp4 \
  -c:v libx264 -preset fast -crf 22 \
  -c:a aac -b:a 128k \
  -movflags +faststart \
  short.mp4

Then publish exactly as above — same call, same format: "reel" flag for Instagram, minus the TikTok profile if that is where the clip came from:

curl -X POST "https://api.postproxy.dev/api/posts" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "post": { "body": "Reposting this week'\''s clip." },
    "profiles": ["youtube", "instagram"],
    "media": ["https://yourcdn.com/short.mp4"],
    "platforms": {
      "youtube": { "title": "This week'\''s clip", "privacy_status": "public" },
      "instagram": { "format": "reel" }
    }
  }'

Picking the right clip

Cropping is mechanical; what to crop is editorial. Common heuristics:

• The first 60 seconds — the YouTube hook is often the best opener for a Shorts version.
• A timestamped highlight — many YouTubers already mark “best moments” in chapters; reuse those timestamps.
• A captioned segment — pick a 30-second answer to a specific question and put the question on screen as a caption.

If you’re piping this through an LLM that picks timestamps from the YouTube transcript, the only thing that changes in the script above is CLIP_START and CLIP_LEN.

Different format per platform

The media array on a single post applies to every targeted profile — there’s no per-platform media swap. If you want a 60-second cut for Shorts and a 90-second cut for Reels and TikTok, run ffmpeg twice and send two requests:

# YouTube Shorts — 60s
curl -X POST "https://api.postproxy.dev/api/posts" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "post": { "body": "Highlights from this week's stream." },
    "profiles": ["youtube"],
    "media": ["https://yourcdn.com/short-60s.mp4"],
    "platforms": {
      "youtube": { "title": "Stream highlights", "privacy_status": "public" }
    }
  }'

# Reels + TikTok — 90s
curl -X POST "https://api.postproxy.dev/api/posts" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "post": { "body": "Highlights from this week's stream." },
    "profiles": ["instagram", "tiktok"],
    "media": ["https://yourcdn.com/short-90s.mp4"],
    "platforms": {
      "instagram": { "format": "reel" },
      "tiktok": { "privacy_status": "PUBLIC_TO_EVERYONE" }
    }
  }'

Why this is worth automating

Manual repurposing is a 30-minute job per video — open editor, scrub for highlights, crop, export, upload three times, write three captions. Doing this on a 3-per-week schedule is 4-5 hours a month of menial work. The script above runs in under 60 seconds and three of the four platforms confirm publish within 2 minutes.

For the cross-platform spec table and partial-success behaviour, see the cross-post guide and the blog post comparing Reels, TikTok, and Shorts publishing flows.