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 ≤60s.
• 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.

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.