To send a variable-frame-rate (VFR) file to YouTube with FFmpeg, pace the file input in real time with -re, then choose output synchronization deliberately. Those are separate jobs: -re does not convert VFR video to constant frame rate (CFR). The command below is an illustrative starting point, not a tested recipe; check your FFmpeg build, source timestamps, encoder, and YouTube Live settings before broadcasting.
Understand the two frame-timing decisions
A VFR file does not necessarily have evenly spaced video frames. Before streaming, decide whether to preserve that irregular cadence or create a fixed output cadence. Then configure input pacing and output synchronization to suit that goal.
Input pacing: -re
FFmpeg documents -re as an input option that reads at the input’s native frame rate, equivalent to -readrate 1. It is useful when sending a file in real time rather than letting FFmpeg process it as quickly as possible. It does not make the file CFR or resample its frames. FFmpeg’s 2022 documentation-change record explains this behavior.
Output synchronization: -fps_mode
-fps_mode[:stream_specifier] controls video synchronization and frame-rate behavior on output, per stream. The cited FFmpeg project record marks the older global -vsync option deprecated. The record dates from June 2022, so do not assume a particular option or value is accepted by every installed version: check ffmpeg -version and ffmpeg -h full.
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For a passthrough-style output, the example uses -fps_mode:v passthrough to avoid deliberately imposing a fixed cadence. If you want CFR instead, select an output mode or filtering approach that deliberately duplicates or drops frames as needed. An output -r or fps filter is not a substitute for real-time input pacing; it can change frame timing. Exact behavior depends on the FFmpeg build and output path.
Check YouTube’s current ingest requirements
YouTube’s Live encoder settings page lists RTMP and RTMPS ingest, H.264, H.265 (HEVC), and AV1 video, frame rates up to 60 fps, constant-bitrate encoding, AAC or MP3 audio, and a recommended two-second keyframe frequency, not exceeding four seconds. Settings can change, so check the page and the options in your Live Control Room before each setup.
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Bitrate is not one-size-fits-all. These examples are YouTube-published recommendations; choose the row matching both your codec and output resolution/frame rate.
| Codec | Output | Minimum bitrate | Recommended bitrate |
|---|---|---|---|
| H.264 | 1080p30 | 5 Mbps | 14 Mbps |
| H.264 | 1080p60 | 6 Mbps | 17 Mbps |
| H.264 | 720p30 | 3 Mbps | 8 Mbps |
| H.264 | 720p60 | 3 Mbps | 8 Mbps |
| AV1 or H.265 (HEVC) | 1080p30 | 4 Mbps | 10 Mbps |
| AV1 or H.265 (HEVC) | 1080p60 | 4 Mbps | 12 Mbps |
| AV1 or H.265 (HEVC) | 720p30 | 2 Mbps | 6 Mbps |
| AV1 or H.265 (HEVC) | 720p60 | 2 Mbps | 6 Mbps |
These figures are YouTube’s guidance, not a guarantee that a particular network or encoder will sustain a stream. Check available upload capacity and leave headroom. YouTube recommends RTMPS, which encrypts data into and through Google’s servers. Use the RTMPS ingest URL when available, and keep your stream key private.
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Prepare the input and stream key
- Inspect your FFmpeg build. Run
ffmpeg -versionandffmpeg -h full. Confirm that the build accepts-fps_mode:v passthrough, includes the chosen encoder and protocol, and supports your intended options. - Inspect the file’s streams and timestamps. Use
ffprobe input.mp4to see stream metadata, then examine timestamp and frame information if the cadence is uncertain. Do not assume a nominal frame-rate label describes every frame interval. - Get the ingest URL and key. In YouTube Live Control Room, use the ingest details for your broadcast. Prefer RTMPS if offered. Store the key privately; anyone who obtains it may be able to send video to your channel.
- Choose your output. Decide whether to preserve irregular timing or convert to a fixed cadence. Set resolution, frame rate, codec, bitrate, and keyframe interval to match the current YouTube guidance and your available upload capacity.
Use this illustrative FFmpeg command
This untested starting point reads a file in real time, maps its first video stream and optional audio, encodes H.264 and AAC, and uses passthrough output synchronization. Replace the environment variables with the RTMPS URL and key format shown in your Live Control Room.
ffmpeg -re -i input.mp4
-map 0:v:0 -map '0:a?'
-c:v libx264 -pix_fmt yuv420p -fps_mode:v passthrough
-c:a aac
-f flv "$YOUTUBE_RTMPS_URL/$STREAM_KEY"The example intentionally omits a fixed -r and bitrate: both depend on the source, chosen cadence, resolution, encoder, network capacity, and YouTube’s current recommendation for that codec. Configure encoder rate control for CBR and set a keyframe interval matching YouTube’s two-second recommendation, without exceeding four seconds. Encoder-specific options vary by build, so verify them with ffmpeg -h encoder=libx264 or the help for your selected encoder.
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The optional-audio map (0:a?) allows a file without an audio stream to pass mapping, but YouTube’s recommendations include audio encoding. Confirm your intended broadcast’s audio behavior rather than assuming an absent track is suitable. The selected protocol, URL syntax, and stream-key format must also match the ingest details shown in Live Control Room.
Test and monitor the live stream
- Run a representative preflight with motion and audio similar to the planned broadcast. YouTube explicitly advises: “Make sure to test before you start your live stream. Tests should include audio and movement in the video similar to what you’ll be doing in the stream.”
- Check YouTube Live Control Room for stream-health messages while the test is running; do not rely only on FFmpeg reporting that it is sending data.
- Confirm that the video and audio play as expected, and that the selected output cadence, resolution, codec, and bitrate match the configuration you intended.
- Repeat the test after changing important encoder, synchronization, or network settings.
Troubleshoot VFR streaming problems
FFmpeg rejects -fps_mode or passthrough
Check ffmpeg -version and ffmpeg -h full for the installed build’s syntax and supported values. Do not copy an old -vsync example without checking it against your version; the FFmpeg project record describes that option as deprecated.
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FFmpeg sends faster than real time or the file runs ahead
For a file input, confirm that -re is placed before the relevant input’s -i. It paces file reading; it does not fix timestamps or convert frame rate. Do not blindly apply it to a real-time capture device or live input—the cited FFmpeg source does not establish a complete rule for every such source and build.
Playback stutters or drifts
First distinguish a timestamp or frame-pacing issue from encoder overload, insufficient upload capacity, codec mismatch, or a YouTube ingest-health warning. Inspect the input timestamps and FFmpeg output, then check Live Control Room messages. Change output synchronization only after identifying whether you intend to preserve source cadence or produce CFR.
YouTube reports an ingest or quality warning
Compare the configured protocol, codec, resolution, frame rate, bitrate, and keyframe interval with the current YouTube Live encoder page. Use the bitrate recommendation for the exact codec and output mode, and test whether the upload connection can sustain it with headroom.
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