For FFmpeg streaming to YouTube Live, target a keyframe interval of 2 seconds and use a closed GOP. With a constant output frame rate, set the GOP length to twice the frame rate: 60 frames at 30 fps or 120 frames at 60 fps. YouTube says not to exceed a 4-second interval. The FFmpeg options below show the relevant libx264 settings; they are not a complete streaming command.
What GOP and keyframe interval should you use?
A GOP, or group of pictures, is the span between keyframes in an encoded video. The interval is a duration, while FFmpeg’s -g value is a number of frames. Convert the recommended duration to frames using the stream’s actual output frame rate.
| Output frame rate | GOP for a 2-second interval | 4-second upper limit in frames |
|---|---|---|
| 30 fps | 60 frames | 120 frames |
| 60 fps | 120 frames | 240 frames |
YouTube recommends a 2-second keyframe frequency and says not to exceed 4 seconds. Its guidance gives the 30 fps example as one keyframe every 60 frames. These are platform configuration recommendations, not a guarantee that a particular encode or ingest path is correct. YouTube’s live encoder settings and live streaming error guidance provide the relevant requirements.
Set the interval in FFmpeg with libx264
For a 30 fps output encoded with libx264, add these video-encoder options to your FFmpeg command:
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-g 60 -keyint_min 60 -flags +cgopFor a 60 fps output, use:
-g 120 -keyint_min 120 -flags +cgopWhat each option does
-gsets the maximum GOP size, measured in frames.-keyint_minsets the minimum keyframe spacing. Matching it to the target GOP length requests regular spacing at that interval.-flags +cgoprequests a closed GOP, so a group does not depend on frames from the preceding group.
YouTube says its pipeline requires a closed GOP for optimal transcoding. FFmpeg documents cgop among its codec flags; the examples here are specifically for libx264. See the FFmpeg codecs documentation.
Put the options in the right place
These are encoder settings, not a full command. Keep the input, video and audio settings, rate control, and RTMP or RTMPS output configuration appropriate to your source and YouTube’s current requirements. Place the options among the output video-encoding options for the output they are meant to configure. If your command has multiple outputs or encoders, verify which output receives them.
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Do not assume the same flag names or behavior apply to another FFmpeg encoder. YouTube accepts multiple codecs, but the settings shown here are for libx264; consult the selected encoder’s FFmpeg options before translating them.
Recalculate if the output frame rate changes
The GOP frame count depends on the encoded output frame rate, not simply the frame rate you intended at the source. For a constant-frame-rate output, multiply its frames per second by two for a 2-second GOP. If you change the output from 30 to 60 fps, change -g 60 to -g 120 and adjust -keyint_min accordingly. Leaving the old frame count in place doubles the interval in seconds.
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If the output is variable-frame-rate or its effective frame rate is uncertain, do not treat a frame count as a fixed duration without checking the emitted stream. YouTube’s guidance notes that changing frame rate requires changing keyframe frequency.
Test and verify the stream
- Use the intended output settings. Confirm the output frame rate and selected video encoder, then calculate the frame count for a 2-second interval.
- Run a representative test before the event. Include audio and movement similar to the planned live stream; a static test image alone may not reveal issues that occur in normal content.
- Check YouTube’s stream health and messages. If YouTube reports that keyframes are too far apart, recalculate using the actual output frame rate and inspect the ingest settings. YouTube notes that ingestion errors can cause an incorrectly observed GOP size.
- Confirm GOP structure. Ensure the chosen encoder is producing a closed GOP; the example flag requests this for
libx264, but verify behavior with the FFmpeg build and encoder you actually run.
Troubleshooting
| Symptom | Likely cause | What to check or change |
|---|---|---|
| YouTube reports keyframes are too far apart. | The GOP frame count was calculated for a different frame rate, or the stream is being ingested with incorrect settings. | Check the encoded output frame rate, set the GOP to twice that rate for a 2-second interval, and inspect the ingest configuration. |
| The interval changes after changing frame rate. | The GOP stayed at the old frame count. | Recalculate both -g and -keyint_min from the new output frame rate. |
| The closed-GOP setting appears ineffective. | The command uses an encoder other than libx264, or the encoder/build interprets options differently. |
Check the selected encoder’s own FFmpeg documentation and confirm the emitted stream with that build. |
| YouTube’s observed GOP size does not match the intended value. | Ingestion errors or other incorrect ingest settings may affect what YouTube observes. | Review YouTube’s stream-health messages and test again with the intended ingest configuration. |
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