How to Detect Variable Frame Rate (VFR) with FFprobe

Variable Frame Rate (VFR) causes significant audio sync and playback issues in video editing software, making detection a critical step in media workflows. This guide explains how to use FFprobe to inspect video stream metadata and analyze frame presentation timestamps to definitively determine whether a video file utilizes Variable Frame Rate or Constant Frame Rate (CFR).

Method 1: The Quick Metadata Check

The fastest way to check for VFR is to compare the stream's reported nominal frame rate (r_frame_rate) with its average frame rate (avg_frame_rate).

Run the following command:

ffprobe -v error -select_streams v:0 -show_entries stream=r_frame_rate,avg_frame_rate -of default=noprint_wrappers=1 input.mp4

Interpreting the Output

  • Constant Frame Rate (CFR): Both values will be identical (e.g., r_frame_rate=30/1 and avg_frame_rate=30/1).
  • Variable Frame Rate (VFR): The values will differ (e.g., r_frame_rate=30/1 and avg_frame_rate=29.412/1).

While a mismatch confirms VFR, matching values can occasionally produce false negatives if container metadata is incomplete. For 100% accuracy, analyze individual frames.

Method 2: Inspecting Frame Durations (Accurate Method)

To verify frame timing directly from the encoded frames, inspect the duration of each packet. In a true CFR video, every frame has the exact same duration.

Run the following command on Linux or macOS:

ffprobe -v error -select_streams v:0 -show_entries frame=pkt_duration_time -of csv=p=0 input.mp4 | sort -u

On Windows Command Prompt (cmd):

ffprobe -v error -select_streams v:0 -show_entries frame=pkt_duration_time -of csv=p=0 input.mp4 | sort /unique

Interpreting the Output

  • Constant Frame Rate (CFR): The output returns a single unique value (for example, 0.033333 for a 30 fps video).
  • Variable Frame Rate (VFR): The output lists multiple different durations, confirming that frame lengths vary across the timeline.

Method 3: Inspecting Timestamp Differences

If the container does not explicitly store duration per frame, you can read the presentation timestamps (pkt_pts_time):

ffprobe -v error -select_streams v:0 -show_entries frame=pkt_pts_time -of csv=p=0 input.mp4

In a CFR file, the numerical difference between each consecutive timestamp remains constant across the entire stream. In a VFR file, the difference between consecutive timestamps fluctuates.