How to Use FFmpeg -t and -to to Cut Videos

This guide explains how FFmpeg handles the -t and -to options when trimming video files. While users often attempt to combine -t and -to to define the start and end of a middle clip, FFmpeg treats these two flags as mutually exclusive duration limiters rather than a start-and-stop pair. To extract a specific middle section of a video, you must combine either -t or -to with the seek flag (-ss). Below is a straightforward breakdown of how these flags interact and the exact commands needed to extract your clip.

Understanding -ss, -t, and -to

To cut a middle segment, you must define where the segment begins and where it ends. The relevant flags serve distinct purposes:

  • -ss (Start Time): Seeks to a specific timestamp (e.g., -ss 00:01:30). This is mandatory for extracting a middle section; without it, FFmpeg begins at the very start of the file (00:00:00).
  • -t (Duration): Specifies the length of the clip to capture from the start point (e.g., -t 30 extracts 30 seconds of footage).
  • -to (Stop Position): Specifies an absolute timestamp where encoding or copying should stop (e.g., -to 00:02:00).

Why You Cannot Combine -t and -to Together

In FFmpeg, -t and -to both define when to stop processing. According to official FFmpeg specifications, -t and -to are mutually exclusive. If you specify both options in the same output context, -t takes precedence, and -to is ignored.

Furthermore, neither -t nor -to defines a start point. Running a command using only -t and -to will cause FFmpeg to start from the beginning of the video (00:00:00) and apply whichever duration flag takes priority, failing to capture a middle section.

Correct Methods to Extract a Middle Section

To isolate a middle section, pair -ss with either -t or -to.

Method 1: Start Time + Duration (-ss and -t)

Use this method when you know where the clip starts and exactly how long it should run.

ffmpeg -ss 00:02:00 -i input.mp4 -t 00:00:45 -c copy output.mp4
  • -ss 00:02:00: Starts trimming at the 2-minute mark.
  • -t 00:00:45: Captures 45 seconds of video from that point.
  • -c copy: Copies audio and video streams without re-encoding, ensuring fast processing.
  • Result: The output contains the video segment between 00:02:00 and 00:02:45.

Method 2: Start Time + End Timestamp (-ss and -to)

Use this method when you know the exact start and end timestamps in the original video.

ffmpeg -ss 00:02:00 -to 00:02:45 -i input.mp4 -c copy output.mp4
  • -ss 00:02:00: Fast-seeks to the 2-minute mark before reading input packets.
  • -to 00:02:45: Stops reading at the 2-minute, 45-second mark of the source file.
  • Result: The output contains the segment between 00:02:00 and 00:02:45.

Important Placement Notes

  • Input Seeking (-ss before -i): Placing -ss before -i makes FFmpeg seek using keyframes, which is nearly instantaneous. When -to is used alongside an input -ss, it refers to the original source timestamp.
  • Output Seeking (-ss after -i): Placing -ss after -i forces FFmpeg to decode frames sequentially from the start until it reaches the specified time. In this configuration, -to is interpreted relative to the new start point rather than the original video timeline.
  • Frame Accuracy: If stream copying (-c copy) cuts slightly before or after your specified time, remove -c copy to re-encode the video. Re-encoding allows FFmpeg to cut precisely on non-keyframes (P-frames or B-frames):
ffmpeg -ss 00:02:00 -i input.mp4 -to 00:02:45 -c:v libx264 -c:a aac output.mp4