Trimming Video — -ss/-to/-t and the Keyframe Problem

What You Will Learn

The meaning and usage of -ss (start position), -to (end position), and -t (duration)
The behavioral difference between placing -ss before vs. after the input (speed vs. accuracy)
Fast trimming with stream copy (-c copy) and its precision limitations
Frame-accurate trimming with re-encoding
How to use time formats (HH:MM:SS.ms and seconds)

Tested version: Verified with FFmpeg 6.1 (ubuntu-latest / CI-validated) Target OS: Windows / macOS / Linux

Basic Options Reference

Option	Position	Description
`-ss position`	Before input or before output	Specifies where processing starts
`-to position`	Before output	Specifies where processing ends (absolute timestamp from file start)
`-t duration`	Before input or before output	Specifies the duration to process

-to and -t cannot be used together. If both are specified, -t takes precedence (per the FFmpeg official documentation).

Time Formats

FFmpeg accepts two styles of time specification.

Format	Example	Description
`HH:MM:SS`	`00:01:30`	1 minute 30 seconds
`HH:MM:SS.ms`	`00:01:30.500`	1 minute 30 seconds 500 milliseconds
Seconds (decimal allowed)	`90` or `90.5`	90 seconds (or 90.5 seconds)

Input-Side -ss vs. Output-Side -ss (Most Important)

The position of -ss is the single biggest factor controlling trimming speed and accuracy.

Input-Side -ss (Fast Seek / Keyframe-Based)

ffmpeg -ss 00:01:00 -i input.mp4 -to 00:02:00 -c copy output.mp4

Placing -ss before -i causes FFmpeg to perform a fast seek (fast seek) to the keyframe immediately before the specified time.

Benefit: Completes in seconds even for large files
Drawback: The result is rounded to a keyframe boundary, so the output may start slightly before the specified time

Output-Side -ss (Slow Seek / Frame-Accurate)

ffmpeg -i input.mp4 -ss 00:01:00 -to 00:02:00 -c copy output.mp4

Placing -ss after -i causes FFmpeg to decode from the beginning of the file up to the specified frame.

Benefit: Cuts from the exact frame at the specified time
Drawback: Very slow for extracting segments from late in a long video, because it decodes from the start

Which Should You Use?

Use Case	Recommendation
Fast extraction with stream copy (-c copy)	Input-side `-ss` (speed priority)
Frame-accurate cut required	Input-side `-ss` + re-encoding (see below)
Quickly extracting a short clip for review	Input-side `-ss`

Fast Trimming with Stream Copy (-c copy)

Extremely fast because no re-encoding occurs. However, accuracy depends on keyframe boundaries.

ffmpeg -ss 00:00:30 -i input.mp4 -to 00:01:30 -c copy output.mp4

ffmpeg -ss 90 -i input.mp4 -t 60 -c copy output.mp4

Notes:

If the start point does not coincide with a keyframe, the beginning of the output may show visual artifacts (block noise)
Because -c copy skips re-encoding, file size depends on the source material

Accurate Trimming (with Re-Encoding)

For frame-accurate cuts, combine input-side -ss with re-encoding. Input-side -ss performs a fast seek, then re-encoding begins from there — a good balance of speed and accuracy.

ffmpeg -ss 00:00:30 -i input.mp4 -to 00:01:30 -c:v libx264 -crf 23 -c:a aac output.mp4

Specifying by duration:

ffmpeg -ss 00:01:00 -i input.mp4 -t 30 -c:v libx264 -crf 23 -c:a aac output.mp4

Choosing Between -to and -t

`-to`: Specify by End Timestamp

ffmpeg -ss 00:01:00 -i input.mp4 -to 00:03:00 -c copy output.mp4

This example extracts 2 minutes from 1:00 to 3:00. -to is an absolute timestamp from the file start.

`-t`: Specify by Duration

ffmpeg -ss 00:01:00 -i input.mp4 -t 120 -c copy output.mp4

This example extracts 120 seconds (2 minutes) starting at 1:00. Convenient because you do not need to calculate the end time.

Common Pitfalls

Output Beginning Looks Corrupted with -c copy

Cause: The cut start point is not a keyframe, so the decoder cannot correctly reconstruct frames. Fix: Re-encode with -c:v libx264 -c:a aac, or adjust the start point to a keyframe position.

-to Timestamp Behavior Is Affected by Input-Side -ss

Behavior may vary depending on the FFmpeg version and situation. If results are unexpected, replacing -to with -t (duration) gives more predictable results.

Output Has No Audio or No Video

Verify that audio and/or video streams exist in the source. Also check that -an (no audio) or -vn (no video) has not been specified unintentionally.

Tested with: ffmpeg 6.1.1 / Ubuntu 24.04 (GitHub Actions runner) Primary sources: ffmpeg.org/ffmpeg.html / trac.ffmpeg.org/wiki/Seeking / ffmpeg.org/ffmpeg-formats.html