SUB subtitle format

Reference · Last updated 2025

Overview

SUB (MicroDVD) is a subtitle format that uses frame numbers rather than timestamps. Because timing is frame-based, the file is only accurate when played at the correct frame rate. This makes it the most fragile of the common subtitle formats.

Extension: .sub
Complexity: Low (but conceptually tricky)
Browser support: None
Format variant: MicroDVD (the "sub" extension also appears in other formats, e.g. VobSub binary subtitles)

Structure

Each line is a cue in the form {start_frame}{end_frame}Dialogue:

{25}{100}Hello and welcome.
{120}{200}This is a MicroDVD file.

The frame rate problem

Frame {25} at 23.976 fps is at a different point in time than frame {25} at 25 fps. Without knowing the frame rate, the timing cannot be computed. Common frame rates:

23.976 fps (film, NTSC-derived)
24 fps (film)
25 fps (PAL broadcast)
29.97 fps (NTSC broadcast)

When converting SUB to SRT, you must specify the correct frame rate. If you specify the wrong one, the resulting SRT will be progressively out of sync.

Other SUB formats

The .sub extension is ambiguous. It can refer to:

MicroDVD SUB — the text format described above.
VobSub — binary image-based subtitles ripped from DVDs. Usually accompanied by an .idx file. Requires OCR to convert to text.

When it appears

Legacy DVD rips — MicroDVD and VobSub were common in the 2000s.
Old subtitle archives — some subtitle databases still host SUB files.
Archival purposes — rarely produced new today.

Conversion

Subtitle Convert does not support SUB conversion. To convert SUB to SRT, use:

FFmpeg — ffmpeg -subfps 25 -i input.sub output.srt (specify the frame rate).
Subtitle Edit — handles both MicroDVD and VobSub.
Aegisub — handles MicroDVD.