What is the SMI file format?
.smi is a plain-text file storing closed-caption data in the SAMI (Synchronized Accessible Media Interchange) format, developed by Microsoft in 1998 for use with Windows Media Player. Despite sharing the .smi extension, SAMI is entirely distinct from the unrelated SMIL multimedia presentation format.
A SAMI file is structured like an HTML/SGML document. Its root element is <SAMI>, containing a <HEAD> block with <STYLE> rules that define CSS-like language classes (for example .ENUSCC for English US closed captions) and a <BODY> block made up of <SYNC Start=N> timestamp elements. Each <SYNC> entry holds a <P Class=...> paragraph with the caption text for that millisecond offset.
One of SAMI's practical strengths is multi-language support: a single .smi file can carry captions in several languages by defining multiple CSS classes, with each <P> assigned to the appropriate class. Basic styling - color, font, and alignment - is controlled through the same <STYLE> block.
SAMI was Microsoft's primary accessibility captioning format for Windows, but it never gained broad cross-platform adoption. Modern subtitle workflows favor SubRip (.srt) or WebVTT (.vtt), which are supported across browsers, devices, and players. .smi files can be opened in any text editor and converted using tools such as Subtitle Edit.
Security & safety
RISK: LOWA SAMI .smi is plain text and cannot execute code, so the file itself is safe to open and read. Because it is HTML-like markup, never paste it into a live web page unsanitized. Two practical cautions: keep your media player updated (historical subtitle-parser vulnerabilities affected several players), and watch encoding - save as UTF-8 so accented or non-Latin (e.g. Korean) characters display correctly. Subtitles from sketchy download sites may contain misleading links in the text; read, don't blindly trust.
Format details
in a nutshellFULL NAMESAMI Captioning Fileaka Synchronized Accessible Media Interchange, Microsoft SAMI subtitles
DEVELOPERMicrosoftsince 1998 (Microsoft SAMI 1.0, alongside Windows Media)
CATEGORYVideo Files
MIME TYPEapplication/smil
TYPEHTML/XML-like timed caption markup (plain text)
STANDARDOpen · royalty-free
This extension is also used by…
- SMIL presentation (.smi) - Synchronized Multimedia Integration Language XML files sometimes use .smi/.smil for timed multimedia layouts (RealPlayer, Ambulant).
- SmartMovie video (.smi) - Older feature-phone video format produced by SmartMovie/SMI converters; a binary video container, not subtitles.
Programs that open SMI files
Windows4 apps
VLC media player
Open-source Play the video; if the .smi shares the video's name and folder it loads automatically, or use Subtitle > Add Subtitle File. 
Subtitle Edit
Open-source Open the .smi to read/fix text and timing, then save or convert to .srt/.vtt. 
PotPlayer
Free Open the video; PotPlayer auto-loads a same-named .smi and renders SAMI styling well. 
macOS2 apps
Linux2 apps
Technical details
deep spec| Developer | Microsoft |
| Format type | Plain-text HTML/SGML-style captioning markup |
| Root element | <SAMI> … </SAMI> (case-insensitive tag parsing) |
| Timing mechanism | Millisecond offsets via SYNC Start= attribute on each <SYNC> element in the <BODY> block |
| Language and track support | Multiple simultaneous languages in one file, each identified by a distinct CSS class (e.g. .ENUSCC for English US closed captions) |
| Character encoding | ASCII/ANSI (Windows code page) for Latin scripts; UTF-8 or UTF-16 with BOM for non-Latin text |
| MIME type | application/smil (also accepted: text/plain, application/x-sami) |
| Compression | None - plain uncompressed text |
| File signature | No binary magic bytes; identified by <SAMI> root tag near the start of file |
| Typical file size | 5 KB - 300 KB, scaling with dialogue length and number of language tracks |
| Styling | CSS-like rules in <STYLE> block control font, color, and alignment per language class |
| Primary use | Closed captioning and accessibility for Windows Media Player video content |
| Cross-platform support | Natively supported by Windows Media Player; also read by VLC, PotPlayer, mpv, and Subtitle Edit |
| Superseded by | .vtt (WebVTT) and .srt (SubRip) for modern web, browser, and device playback |
| Released | 1998 (Microsoft SAMI 1.0, alongside Windows Media) |
| Latest version | SAMI 1.0 (the only published version; never formally revised) |
| Open standard | Yes · royalty-free |
| Specification | learn.microsoft.com |
SMI conversions
Possible
.SMI .SRTGUIDE
Yes - open the .smi in Subtitle Edit and save as SubRip (.srt); the whole process takes under a minute.
How to convert
.SMI .SSAGUIDE
Yes - Subtitle Edit converts a SAMI file to SSA in a few clicks, and it is free.
How to convert
.SMI .TXTGUIDE
Yes -- a SAMI file is plain text with HTML-like markup, so extracting the dialogue is straightforward: open…
How to convert
.SMI .VTTGUIDE
Yes - open the .smi in Subtitle Edit and save it as WebVTT; the whole process takes under a minute.
How to convert
.SMI .SUBGUIDE
Yes, Subtitle Edit (free, Windows/macOS/Linux) converts SAMI to MicroDVD SUB directly - open the file, save…
How to convert
Not possible
Community Q&A
asked by users
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Frequently asked questions
How do I open a SAMI .smi subtitle file?
To watch it with a video, give the .smi the same base name as the video, keep them in the same folder, and play in VLC or PotPlayer (it auto-loads). To read/edit the text, open it in Subtitle Edit or any text editor.
How do I convert a SAMI .smi file to SRT?
Open the .smi in Subtitle Edit and choose Save As > SubRip (.srt). If the SAMI file has several language classes, select the language you want to export first.
Why won't my .smi subtitles show up?
Either the player doesn't support SAMI (use VLC/PotPlayer), the file isn't named to match the video, or your .smi is actually a SMIL playlist or a SmartMovie video rather than SAMI captions.
Is .smi the same as .srt?
No. SAMI (.smi) is Microsoft's HTML-like caption format; SubRip (.srt) is the simpler, far more widely supported format. Subtitle Edit converts between them in seconds.
My .smi shows garbled characters - how do I fix it?
It's an encoding mismatch. Re-open in Subtitle Edit or Notepad++ and re-save as UTF-8 so accents and non-Latin scripts (e.g. Korean) display correctly.
I opened a .smi and it's not subtitles - what is it?
The .smi extension is shared. Besides SAMI captions it's used by SMIL multimedia playlists (XML) and old SmartMovie phone videos (binary). Check whether the file is text starting with <SAMI>.
References
1Microsoft - Understanding SAMI 1.0learn.microsoft.com
2Wikipedia - SAMI (caption format)en.wikipedia.org
