What is the EOT file format?
.eot files store OpenType fonts in a compact, subsettable wrapper developed by Microsoft. .eot fonts are not accessed directly but are instead meant to be embedded on websites - the format was created for Internet Explorer and first appeared in IE 4 (1997). Modifying or deleting an .eot file may prevent fonts from displaying correctly on any page that depends on it.
Font embedding
EOT font files are not distributed with operating systems and cannot be installed as a regular desktop font - they function as a delivery wrapper, not an installable typeface. Only fonts whose license permits web embedding can be packaged into .eot; the format enforces light copy-protection via a *rootstring*, a list of domain URLs baked into the file header that ties the font to specific websites. .eot files are stored in the browser cache so that they need not be downloaded each time the given fonts are required.
.eot was consumed exclusively by Internet Explorer; no other major browser ever supported it. With Internet Explorer retired on 15 June 2022 and WOFF and WOFF2 now universally supported across all browsers, .eot is obsolete and should be dropped from new @font-face declarations.
Security & safety
RISK: LOWAn .eot is a font wrapper, not executable, so the file itself is low risk to handle. Two real cautions: (1) licensing - EOT often wrapped commercial fonts with a 'rootstring' URL lock as light anti-piracy, so converting an EOT back to TTF/OTF and reusing it elsewhere may breach the font's license; check the EULA. (2) Avoid sketchy 'EOT converter' download portals that bundle adware - FontForge (open source) or a reputable web converter is safer. Historically, font parsers had vulnerabilities, so keep tools updated.
Format details
in a nutshellPrograms that open EOT files
Windows2 apps
FontForge
Open-source Open the .eot to inspect it, then File > Generate Fonts as TTF/OTF to get an installable, usable font. 
Web browser (any)
Free EOT is consumed via CSS @font-face by Internet Explorer only; there is no useful standalone 'open'. Modern browsers ignore it - use WOFF2 instead. 
macOS1 app
Linux1 app
Technical details
deep spec| Developer | Microsoft |
| MIME type | application/vnd.ms-fontobject |
| Encoding | Binary |
| Byte order | Little-endian (EOT header fields); embedded sfnt payload is big-endian |
| Container structure | Binary EOT header (sizes, version, font metadata, rootstring URL list) followed by the embedded OpenType/TrueType sfnt font block |
| Compression | Optional MicroType Express (MTX) lossless compression of the sfnt payload; often stored uncompressed in practice |
| Font subsetting | Supports glyph subsetting to include only characters required by the page, keeping file size small |
| Copy protection | Rootstring mechanism: a list of permitted domain URLs embedded in the file header that ties the font to specific websites |
| Glyph capacity | Up to 65,535 glyphs (bound by the embedded sfnt format limit) |
| Magic number | 32-bit little-endian value 0x504C at byte offset 34 (hex bytes: 4C 50); no fixed ASCII signature at offset 0 |
| Format versions | 0x00010000, 0x00020001, 0x00020002; EOTLite variant omits MTX compression and rootstrings |
| Typical file size | 10 KB - 1 MB; subsetting to required glyphs keeps web-delivered EOT small |
| Wrapped format | Embeds a standard sfnt (OpenType or TrueType) font as its payload |
| Browser support | Internet Explorer only (IE 4 - IE 11); never supported by Chrome, Firefox, Safari, or Chromium-based Edge |
| Status | Obsolete since Internet Explorer was retired on 15 June 2022; superseded by WOFF and WOFF2 |
| Released | 1997 (Internet Explorer 4 / Microsoft WEFT); W3C submission 2008 |
| Latest version | EOT / EOTLite as submitted to W3C (2008); never standardized as a Recommendation |
| Open standard | Yes · royalty-free |
| Specification | www.w3.org |
EOT conversions
Community Q&A
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