.doc and .docx look almost identical from inside Word — same toolbar, same editing experience, same document. The real difference is entirely underneath the surface, in how the file itself is actually built, and it's a bigger change than the extra letter in the extension suggests.
What Changed, Technically
.doc (used by Word 97–2003) is a binary format — the document is stored as a single, densely packed block of proprietary binary data that only specific software knows how to parse. It's not human-readable, not easily inspectable, and notoriously prone to corruption: if part of that single binary blob gets damaged, the whole file can become unreadable.
.docx (Word 2007 onward) is actually a zip archive containing a set of XML files — you can rename a .docx to .zip and open it directly to see the structure: separate XML files for the document body, styles, embedded images, and metadata, all referencing each other. This is why .docx files are typically smaller (XML compresses well) and more resilient — damage to one internal XML file doesn't necessarily destroy the whole document the way binary corruption does.
| .doc | .docx | |
|---|---|---|
| Format type | Proprietary binary | Open XML inside a zip archive |
| Introduced | Word 97 | Word 2007 |
| Typical file size | Larger for the same content | Smaller — XML compresses efficiently |
| Corruption resilience | Poor — one damaged section can break the whole file | Better — internal structure is more separable |
| Openness | Closed, reverse-engineered by other software | Documented open standard (Office Open XML) |
| Default in modern Word | No | Yes, since 2007 |
Does It Actually Matter Day-to-Day?
For most people, rarely — modern Word, Google Docs, and LibreOffice all open and save both formats without much friction. Where it genuinely matters:
- Compatibility with very old software — some legacy systems and older Office installs only handle .doc
- File size at scale — if you're archiving or emailing many documents, .docx's better compression adds up
- Macros — a .docx containing macros is actually saved as .docm, a distinct extension, specifically so recipients get a clear signal before opening a file capable of running code — .doc had no such separation, which macro-based malware exploited for years
- Long-term archival — .docx's open, documented structure makes it a safer bet for a file you need to still be readable in twenty years, since the format itself isn't dependent on one company's proprietary parser surviving that long
Converting Between Them
Opening a .doc in a modern version of Word and using "Save As → .docx" converts it cleanly in almost all cases, since Word itself handles the underlying restructuring. The content, formatting, and layout carry over reliably — this isn't the same kind of lossy, structure-guessing conversion that PDF-to-Word requires, since both .doc and .docx come from the same editable-document lineage.
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