DOCX vs DOC: What's the Difference? (Word File Formats Explained)

Both DOC and DOCX open in Microsoft Word. Both look identical on screen. But internally, they are completely different formats from different eras of software design — and knowing the difference explains why DOCX is smaller, more cross-compatible, and easier to recover when corrupted.

DOC: The Binary Era (1983–2006)

The .doc format dates back to Word for DOS in 1983, with the version most people know — Word 97–2003 format — standardized in 1997. DOC stores all document content — text, formatting, images, styles, macros — in a single proprietary binary file using Microsoft's Compound Document File Format (also called OLE/COM Structured Storage).

The binary format had significant problems:

• Proprietary black box: Only Microsoft's code reliably read and wrote DOC. Third-party applications (LibreOffice, OpenOffice) spent years reverse-engineering the format with imperfect results.
• Corruption vulnerability: A single byte flip in the binary could corrupt an entire document. Recovery was difficult.
• Security risk: DOC embeds macros directly in the binary, making it a common vector for malware (the "macro virus" era of the 1990s–2000s).
• Large file size: Binary representation is space-inefficient for text content.
• No versioning: Impossible to diff DOC files in version control — binary changes are opaque.

DOCX: The Open XML Era (2007–Present)

Microsoft introduced DOCX with Office 2007 as part of the Office Open XML format specification. It is fundamentally different from DOC in its architecture:

A DOCX file is a ZIP archive containing multiple XML files and folders. The structure inside a typical DOCX:

[Content_Types].xml
_rels/
  .rels
word/
  document.xml        ← Your actual text and formatting
  styles.xml          ← Style definitions
  settings.xml        ← Document settings
  fontTable.xml       ← Fonts used
  media/              ← Embedded images
docProps/
  app.xml             ← App metadata
  core.xml            ← Author, created date, etc.

The word/document.xml file is human-readable XML containing every paragraph, run of text, and formatting instruction in your document. You can open this file in a text editor and read it.

File Size: Why DOCX Is Smaller

A document with 10 pages of text and no images:

• DOC: ~200–500 KB (binary overhead even for simple documents)
• DOCX: ~15–50 KB (compressed XML; text compresses extremely well in ZIP)

A document with many embedded high-resolution images is more comparable since image data dominates the file size in both formats. But for text-heavy documents, DOCX is dramatically smaller.

Security Differences

DOC files can contain Word macros (VBA code) that execute automatically when the document opens — the primary vector for macro malware in the 1990s–2000s. DOCX separates documents from macros: a plain .docx file cannot contain executable macros. Files with macros must use the .docm extension, which makes it immediately clear to users (and security software) that the file contains executable code. Many email systems now block .docm attachments by default.

When to Use DOC vs DOCX

• Use DOCX for all new documents. It is the default in every version of Word from 2007 onward and has better compatibility with all modern tools.
• Use DOC only when the recipient is using Word 97–2003 and cannot install the free Microsoft Office Compatibility Pack. This is an extremely rare scenario in 2026.
• Use PDF when document layout must be preserved exactly for printing or read-only distribution — DOCX formatting can shift across different Word versions and platforms.
• Use plain text (.txt) or Markdown when the content doesn't need formatting — maximum compatibility, smallest size, version-control friendly.

How to Convert DOC to DOCX

In Microsoft Word:

1. Open the .doc file in Word
2. Click File → Info → Convert (converts in place) — or —
3. Click File → Save As and choose Word Document (.docx)

In LibreOffice Writer:

1. Open the .doc file
2. File → Save As → choose "Microsoft Word 2007-365 (.docx)"

Online via Convertlo: use the document converter below to convert DOC to DOCX or DOCX to PDF without installing any software.

Frequently Asked Questions