---
title: "W3C Log Format"
description: "The W3C Extended Log File Format is a customizable, header-defined log format used primarily by IIS and some CDNs, where field names are declared at the top of the file."
category: "Server Log Analysis"
date: "2026-03-05"
url: "https://getbeast.io/glossary/w3c-log-format/"
type: "glossary"
---

# W3C Log Format

**Category:** Server Log Analysis | **Updated:** 2026-03-05

The W3C Extended Log File Format is a customizable, header-defined log format used primarily by IIS and some CDNs, where field names are declared at the top of the file.

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## What Is the W3C Log Format?

The W3C Extended Log File Format is a flexible log format standardized by the World Wide Web Consortium. Unlike the Common and Combined formats that have fixed field positions, W3C logs include a `#Fields:` header directive that declares which fields appear and in what order. This makes the format self-describing and customizable.

## Why It Matters

The W3C format is the default for Microsoft IIS web servers and is also used by many CDN providers (Cloudflare, Akamai, CloudFront). If you are analyzing logs from these sources, you need a parser that reads the `#Fields:` directive to correctly interpret each line. The format supports fields not available in Combined format, such as time-taken (server processing time) and custom headers.

## How to Work with W3C Logs

The key advantage of W3C format is the `time-taken` field, which records server-side processing time in milliseconds — invaluable for performance analysis. [LogBeast](/logbeast/) auto-detects W3C format by reading the `#Fields:` header and maps each field accordingly.

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## Related Terms

- [Combined Log Format](/glossary/combined-log-format/)
- [Common Log Format](/glossary/common-log-format/)
- [Access Log](/glossary/access-log/)
- [Log Parsing](/glossary/log-parsing/)
- [Server Response Time](/glossary/server-response-time/)

## Further Reading

- [Understanding Server Log Formats](/blog/log-formats/)

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*Part of the [GetBeast SEO Glossary](/glossary/). Visit [GetBeast.io](https://getbeast.io) for professional SEO and log analysis tools.*