Google addresses the subject of robots.txt files and whether it’s a good SEO practice to keep them within a reasonable size.

This topic is discussed by Google’s Search Advocate John Mueller during the Google Search Central SEO office-hours hangout recorded on January 14.

David Zieger, an SEO manager for a large news publisher in Germany, joins the livestream with concerns about a “huge” and “complex” robots.txt file.

How huge are we talking here?

Zieger says there’s over 1,500 lines with a “multitude” of disallows that keeps growing over the years.

The disallows prevent Google from indexing HTML fragments and URLs where AJAX calls are used.

Zieger says it’s not possible to set a noindex, which is another way to keep the fragments and URLs out of Google’s index, so he’s resorted to filling the site’s robots.txt with disallows.

Are there any negative SEO effects that can result from a huge robots.txt file?

Here’s what Mueller says.

SEO Considerations For Large Robots.txt Files

A large robots.txt file will not directly cause any negative impact to a site’s SEO.

However, a large file is harder to maintain, which may lead to accidental issues down the road.

Mueller explains:

“No direct negative SEO issues with that, but it makes it a lot harder to maintain. And it makes it a lot easier to accidentally push something that does cause issues.

So just because it’s a large file doesn’t mean it’s a problem, but it makes it easier for you to create problems.”

Zieger follows up by asking if there are any issues with not including a sitemap in the robots.txt file.

Mueller says that’s not a problem:

“No. Those different ways of submitting a sitemap are all equivalent for us.”

Zieger then launches into a several more follow-up questions that we’ll take a look at in the next section.

Does Google Recognize HTML Fragments?

Zieger asks Mueller what would be the SEO impact of radically shortening the robots.txt file. Such as removing all the disallows, for example.

The following questions are asked:

  • Does Google recognize HTML fragments that aren’t relevant to site visitors?
  • Would HTML fragments end up in Google’s search index if they weren’t disallowed in robots.txt?
  • How does Google deal with pages where AJAX calls are used? (Such as a header or footer element)

He sums up his questions by stating most of what’s disallowed in his robots.txt file are header and footer elements that aren’t interesting for the user.

Mueller says it’s difficult to know exactly what would happen if those fragments were suddenly allowed to be indexed.

A trial and error approach might be the best way of figuring this out, Mueller explains:

“It’s hard to say what you mean with regards to those fragments

My thought there would be to try to figure out how those fragment URLs are used. And if you’re unsure, maybe take one of these fragment URLs and allow its crawling, and look at the content of that fragment URL, and then check to see what happens in search.

Does it affect anything with regards to the indexed content on your site?
Is some of that content findable within your site suddenly?
Is that a problem or not?

And try to work based on that, because it’s very easy to block things by robots.txt, which actually are not used for indexing, and then you spend a lot of time maintaining this big robots.txt file, but it actually doesn’t change that much for your website.”

Other Considerations For Building A Robots.txt File

Zieger has one last follow-up regarding robots.txt files, asking if there are any specific guidelines to follow when building one.

Mueller says there’s no specific format to follow:

“No, it’s essentially up to you. Like some sites have big files, some sites have small files, they should all just work.

We have an open source code of the robots.txt parser that we use. So what you can also do is get your developers to run that parser for you, or kind of set it up so that you can test it, and then check the URLs on your website with that parser to see which URLs would actually get blocked and what that would change. And that way you can test things before you make them live.”

The robots.txt parser Mueller refers to can be found on Github.

Hear the full discussion in the video below:

Featured Image: Screenshot from, January 2022.

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By Rose Milev

I always want to learn something new. SEO is my passion.

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