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Do the search engines take the number of 404 errors returned by a site into account when calculating the site rank? For example I have 8,000 404 errors and I am thinking that that's affecting my SEO in a negative way.

However, I read this Google Webmaster Central article about 404 errors which suggests that they are not always a negative quality indicator.

So I am confused...are high numbers of 404 errors harmful to a site or not?

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up vote 5 down vote accepted

It depends on why the 404 errors are coming up but in general terms, yes, high numbers of 404 errors can count against the quality metrics of your site and hurt your rankings.

Whenever Google spiders the web, they are following links from external sites to yours, your own internal structure, and your past indexed pages. While 404s themselves are not a negative indicator according to the linked article above, 404s of expected content due to errors in your internal linking or server configuration (i.e., a bad redirect rule) will lower the number of documents you have indexed and that is the "penalty" applied.

There are several things you can do to mitigate the effect of 404 errors:

  1. Use a custom 404 error page to suggest alternative navigation. The inbound user or robot should be presented links and a menu to assist them in finding other content on your site. Never leave a default 404 server-generated error page in place
  2. If the 404s are caused from external links, use 301 redirects to get the inbounds to the desired location
  3. Use an XML sitemap to tell search engines what exists. This helps to drop dead pages out of their indexes

Follow those simple steps and any penalty caused by excessive 404 errors should fall away over time.

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1  
Thank you man, that's a nice answer – Johan Aug 7 '13 at 22:52
    
You're welcome. – JCL1178 Aug 7 '13 at 22:52

Google's John Mueller says:

404 errors on invalid URLs do not harm your site’s indexing or ranking in any way. It doesn’t matter if there are 100 or 10 million, they won’t harm your site’s ranking.

(as long as the 404 isn't coming from pages you want to see ranking)

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They are not pages, they are non-existing posts. Thanks Stephen! Nice! – Johan Aug 8 '13 at 5:54
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If someone is linking to your site say CarA.html but they are linking using carA.html then you won't be getting the juice from their link so you could as a audience stand point and SEO be beneficial to 301 redirect to the right page. Otherwise if they are linking to pages about Cakes, but your site is about Cars then you should simply ignore it and leave it as 404 as this would not benefit you or your audience. Be sure to check out the page that says LINKED FROM to see if you have an internal link or external link doing it. – Simon Hayter Aug 8 '13 at 7:38
    
John also said recently if they see a lot of errors they slow down crawling – Pepone Jan 9 at 0:15

In Google Webmaster Tools it says 'they generally don't hurt your ranking.'

However, they are bad for user experience and search engines don't buy things.

To quote the Google Intro to SEO .pdf:

Blockquote ...you should base your optimization decisions first and foremost on what's best for the visitors of your site. They're the main consumers of your content and are using search engines to find your work.

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404 errors on your site don't affect SEO in the sense that having too many 404's will reduce your ranking, there is however a caveat to this and that third party links to your site which terminate at 404 pages won't pass link juice and so your rank won't improve from that link, certainly not the end of the world for only one or two links but when you are talking about a large number of links that's when it can have a drastic effect on your site.

The important points to remember are...

  • Always return a proper 404 header and not a soft 404 otherwise the search engine will think the 404 not found page is the actual content page.
  • Make sure that if you rename or move content where previous links will no longer work instead of returning a 404 error you use a 301 redirect or a HTTP Moved header to tell the crawler where the content is now and have the link updated in the index as well as still get the link juice.
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