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A robots.txt file size checker tells you how close your file is to the hard limit Google enforces on how much of it gets read. Paste your robots.txt content into the box above and it counts the actual bytes, not just characters, and compares that against Google's documented cutoff.
Google's Search Central documentation states that Google enforces a robots.txt file size limit of 500 kibibytes, or 512,000 bytes. Content after that point in the file is ignored entirely, as if it were never written. This is not a soft warning, it is a hard cutoff applied during crawling, so any Allow or Disallow directive that falls past the limit simply does not apply, whether or not you intended it to.
Google has noted that very few robots.txt files across the web come anywhere near 500 KiB, since a well-structured file listing rules by pattern rather than by individual URL typically runs a few kilobytes. The sites that do run into trouble tend to be very large ecommerce or listing platforms whose robots.txt is generated programmatically and ends up with one Disallow line per product, filter combination, or user-generated page, sometimes running into the tens of thousands of lines.
The fix almost always comes down to consolidation. Rather than listing every blocked URL individually, use wildcard patterns to cover whole classes of URLs in a single rule, for example blocking an entire parameter-driven filter path with one pattern instead of one line per filter combination. For content you want excluded from the index rather than blocked from crawling, a noindex meta tag or header on the page itself is often the better tool anyway, since a disallowed page can still appear in search results without a snippet if other sites link to it. Getting a large site's crawl directives right is exactly the kind of technical foundation that determines whether the rest of your SEO work can even take effect, if you want a second set of eyes on it, a free SEO audit is a good place to start.
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