A website crawler is a tool that automatically discovers and visits web pages by following links and collecting information about their content, structure, and technical elements. Search engines use crawlers to discover pages for indexing, while SEO professionals use crawling tools to identify technical issues, analyze internal links, and understand how a website is organized.
Whether you manage a small business website, an online store, or a large content site, understanding how website crawlers work can help you improve your site's accessibility, technical SEO, and search visibility.
This article explains what website crawlers are, how they work, the different types of crawlers, and which tools you can try for crawling and auditing your website.
What Is a Website Crawler?
A website crawler, also called a web crawler, spider, or search bot, is an automated program that systematically visits web pages and follows links to discover other pages. It retrieves information from websites and processes it according to its intended purpose.
Search engines such as Google use crawlers to discover new and updated pages. SEO professionals use website crawling software to examine a site's structure, identify technical problems, and collect data about URLs, metadata, links, and other on-page elements.
For example, when a crawler visits a website's homepage, it can extract the links found on that page and follow them to other URLs. It repeats this process to discover more pages, depending on its configuration, access permissions, and crawl limits.
Website crawlers can serve different purposes:
- Search engine crawling: Discovering web pages and retrieving content for potential indexing.
- Technical SEO auditing: Finding broken links, redirect chains, missing metadata, and other technical issues.
- Website monitoring: Checking pages for changes, errors, and availability.
- Content analysis: Collecting page titles, headings, word counts, canonical tags, and other content details.
- Website research: Understanding site structures, internal linking, and publicly accessible information.
A crawler does not automatically make a page appear in search results. Crawling, indexing, and ranking are separate processes. A search engine must first discover and crawl a page, then determine whether it should be indexed. Ranking systems decide how eligible pages appear for particular searches.
How Does a Website Crawler Work?
A website crawler typically starts with a list of URLs called seed URLs. These are the initial pages the crawler is instructed to visit. From there, it discovers additional pages by extracting links and following them according to its crawling rules.
The exact process depends on the crawler. A search engine bot and a desktop SEO crawler may visit the same website but have different goals, crawling limits, and data collection methods.
How Website Crawling Works
From the first URL to a structured crawl report
1. The crawler starts with a seed URL
The first step is providing the crawler with one or more starting URLs. For a typical website audit, this is often the site's homepage.
For example, a crawler might begin at:
https://example.com/
The crawler adds this address to its crawl queue, a list of URLs waiting to be visited. Depending on its settings, it may also use an XML sitemap or a manually supplied list of URLs as additional starting points.
2. The crawler requests the page
The crawler sends a request to the web server hosting the page. The server responds with information such as an HTTP status code and, if successful, the page's HTML.
Common HTTP status codes include:
| Status code | Meaning |
|---|---|
| 200 | The request succeeded. |
| 301 | The page has permanently moved. |
| 302 | The page has temporarily moved. |
| 403 | Access to the requested resource is forbidden. |
| 404 | The requested page was not found. |
| 500 | The server encountered an internal error. |
The response helps the crawler determine whether it can access the page and what to do next. A crawler may follow redirects, record errors, or stop processing a URL based on its configuration.
3. The crawler analyzes the page
After retrieving a page, the crawler extracts the information relevant to its purpose. An SEO crawler, for example, may collect:
- Page title and meta description
- H1 and other heading tags
- Canonical URL
- Meta robots directives
- HTTP status code
- Internal and external links
- Image URLs and alt text
- Word count and other content details
- Structured data, where supported
The crawler may also examine the HTML for directives that affect
crawling, such as noindex and nofollow.
Some tools can render JavaScript to discover content and links that
are not present in the initial HTML response.
4. The crawler follows links to discover more pages
After analyzing a page, the crawler identifies eligible links and adds new URLs to its crawl queue.
For example, a homepage might link to these pages:
/about//services//blog/
The crawler can visit those pages, extract their links, and continue discovering additional URLs. This process repeats until it reaches its stopping conditions, such as exhausting the crawl queue, reaching a maximum URL limit, or encountering a configured crawl restriction.
5. The crawler organizes the collected data
The final step is processing the information collected during the crawl. An SEO auditing tool may group pages by status code, identify missing title tags, flag broken links, and show how pages connect through internal links. More advanced tools can compare crawl results over time to help website owners monitor technical changes and measure improvements.
Types of Website Crawlers
Not all website crawlers perform the same tasks. Some are designed to discover pages for search engines, while others collect information for auditing, monitoring, or research.
1. Search engine crawlers
Search engine crawlers are automated bots operated by search engines to discover and retrieve publicly accessible web pages. They help search engines keep their indexes up to date.
- Googlebot: Google's crawler for discovering and retrieving pages for Google Search.
- Bingbot: Microsoft's crawler for discovering pages for Bing.
- Other search engine bots: Crawlers operated by other search providers and services.
Search engine crawlers generally discover URLs through links, sitemaps, and previously known URLs. Their crawling frequency and behavior depend on their own systems and the site's accessibility.
2. SEO website crawlers
SEO crawlers are tools that simulate the process of visiting pages and collecting technical information about a website. They are commonly used by SEO specialists, web developers, and site owners to identify problems with crawling, indexing, and user experience.
An SEO crawler can help identify:
- Broken internal and external links
- Redirect chains and loops
- Missing or duplicate title tags
- Missing meta descriptions
- Pages with incorrect canonical tags
- Pages with noindex directives
- Missing image alt attributes
- Orphan pages, when combined with suitable URL discovery sources
- Internal linking and crawl depth issues
Unlike search engine crawlers, these tools are usually operated on demand by a user. Their results are intended for analysis and troubleshooting, not for adding pages to a search engine's index.
3. Website monitoring crawlers
Monitoring crawlers regularly check specified pages or websites to detect changes and technical problems. Depending on the software, they may check whether a page is available, whether its content has changed, whether links are still working, or whether certain elements have disappeared.
For example, a website owner could schedule a daily check of important landing pages to catch unexpected 404 errors or changes to page titles.
4. Data collection and research crawlers
Some crawlers are designed to collect publicly available information from websites for research, analytics, or other data processing tasks. These crawlers may focus on particular types of information, such as product listings, public directories, or news articles.
Their permitted use depends on the website's access restrictions, applicable terms, and relevant laws. For SEO work, a dedicated auditing crawler is generally more useful than a general-purpose data collection crawler because it provides structured reports about technical website issues.
Website Crawling vs. Indexing: What's the Difference?
Crawling and indexing are connected but separate stages of search engine processing. A page can be crawled without being indexed, and a URL can sometimes be known to a search engine even if its content has not been crawled.
| Crawling | Indexing |
|---|---|
| Discovers and retrieves URLs and page content. | Analyzes and stores eligible page information in a search index. |
| Uses links, sitemaps, and other URL discovery methods. | Evaluates content, canonicalization, and indexing directives. |
| Checks whether a page can be accessed. | Determines whether a page is suitable for inclusion in the index. |
| Can identify technical access problems. | Can reveal issues that prevent a page from being indexed. |
A page may return a successful HTTP 200 response but contain a
noindex directive. A crawler can access and report on
that page, but the directive tells search engines not to include it
in their index.
Likewise, a page may be blocked from crawling through
robots.txt. That prevents the crawler from retrieving
the page's content, but it does not necessarily prevent the URL
from appearing in search results.
What Does a Website Crawler Check During an SEO Audit?
A technical SEO crawler collects data that helps you understand how your website is structured and whether its pages have common technical problems. The specific checks vary by software, but most dedicated SEO crawlers offer several core features.
1. HTTP status codes
A crawler checks the HTTP response returned by each URL. This helps identify pages that load successfully, pages that redirect, and pages that return errors. A large number of broken URLs may indicate problems with internal links, deleted content, or incorrect URL references.
2. Broken links
Broken links point to URLs that cannot be accessed successfully. They can lead to poor user experiences and make it harder for crawlers to discover relevant pages. An SEO crawler can identify the broken destination and, in many cases, the source page containing the link. Read more in Broken Link Audit: How to Find and Fix Errors.
3. Title tags and meta descriptions
Title tags and meta descriptions provide information about a page's content. Crawlers can check whether these elements are missing, duplicated, or unusually long or short. These checks are useful when reviewing large websites with many pages.
For additional troubleshooting, see Missing Title Tags: Causes and How to Fix Them and Missing Meta Descriptions: How to Find and Fix Them.
4. Canonical tags
Canonical tags indicate the preferred URL for a page when duplicate or similar content exists. A crawler can collect canonical URLs and compare them with the pages being crawled. This helps reveal self-referencing canonical tags, canonical URLs pointing to redirects or errors, and inconsistencies between pages.
Learn more in Canonical Tag Issues: Common Problems and Fixes.
5. Internal links and crawl depth
Internal linking helps visitors and crawlers navigate a website. Crawlers can analyze the links between pages and calculate how many clicks away a page is from a starting URL. This can help identify pages that are difficult to reach, important pages with few internal links, and sections of a website with disconnected navigation.
Pages without internal links pointing to them may be orphan pages. See Orphan Pages: How to Find and Resolve Them for more on identifying and addressing them.
6. Robots directives and indexability
SEO crawlers can check robots meta tags, X-Robots-Tag headers, and robots.txt restrictions to help establish whether pages are accessible and intended for indexing. It's important to interpret these directives together. A crawler may be unable to see a page's noindex tag if access to that page is blocked by robots.txt.
For a deeper look at this issue, read Noindex Issues: How to Find and Fix Them.
7. JavaScript-rendered content
Modern websites may rely on JavaScript to generate page content, navigation, or internal links. A crawler that only retrieves the initial HTML might not discover everything that appears in a browser. JavaScript rendering can help SEO crawlers inspect content after the browser has executed scripts.
Website Crawler Tools to Try
There are several website crawling tools available, ranging from free search engine utilities to dedicated desktop applications and cloud-based auditing platforms. The right tool depends on the size of your website, the type of audit you need, and whether you prefer a desktop or browser-based workflow.
1. Google Search Console
Google Search Console is a free tool for monitoring how Google discovers, crawls, and indexes your website. It is not a general-purpose crawler that you can run across an entire site on demand, but it provides important information about Google's own crawling and indexing activity.
Useful features:
- URL Inspection for checking individual URLs
- Page indexing reports
- Sitemap submission and monitoring
- Crawl statistics and crawl-related information
Best for: Website owners who want to understand how Google handles their pages and investigate indexing issues.
2. Screaming Frog SEO Spider
Screaming Frog SEO Spider is a desktop website crawler for technical SEO audits. It can crawl websites by following links, collect detailed page data, and identify a wide range of technical SEO issues.
Useful features:
- Broken link and server error detection
- Title tag and meta description analysis
- Redirect and canonical checks
- XML sitemap generation
- JavaScript rendering and custom extraction
Pricing: A free version is available for crawling up to 500 URLs. A paid license removes that crawl limit and unlocks additional features.
Best for: SEO professionals and website owners who need detailed crawl data and customizable auditing.
3. Sitebulb
Sitebulb is a website auditing tool available as desktop software and a cloud service. It combines crawling data with visual reports and prioritized technical SEO recommendations.
Useful features:
- Technical SEO issue detection
- Internal link analysis and site visualizations
- JavaScript crawling
- Prioritized audit hints
- Exportable reports for communicating findings
Pricing: Sitebulb offers a free trial. Its desktop product advertises audits of up to 500,000 URLs, while cloud plans offer different scale and collaboration options.
Best for: Users who want visual reports and explanations that help them understand and communicate audit findings.
4. Netpeak Spider
Netpeak Spider is a desktop SEO crawler designed to audit websites and analyze technical issues. It can crawl websites and provide structured information about URLs, links, and on-page elements.
Useful features:
- Broken link detection
- Redirect analysis
- Metadata and heading checks
- Duplicate content analysis
- Internal linking analysis
Best for: Users who want a desktop SEO crawler with a range of technical audit checks and configurable crawling.
5. SiteAuditLint
SiteAuditLint is a Windows desktop SEO crawler designed to help website owners and SEO professionals identify technical issues and review website performance over time.
Useful features:
- Technical SEO website crawling
- Detection and review of common website issues
- Historical crawl data and comparisons
- Tracking changes in reported issues across audits
Pricing: The free option allows crawling up to 100 URLs per crawl. The Pro version is a $99 one-time purchase with unlimited URLs, audit history, comparisons, and additional reporting features.
Best for: Windows users who want a desktop crawler, an issue-focused action plan, and a way to compare audit results over time.
Explore the SiteAuditLint features or see how it compares with other tools in the desktop SEO crawler comparison.
How to Choose a Website Crawler
Before selecting a tool, consider what you want to achieve with your crawl. A small website with a few dozen pages may only need basic checks, while a large ecommerce site may require advanced crawling settings, JavaScript rendering, and detailed reporting.
| What to consider | Why it matters |
|---|---|
| Crawl limit | Determines how many pages you can inspect in a single crawl. |
| JavaScript rendering | Helps reveal content and links generated by JavaScript. |
| Technical SEO checks | Determines which issues the crawler can detect and report. |
| Internal link analysis | Helps you understand how pages are connected. |
| Historical comparisons | Lets you track whether reported issues change after updates. |
| Export options | Makes it easier to investigate issues and share findings. |
| Operating system | Ensures the software is compatible with your computer. |
| Cost | Helps you select a tool that fits your budget and audit needs. |
For a first audit, start with the basic requirements: crawl the pages you want to inspect, review the status codes, check the page titles and metadata, and examine the internal links. You can add more advanced checks as your needs grow.
What Can Prevent a Website Crawler From Accessing Pages?
Even when a website is online, crawlers may not be able to access every page. Several technical and configuration issues can limit crawling.
Robots.txt restrictions
The robots.txt file contains rules that tell compliant
crawlers which parts of a website they are allowed to request.
User-agent: *
Disallow: /private/
Allow: /
This example asks compliant crawlers not to crawl URLs under
/private/ while allowing crawling elsewhere.
Robots.txt is not a security mechanism, and it does not guarantee
that blocked URLs will be excluded from search results.
Broken internal links
If a page contains a link to a URL that no longer exists, a crawler may discover the broken destination and report it as an error. Broken links can also make important pages harder to reach through the site's internal navigation.
Server errors and access restrictions
Server errors, network failures, authentication requirements, and rate limiting can interfere with crawling. When a crawler repeatedly encounters these problems, some URLs may remain unprocessed or be recorded as inaccessible.
JavaScript rendering problems
Websites that depend heavily on JavaScript may present different content to users and crawlers that do not execute scripts. If important links or page content are only available after rendering, a crawler without JavaScript support may not find them.
Crawl limits and configuration
A crawl may stop before discovering every page because of a URL limit, maximum crawl depth, excluded URL patterns, or other settings. The crawl results therefore represent the pages the tool actually discovered and processed, not necessarily every URL on the website.
How to Run Your First Website Crawl
Running a basic website crawl is a straightforward way to start investigating your site's technical SEO.
- Choose a crawler and enter your website URL. Install a desktop crawler or open a suitable web-based auditing tool. Start with your homepage or another important page that links to the rest of your site.
- Configure the crawl settings. Set a reasonable URL limit, choose whether to follow redirects, and decide whether to enable JavaScript rendering. If you are crawling a live website, avoid unnecessarily aggressive settings.
- Start the crawl and monitor progress. Watch the number of discovered and completed URLs, and check whether the crawler encounters access errors or unexpected redirects.
- Review the crawl reports. Start with pages returning 4xx or 5xx status codes, then inspect broken links, redirect issues, missing titles, and other relevant findings.
- Investigate and fix the issues. Check the affected URLs and their source pages before making changes. Not every warning requires a fix, so consider the purpose and importance of each page.
- Run another crawl to check your changes. Repeat the crawl using the same settings and compare the results. Reviewing the changes can help you determine whether the fixes worked and whether new problems appeared.
How to Use Crawl Data to Improve Your Website
A crawl report is most useful when you turn the findings into a practical improvement process.
For example, imagine an audit finds 42 broken links, 25 pages missing title tags, and 16 redirect errors. Instead of attempting to fix everything at once, group the findings by issue type and investigate their potential impact.
You can start by checking whether broken links affect important pages, then review metadata on key landing pages and investigate redirect chains that create unnecessary requests.
For ongoing website maintenance, save the crawl results and compare future audits. Historical comparisons can help you see which issues have been resolved, which remain, and whether new problems have emerged after a website update.
Example: Tracking SEO Audit Improvements
Illustrative issue counts from an initial and follow-up audit
A comparison like this makes it easier to review progress after technical fixes and identify which issues still need attention. If the same categories are checked consistently, historical data can help reveal trends that a single crawl cannot show.
Frequently Asked Questions
Is a website crawler the same as a web scraper?
No. A website crawler primarily discovers and visits URLs, often by following links, and collects information about pages. A web scraper extracts specific data from web pages, such as product details, prices, or article information. Some tools combine crawling and scraping capabilities, but the terms describe different tasks.
Can a website crawler improve SEO?
A crawler does not directly improve rankings. It helps identify technical issues that may affect how search engines access and understand a website. Fixing relevant problems, improving internal links, and maintaining accessible content can support a site's SEO efforts.
Does Google crawl every page on a website?
No. Google may not crawl every discovered URL. Access restrictions, server conditions, crawl prioritization, and other factors can affect which pages Googlebot visits. Google also does not guarantee that every crawled page will be indexed.
How often should you crawl your website?
The ideal frequency depends on how often your website changes and how important it is to detect technical problems quickly. A small website that rarely changes may only need periodic audits, while a large site with frequent releases may benefit from more regular crawling.
Can a crawler find orphan pages?
A crawler that only follows internal links may miss pages that have no links pointing to them. To discover potential orphan pages, combine crawl data with other URL sources, such as XML sitemaps, analytics exports, or a list of known URLs. Compare these sources with the pages discovered during the crawl.
Does crawling a website affect its performance?
Crawling generates requests to a website's server. A high crawl rate or a large number of simultaneous requests can create additional load, particularly on smaller sites. Configure crawling responsibly, especially when auditing websites that you do not own or administer.
Final Thoughts
Website crawlers are essential tools for understanding how pages are discovered, accessed, and connected. Search engines use their own crawlers to find content, while SEO professionals use dedicated crawling software to audit websites and investigate technical problems.
Knowing how crawling works can help you recognize the difference between crawling and indexing, understand why some pages are inaccessible, and identify opportunities to improve your website's structure.
Whether you are performing your first technical SEO audit or monitoring an established website, start with a clear crawl, review the findings, fix the issues that matter, and compare subsequent audits to track your progress.
SiteAuditLint is a Windows desktop SEO crawler with a free option for up to 100 URLs per crawl and a $99 one-time Pro version with unlimited URLs. It includes an issue-focused Action Plan and historical audit comparisons to help you review changes between crawls.