What is the maximum length of a URL in different browsers?
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What is the maximum length of a URL in different browsers? Does it differ between browsers? 

Does the HTTP protocol dictate it?
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Krystina Shields Created at: 2013-11-13 17:07:44 UTC By Krystina Shields
FWIW, for Windows users, server paths exceeding 250 characters may cause grief when building URLs, for example, see HttpContext.Current.Server.MapPath fails for long file names at forums.asp.net. bottom line: if one restriction does not get you, another one may. - Casey Torp
@gerryLowry: Windows tools already tend to have problems even compiling with long file names, so this usually isn't a concern if you only use MapPath for actual file names.  One hopes that the extra-long url names are not paired with extra-long file paths, since the latter results in an unusable mess whereas the former may just be a developer abusing get requests where he should be using post requests. - Verda Jaskolski
From support.microsoft.com/kb/208427 "Maximum URL length is 2,083 characters in Internet Explorer" - Tobin Russel
May I ask why did you need to know that? I.e. what's the use-case for having a long URL? - Alan Kilback
Related ServerFault question about Tomcat: serverfault.com/questions/56691/… - Dr. Efrain Lowe
11 Answers
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Limit request line directive sets the maximum length of a URL. By default, it is set to 8190, which gives you a lot of room. However other servers and some browses, limit the length more.

Because all parameters are passed on the URL line,items that were in password of hidden fields will also be displayed in the URL of course. Neither mobile should be used for real security measures and should be considered cosmetic security at best.....
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There is really no universal maximum URL length. The max length is determined only by what the client browser chooses to support, which varies widely. The 2,083 limit is only present in Internet Explorer (all versions up to 7.0). The max length in Firefox and Safari seems to be unlimited, although instability occurs with URLs reaching around 65,000 characters.
Opera seems to have no max URL length whatsoever, and doesn't suffer instability at extremely long lengths.
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WWW FAQs: What is the maximum length of a URL? has its own answer based on empirical testing and research. The short answer is that going over 2048 characters makes Internet Explorer unhappy and thus this is the limit you should use. See the page for a long answer.
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The longest URLs I came across are data URLs

Example image URL from Google image results (11747 characters)

data:image/jpeg;base64,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

0
The URI RFC (of which URLs are a subset) doesn't define a maximum length, however, it does recommend that the hostname part of the URI (if applicable) not exceed 255 characters in length:


  URI producers should use names that
  conform to the DNS syntax, even when
  use of DNS is not immediately
  apparent, and should limit these names
  to no more than 255 characters in
  length.


As noted in other posts though, some browsers have a practical limitation on the length of a URL.
0
Short answer - de facto limit of 2000 characters

If you keep URLs under 2000 characters, they'll work in virtually any combination of client and server software.

Longer answer - first, the standards...

RFC 2616 (Hypertext Transfer Protocol HTTP/1.1) section 3.2.1 says


  The HTTP protocol does not place
  any a priori limit on the length of
  a URI. Servers MUST be able to handle
  the URI of any resource they    serve,
  and SHOULD be able to handle URIs of
  unbounded length if they    provide
  GET-based forms that could generate
  such URIs. A server    SHOULD return
  414 (Request-URI Too Long) status if a
  URI is longer    than the server can
  handle (see section 10.4.15).
  
  Note: Servers ought to be cautious
  about depending on URI lengths
        above 255 bytes, because some older client or proxy
        implementations might not properly support these lengths.


...and the reality

That's what the standards say. For the reality, see this research over at boutell.com to see what individual browser and server implementations will support. It's worth a read, but the executive summary is:


  Extremely long URLs are usually a
  mistake. URLs over 2,000 characters
  will not work in the most popular web
  browser. Don't use them if you intend
  your site to work for the majority of
  Internet users.


Search engines like URLs < 2048 chars...

Be aware that the sitemaps protocol, which allows a site to inform search engines about available pages, has a limit of 2048 characters in a URL. If you intend to use sitemaps, a limit has been decided for you! (see Calin-Andrei Burloiu's answer below) 

There's also some research from 2010 into the maximum URL length that search engines will crawl and index. They found the limit was 2047 chars, which appears allied to the sitemap protocol spec. However, they also found the Google SERP tool wouldn't cope with URLs longer than 1855 chars.

Internet Explorer's limitations...

IE8's maximum URL length is 2083 chars, and it seems IE9 has a similar limit. 

I've tested IE10 and the address bar will only accept 2083 chars. You can click a URL which is longer than this, but the address bar will still only show 2083 characters of this link.

Is this information up to date?

This is a popular question, and as the original research is nearly 6 years old I'll try to keep it up to date: As of July 2013, the advice still stands, as the IE10 address bar will still not accept a longer URL.
0
The HTTP 1.1 specification says:


  URIs in HTTP can be represented in
  absolute form or relative to some
  known base URI [11], depending upon
  the context of their use. The two
  forms are differentiated by the fact
  that absolute URIs always begin
  with a scheme name followed by a
  colon. For definitive information on
  URL syntax and semantics, see "Uniform
  Resource Identifiers (URI):    Generic
  Syntax and Semantics," RFC 2396 [42]
  (which replaces RFCs    1738 [4] and
  RFC 1808 [11]). This specification
  adopts the    definitions of
  "URI-reference", "absoluteURI",
  "relativeURI", "port",
  "host","abs_path", "rel_path", and
  "authority" from that
  specification.
  
  The HTTP protocol does not place
  any a priori limit on the length of
  a URI. Servers MUST be able to handle
  the URI of any resource they    serve,
  and SHOULD be able to handle URIs of
  unbounded length if they    provide
  GET-based forms that could generate
  such URIs.* A server    SHOULD return
  414 (Request-URI Too Long) status if a
  URI is longer    than the server can
  handle (see section 10.4.15).
  
  Note: Servers ought to be cautious about depending on URI
  lengths
        above 255 bytes, because some older client or proxy
        implementations might not properly support these lengths.


As mentioned by @Brian, the HTTP clients (e.g. browsers) may have their own limits, and HTTP servers will have different limits.
0
Microsoft Support says "Maximum URL length is 2,083 characters in Internet Explorer".

IE has problems with URLs longer than that. Firefox seems to work fine with >4k chars.
0
Sitemaps protocol which is a way for webmasters to inform search engines about pages on their sites (also used by Google in Webmaster Tools) supports URLs with less than 2048 characters. So if you are planning to use this feature for Search Engine Optimization take this into account.
0
In URL as UI Jakob Nielsen recommends:


  the social interface to the Web relies on email when users want to recommend Web pages to each other, and email is the second-most common way users get to new sites (search engines being the most common): make sure that all URLs on your site are less than 78 characters long so that they will not wrap across a line feed.


This is not the maximum but I'd consider this a practical maximum if you want your URL to be shared.
0
ASP.net 2 and SQL Server reporting services 2005 have a limit of 2028. I found this out the hard way, where my dynamic URL generator would not pass over some parameters to a report beyond that point. This was under IE8.
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