If the server supports it you can make a request to just get the headers (HEAD, as opposed to GET) without the actual data payload and this should include the Content-Length.

If you can't do that then you need to start the download and use expectedContentLength of the NSURLResponse.

Basically, create an instance of NSMutableURLRequest and call setHTTPMethod: with the method parameter set to @"HEAD" (to replace the default which is GET). Then send that to the server as you currently request for the full set of data (same URL).