This page is about Rate Adaptive Digital Subscriber Line (RADSL) as provided by BT Wholesale to ISPs for them to resell to most UK ADSL users. DSL users in other countries may find some of this applies to them.

How to test your line speed

You can test your line speed and compare it with BT's predicted speed. Web based ADSL speed testers are prone to substantial errors. Tests conducted with a browser are influenced by factors unconnected with the ADSL line. Simple mail clients are better but not as good as FTP clients.

The best method I have found is to download a large file with a simple FTP client from a fast FTP site and measure its download time. I use a 22 M byte file from which has the following full URL:

To calculate speed:

  1. obtain the downloaded file size in bytes
  2. multiply by 8 to convert to bits
  3. multiply by 1.15 to allow for a 15% network overhead
  4. divide by the download time in seconds to convert to bits per second
  5. divide by 1024 to convert to K bits per second
  6. divide by a further 1024 to convert to M bits per second.

Use this Google calculator - adjust file size at beginning and time at end.

I am getting 14 M bits per second about a mile from the telephone exchange with my router set to ADSL2+ and this agrees well with BT's speed test. My upline speed is about 700 k bits per second.

Home internet connections are normally shared by 50 subscribers so you will have to repeat the speed test at various times during the day. Your line speed will probably be slightly faster than the highest speed you have measured. To ensure that you are measuring line speed you should perform the test without anything else running on your computer or the computers of others connected to your modem/router. Most wireless connections will limit the speed so a wire connection to your modem/router is preferable.

How to obtain BT's predicted speed for your line

  1. put your telephone number and postcode into this form
  2. click 'check'
  3. click on 'BT ADSL' on the results page.


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