How Fast is Fast?

To find out how long it will take to transfer a file on your computer from one place to another, you need to know two things: the size of your file, and the size of the 'pipe' through which the file needs to travel. The pipe analogy is often used when discussing bandwidth and is a good one. Bandwidth is the rate at which computer data can be transmitted between two points. It's just like a water pipe, the more water (data), the bigger the pipe (bandwidth) needs to be to shift the water in a given time.

In the case of a home broadband connection, the first pipe is between you and your ISP. The ISP also has pipes that will connect you to the final destination. Where there are several pipes involved, the bandwidth will be reduced to the size of the smallest pipe.

The pipe analogy needs a little extension. It's like your ISP is on the top of a hill, so getting data (downhill) is quicker than pushing it (uphill). Broadband connections are controlled so that more common downloading (downhill) tasks such as loading webpages, audio, video etc are faster than the uploading (uphill) tasks such as sending an email or sharing photos from your computer. These connections are called asymmetricSymmetric connections are the same in both directions but are generally more expensive. The 'A' in ADSL is for asymmetric.

To talk about bandwidth speeds, we need to start with some units of conversion:

8 bits = 1 byte
1024 bytes = 1 Kilobyte (KB)
1024 Kilobytes = 1 Megabyte (MB)
1024 Megabytes = 1 Gigabyte (GB)
1024 Gigabytes = 1 Terrabyte (TB)

Putting these units into perspective: you can think of 1KB as being roughly 1 A4 page of text. A digital photograph is about 3MB and 1GB will give you a couple of hours of DVD quality video.

Bandwidth is commonly measured in Megabits per second. One Megabit is one million bits and using the table above you could work out that one million bits = 100KB, or about 100 pages of A4 text. So if you have an '8 meg' broadband connection at home, that means you can download about 800KB per second, or a digital photo every few seconds. Lots of other factors might slow it down though! Because your connection is likely asymmetric, you will only be able to upload at a fraction of that speed.

Finally, the table below shows various line speeds in the left hand column followed by expected speeds in KB, MB etc. So now you know what having a 2 megabit broadband connection really means!

Line Speed Per Sec Per Min Per Hour
1Mbit 100 KB/sec 6MB/min 360 MB/hour
2Mbit 200 KB/sec 12 MB/min 720 MB/hour
4Mbit 400 KB/sec 24 MB/min 1.1 GB/hour
10Mbit 1 MB/sec 60 MB/min 2.8 GB/hour
100Mbit 10 MB/sec 600 MB/min 28 GB/hour
1000Mbit 100 MB/sec 6 GB/min 280 GB/hour

If you want to measure the bandwith of your connection, is a good place to start. This tests the bandwidth, up and down, between you and one of speedtest's servers.

That doesn't tell the whole story though. E.g. if the website you're trying to get to is being slow, it might be that the website is on a poor connection while your own connection is fine. provide a nice set of tools to test things like this, so don your detective hat and troubleshoot like a pro.