Just how long should a download take?

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Summary: How quickly a download takes depends on many things including the size of the file and the speed of your connection, among other things.

You've got the opportunity to download a file from the internet. Perhaps a music file (a legal one of course), a picture, a document, or a new peice of software. All you have is its size in bytes, or perhaps "k" or "m", but how do you translate that into knowing wether or not you should wait or go get another cup of coffee while that download happens?

Of course there's no one answer because it depends mostly on your connection speed and perhaps what else you're doing at the time. There are some tools that will help you measure your connection speed and then a little math will give you a good idea of how long it will take. Not much math, I promise.

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You may have purchased a 56k modem, ordered a 768k DSL connection, or perhaps gotten some kind of speed promise from your cable company. Chances are they're all wrong.

First, let's make sure we all know what that "k" and "m" mean. "k" stands for one thousand (or 1024, which is a computer's version of "k"), and "m" for one million (1024x1024, or 1,048,576 to a computer). A 56k modem is supposed to be able to deliver about 56,000 bits of information per second. Each character or byte is 10bits (8 bits of data and two bits of overhead), so 56k should be around 5,600 bytes per second. Similarly, a promised 768k download speed implies 76,800 bytes per second.

I did say "supposed to" and "promised", because in reality those numbers are the maximum or best you might expect. In practice 56k modems rarely deliver 56k. More often than not, it's half that, or 28k. Even 768k DSL download speeds are infrequent - though you're typically at least in that 700k ballpark.

One way to tell the true speed of your download connection is to visit one of the many speed test sites. These work by downloading a file, measuring how long it takes, and then reporting the results back to you. Some speed test sites include: c|net, dslreports (even if you don't have DSL) and PC Pitstop.

So now you have this number. How long will the download take? Take the filesize, in bytes, and divide it by your connection speed in bytes per second. That's roughly the number of seconds your download should take.

For example, say the speed tests show your DSL connection is at 658k. That's 65,800 bytes per second. You want to download a 1 megabyte file - that's roughly 1,000,000 bytes, so 1,000,000 divided by 65,800 is right around 15 seconds. What if your 56k modem measures out to be 28k? Well, that 1,000,000 byte file divided by 2,800 bytes per second gives us 357 seconds, or about 6 minutes. That's all there is to it.

The important thing to remember is that these numbers are still just estimates. Your speed will vary based on several other factors including how good your ISP's connection is to the internet, how "busy" the internet is, the load at server you're accessing, and whether or not there's something else your computer (or another if you have more than one sharing the same connection) is doing on the internet.

But the biggest variable in that estimate is your connection speed and now you know how to make a pretty reasonable download time estimate.

Article C1886 - January 25, 2004