If the United States had converted to the metric system as promised back in the 1970s, Americans would have a much easier time understanding the base10 notation of data size and transfer rates. Each step up in notation is a difference of 1,000 units. When you reach 1,000 bytes it equals one kilobyte. When you reach 1,000 kilobytes it equals one megabyte. The pattern remains consistent through gigabytes, terabytes, petabytes and beyond. There are two tricky little inconsistencies, however. First, a byte is made up of eight, not 10, bits. Second, throughput, or the amount of data passing through a connection per unit of time (usually one second), is noted in bits, while download speed is noted in bytes. That confuses the math a little, but as long as you know what unit is being used, you can convert freely between them.
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Pick a file to download and keep an eye on the status bar. Your computer reports the actual download in kilobytes (KB) per second. The capital “B” indicates bytes while a lowercase “b” indicates bits. That comes in to play later. Note the reported speed in KB and divide it by 1,000. The easiest way to do this is to move the decimal three places to the left. Think of 10 to 100 to 1,000 representing three steps. Your KB number, for example 430, is more accurately represented as 430.0, so just slide that decimal over and you get .43 megabytes (MB).

Check the documentation that came with your Internet service. It should tell you what the maximum downstream and upstream speeds are. This is a measure of throughput. Most consumerlevel DSL services offer asymmetrical throughput, so you may see something like 6Mb down, 1.5Mb up. Note the lowercase “b.” This means that your maximum downstream throughput is 6 megabits, or 6,000 kilobits. To first convert this to bytes, divide 6,000 by eight (there are eight bits per byte) and you get 750 KB (capital “B” for bytes) download speed. Move your decimal over three places and your service has a maximum download speed of .75 MB.

Check the real world numbers with an Internet speed test. You will never get the maximums out of your service, so there are online tests to show you just what you are getting. Chances are, it will be somewhere between the service tier below yours and the maximum speed of your tier. The test works by downloading a small file and monitoring the progress. Again, be wary of the lowercase “b” and convert the information you receive to bytes first, then slide your decimal.
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