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 The Data-Transfer-Rate Conversion Table

How kilobits per second stack up to Kilobytes per second.

By Scot Finnie

The Internet measures data-transfer rates one way — bits per second — but all the more popular Web browsers show file download performance in bytes per second. How do you figure this out? This article explains the difference, and gives you real-world examples and conversions. Here's the Data Transfer Rate Conversion Table.

Broadband Data-Transfer Rates
Typical Service Level kilobits per second (kbps) Kilobytes per second (KB/sec) Estimated Ideal Throughput
  "28.8K" Analog Modem 28.8-kbps 3.6-KB/sec 2.8-KB/sec
  "33.6K" Analog Modem 33.6-kbps 4.2-KB/sec 3.3-KB/sec
  "56K" Analog Modem 53.3-kbps 6.6-KB/sec 5.2-KB/sec
  1-channel ISDN 56-kbps 7-KB/sec 5.4-KB/sec
  1-channel ISDN 64-kbps 8-KB/sec 6.2-KB/sec
  2-channel ISDN 115.2-kbps 14.4-KB/sec 11.2-KB/sec
  2-channel ISDN / IDSL 128-kbps 16-KB/sec 12.5-KB/sec
  Fractional T-1 256-kbps 32-KB/sec 25-KB/sec
  384 S-DSL 384-kbps 48-KB/sec 37.5-KB/sec
  Satellite 400-kbps 50-KB/sec 39-KB/sec
  Fractional T-1 512-kbps 64-KB/sec 50-KB/sec
  Cable / DSL 768-kbps 96-KB/sec 75-KB/sec
  1-Mbps / Cable / DSL 1,000-kbps 125-KB/sec 98-KB/sec
  T-1 / Cable / DSL 1,544-kbps 193-KB/sec 151-KB/sec

Transfer Rates and Throughput

Bits and bytes. That's where this starts. Bit is short for BInary Digit, and it's the smallest unit of binary measure, usually either a 0 or a 1. A byte consists of eight bits (in mainstream computing). Two of many common areas where the terms bits and bytes are applied are data storage and network data-transfer rates. It's that second one being described here, and the definitions that follow don't in every case apply to the data storage definitions of the same terms.

Data-transfer rate is the speed at which data transmits from one device to another. Among the best known data-transfer rates is the one for conventional analog modems, whose top speed is currently "56K." Actually, its fastest download data-transfer rate is governed at 53.3-kbps. 53.3-kbps literally means 53,300 bits per second. There are 1,000 bits in a kilobit. Measuring bits per second is the traditional way to convey network data- transfer rates. Although technically speaking, the term kilobit should have a lowercase initial letter, most published reports capitalize it in abbreviation, resulting in "56-Kbps," or even the really confusing "56K."

What complicates matters greatly is the fact that most Web surfers see data-transfer rates measured on a different scale on their browser download dialog boxes. The newer versions of Internet Explorer and Netscape Navigator both display data-transfer rates measured in Kilobytes-per-second (KB/sec). A Kilobyte is 1,024 bytes, and it is more properly used to measure data storage, not network data-transfer rates. The numbers in the table show pure mathematical data-transfer rates. I used a shareware program called Master Converter from Savard Software to double check my math.

A lot of my newsletter readers have written to me insistently that the numbers in the table above are wrong. And in a way they're right, and in a way they're wrong. There's a difference between a data-transfer rate and throughput. When you're making a kilobit-per-second conversion to a Kilobyte-per-second conversion, the question you have to ask is what bits and bytes are you counting.

Even though there are 8 bits in every byte, the nature of asynchronous transmission (used by most connection methods for accessing the Internet) requires the receiving of two "dead" bits for every 8 bits of useful data transferred: the start and stop bits. Bottom line, it takes 10 bits of transmission horsepower to deliver 8 bits of useful information. The question this begs is, how much data actually moves from one place to another in a given amount of time. And that's the definition of throughput.

Other factors also have an effect on throughput, things like packet loss, data compression, line quality, and server load. The very essence of asynchronous data transfer is that it is very flexible, able to deal with variations in performance. That's why throughput is an inexact science, and why the table shows guesstimates only, which use the "divide by 10" method. Some people say it's more proper to divide by 9, but in my experience, something more like 12 might come closer to approximating what people actually see on an ideal basis. Still, I've chosen to divide by 10 because that's the number most people consider the best one. In the end, you'll have to make your own judgment.

When you're measuring data-transfer rates, however, that is precise. Whether you're sending data as kilobits per second or as Kilobytes per second, many of the throughput-affecting factors affect both rates equally. So, while a received Kilobyte may not actually equally a full Kilobyte of useful information, the same is true if you measure it as kilobits pers second. What I don't know, though, and have endeavored in vain to find out, is just exactly how the Web browsers are figuring their Kilobyte per second download rates. Are they counting 10-bits to a byte, or are they counting 8-bits to a byte? Or are they figuring it another way entirely? If I ever learn something that changes any of the information on this page, I will come back and update it. And point that out to you in Scot’s Newsletter.

The best way to test the ideal throughput of your Internet connection is to download a large file from your ISP's server. Ask your ISP whether it offers this kind of test. A few still do. While you can download large files from other sources, such a download is much more subject to the vagaries of the Internet because you are directly connected to you ISP, but not so other sites. Even so, downloading a multiple-megabyte file from a variety of servers around the Internet, and doing so repeatedly over several days, and then averaging the times would give a very good approximation of real-world throughput. For that kind of testing, start with TestMySpeed.com site, which links to performance tests that work that way. Try to pick sites closest to your physical location.

Fred Langa's BrowserTune 2000 does an excellent job of measuring your Internet throughput. It's another good source of information on this subject. I'm also partial to Dave Methvin and Martin Heller's PCPitStop.com, which also offers an excellent performance test.