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When buying network hardware the specs usualy contain "Bits". Software usualy uses "Bytes".

What are the formulas for converting bandwidth measures?

Examples (assuming perfect conditions, ignoring side effects):

  1. How mow many data (Megabytes per second) can I transfer on my 100 MBit network?
  2. What bandwidth (in KBit) does my ISP provide, when my torrent client shows an upstream of 300 KByte/s (and I am sure, the limit is from my ISP)?
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2 Answers 2

up vote 2 down vote accepted

It's a marketing trick to advertise with bits instead of bytes. 100 Mb sounds bigger than 12.5 MB.

  1. 100 Mbit (100 Mb) = 100 ÷ 8 = 12.5 MB per second. Note that "how many data can I transfer" (data limit) is not equal to "what is my internet speed" (bandwidth)
  2. 300 kBps (300 kilobytes per second) is 300 × 8 = 2400 kbit (= 2.4 Mbit). Your speed is limited by your ISP. You can reach the 12.5 MB/s speed inside your network (providing that your other network equipment also works with 100Mb/s or higher).

    • b = bits
    • B = bytes (one byte contains eight bits)
    • Mb = Megabits
    • MB = Megabyte
    • MBit should not be shortened to MB, it should be Mbit (Mb)

The "formula" is:

bytes = bits ÷ 8

That's all. The opposite should not be difficult: bits = bytes × 8

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In my second example, I meant 300 KBytes of course. According to Wikipedia "kbps" means Bits and "kBps" means Bytes. I think this is the source of my confusion. –  Witek Jun 23 '11 at 11:02
3  
I can't help but comment on this. I really don't think it's a marketing trick. Networks transmit information 1 bit at a time, so it's more accurate to talk about bits per second rather than bytes per second. Further, bytes tends to carry with it the idea of payload. Networks introduce overhead, bringing your overall payload transfer rate lower than simply 100Mbps/8. –  Tyson May 2 '12 at 14:28
    
A bit is a 0 or a 1, which is transmitted via a variations in voltage levels (at least on a typical TCP/IP transmission over Ethernet using twisted copper, which is what the bulk of home networks use). A sequence of 8 bits is a byte. Furthermore, all information on a computer's HDD is ultimately stored as a sequence of bits, represented by a 1 or a 0. This is just how computers work, it has NOTHING to do with marketing. –  MaQleod Apr 10 at 2:41
    
Your formulas are wrong. bytes = 8 × bits would mean bytes is 64 for bits = 8. That should instead be bits = 8 × bytes –  Zabba Dec 17 at 19:03
    
@Zabba fixed, the given example was correct though. –  Lekensteyn Dec 17 at 21:03

There are 8 Bits to a Byte.

How mow many data (Megabytes per second) can I transfer on my 100 MBit network?

100 MBit per second, or 12.5 MB per second, theoretically. The real transmission rate is lower. It depends on several factors, such as electromagnetic interferences, transmission protocols used, lossy networks in between, packets being held by routers, et cetera.

What bandwidth (in KBit) does my ISP provide, when my torrent client shows an upstream of 300 Kbps?

kbps means Kilobit per second, therefore it's 300 KBit per second. You later changed your question to ask for 300 KB/s. That's different. 300 multiplied by 8 is 2400 KBit/s.

Your ISP might provide a much higher (upload) bandwidth. You can't just tell that from the upload speed you are currently getting, because it might be influenced by other factors as well (e.g. the bandwidth of the receiving end). Although it's a good approximation, it's most probably not the actual speed your ISP provides.

An ISP will always offer a nominal bandwidth that you can't practically reach.

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Could you provide the formula, how you computed the 12.5 MB per socond from the 100 MBit per second? And another one for the second example? –  Witek Jun 23 '11 at 10:56
    
@Witek Now I saw you had changed your question. Updated my answer. For the first one: What is 100 divided by 8? –  slhck Jun 23 '11 at 11:04

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