Calculating data transfer rates
Introduction
The data transfer rate is the rate that a known amount of data is transferred in a given period of time. It is measured in terms of bits per second.
Units of data storage and simplifications
The rate can be calculated by dividing the amount of data transfered in bits by the time it took. Before we look at some examples, it is worth reminding ourselves about a few things!
1 Kilobyte (1 Kbyte) is 1 024 bytes exactly, or 210 bytes exactly, or about 1000 bytes, or about a thousand bytes.
1 Megabyte (1 Mbyte) is 1 048 576 bytes exactly, or 220 bytes exactly, or about 1000 000 bytes, or about a million bytes.
1 Gigabyte (1 Gbyte) is 1 073 741 824 bytes exactly, 230 bytes exactly, or about 1000 000 000 bytes, or about a thousand million bytes.
1 Terabyte (1 Tbyte) is 1 099 511 627 776 bytes exactly, 240 bytes exactly, or about 1000 000 000 000 bytes, or about a million million bytes.
So 15 Kbytes is about 15 thousand bytes. 128 Mbytes is about 128 million bytes. 20 Gbytes is about 20 thousand million bytes and so on. More often than not, you don't need to know the exact number of bytes, just an approximation! We will also assume a byte has 8 bits of data. Strictly speaking, it may have 8 bits but it can also be other values. However, it is generally considered to have 8 bits so that is the value we will always use.
Because a byte is a very small unit, we nearly always end up talking about Megabytes or Gigabytes, for example. We should always try to put a number in the most appropriate units. For example:
32 000 bytes is okay, but better is 32 Kbytes.
4 000 000 bytes is okay, and 4000 Kbytes is okay, but better is 4 Mbytes.
We should note, that these are approximations. 1 Mbyte is not exactly 1000000 bytes. It's actually 1048576 bytes (see above). It might be better to write the above as:
32000 bytes is okay, but better is 32 Kbytes.
4000000 bytes is okay, and 4 000 Kbytes is okay, but much better is 4 Mbytes.
The same idea applies to bits. We wouldn't talk about 50 000 000 bits when we can talk about 50 Mbits. We wouldn't talk about 30 000 bits when we can talk about 30 Kbits.
You also need to look out for how the units are written. You will see KB (and MB, GB, TB etc) and Kilobyte when talking about bytes of data. You will usually see kb or kilobits when talking about bits, and when talking about transfer rates, you will typically see bits per second written as bps, or kilobits per second written as kbps. To avoid any confusion, just write out the whole answer!
Baud rate
Another way of writing down the transfer rate is to use the baud rate. 1 baud = 1 bit per second so 56 kbaud is 56 000 bits per second
Example 1
If you transferred 50 MBytes of data in three minutes, what is the data transfer rate? 50 MBytes is the same as 50 000 000 bytes approximately. There are 8 bits per byte, so we transferred 400 000 000 bits. Three minutes is 3 * 60 seconds = 180 seconds. The data transfer rate is therefore given by 400 000 000 divided by 180 which equals 222 222 bits per second, which we will round to 220 000 bps. We should always use appropriate units as this is not an exact calculation we are doing. We can put 220,000 bits per second into kilobits per second by dividing 220,000 by 1000. This gives us a final answer of 220 kilo bits per second, or 220 kbits/s, or 220 kbps or 220 kb/s or 220 kbaud.
Example 2
An Internet connection is working at a data transfer rate of 768 kbps. How much data is downloaded in one minute?
768 kbps = 768 000 / 8 bytes per second = 96 000 bytes per second
In 60 seconds, you can therefore transfer 96 000 * 60 = 5 760 000 bytes per minute, or just under 6 Mbytes of data can be transferred in one minute.