Bits, Bytes, Megabytes and Gigabytes


Bits, Bytes, Megabytes and Gigabytes
Mac Tip#52/24-Apr-2002

The last few Tips have looked at making backups and have often mentioned the storage capacity in Megabytes or Gigabytes of various discs such as floppies, CDs, DVDs and so on.

Many of my clients find all these units of measurement a bit bewildering, so here’s some help.

A computer is really just a collection of switches. Each switch can be on or off. This is represented by a 1 for On and a 0 (zero) for Off. This is called Binary as each switch has only two positions: on or off, 1 or 0. These are Binary Digits or Bits (B from binary and ITS from digits.) There are 2 possible combinations of on and off with 1 switch.

If you put 8 Bits (switches) in a row you get a Byte. With 8 switches there are 256 possible combinations. This is enough to represent all the letters of the alphabet, upper and lower case and numbers and symbols such as ! and ?as well.

A collection of 1024 Bytes is called a Kilobyte (Kb). 1024 Kb makes a Megabyte (Mb) — that’s 8,388,608 bits. 1024 Mb makes a Gigabyte (Gb) and 1024 Gb makes a Terabyte. That Terabyte is a massive 1,099,511,627,776 bytes!

When you want to measure things in a computer you will often use these units. Things you can measure with Kb, Mb, Gb etc are the amount of RAM, the capacity of the hard drive, the capacity of a floppy or a CD or a Zip disc.

You might measure how quickly information is moved by talking about bits per second. For example, a common modem speed these days is 56,000 bits per second, or 56 Kilobits per second. Note: that’s bits not bytes. There are 8 bits per byte. At top speed a 56K modem can transfer about 10 Megabytes every 25 minutes.

Firewire on the other hand, used to connect a movie camera or iPod music player to your recent Mac, can transfer data at up to 400 Megabits per second — that’s 400,000,000 bits per second. At top speed that means you could transfer about 100 megabytes every 2 seconds. That’s fast, and it needs to be for video.

So if you want to email a photo to someone check its size, and remember, if it’s 1 Megabyte or more it will take them at least 2 or 3 minutes per Megabyte to receive just your picture by email if they’ve got a good, fast modem. Do them a favour and prepare your photo specially for email so as to keep the file size small.

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