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What is a Megabyte?

(And why does my cousin complain when I send him 11 of them?)


The "B" in KB or MB is a byte. It is the basic unit of computer storage. Years ago, it was one column on an 80-column IBM card. Today it is a magnetic flyspeck on a hard drive or in memory. A byte can have a value from 0 - 255. In a text file, such as you make with Notepad, a byte is one character. Computers use a code for characters. You may have used a code where 1 = A , 2 = B and so on in grade school. Computers do the same thing for text, but they use 0 - 255. In most computers, upper-case A - Z are 065 to 090, lower-case a - z are 097 - 122 and the space character is 020. The other characters (@, #, %, 1, 2, 3, 4, TAB, etc.) have their codes too. The number code for some punctuation marks are lower than 065, some are higher than 122, and some fall in between the letters.

The more elaborate things you do, the more space a computer needs to store your thing. A text file will take up less space than the same words in a Word document with headings, bold, underlines and so on. You can see the difference if you copy all the words out of a Word document, open Notepad, paste them and save it as the same name but a different file type - text - then look at them both with your windows explorer. Word has to use bytes for the characters in your sentences, but it also has to use bytes to say "cast this next sentence in 14-point Times New Roman", and "Indent the next paragraph one tab space".

(You can see how big a file is in windows explorer. Right-click on "My Computer", click on "Explore". In the Explorer window, click on View -> Details. You should have a heading labeled "Size" over on the right.)

Bytes again. A KB is a kilobyte. Actually it is 1,024 bytes, but Kilo (1,000) is easier to remember. MB (Megabyte) is a thousand kilobytes. A Gigabyte (GB) is a thousand megabytes.


Computers measure pictures in pixels. The more pixels you have per square inch, the more detailed your picture will be. (In the old days, they talked about fine grain film and coarse grain film.) Scanners measure their performance in "dots per inch". Each dot is a pixel. Digital cameras measure their storage in pixels, usually mega-pixels.

It may take one, two or more bytes to define a single pixel. If your picture has just 256 colors in it, each byte can be a pixel. If it has more colors, you'll need more bytes to describe each pixel. File format will affect the size too. If you draw a solid red box 1,000 pixels by 500 pixels in your Paint program and save it as a .bmp, the file will tell your computer "I'm a red dot, I'm a red dot, I'm a red dot . . ." 500,000 times. If you save the same picture as a .gif or .jpg, the file will tell your computer "the next 500,000 dots are red." BMP files are much bigger than JPG or GIF files.

Pictures of documents (showing the light brown of aged paper and the spidery handwriting of 1846) can take up a megabyte or two, while a transcription of the same document might be just a kilo-byte or two. The letter "A" is just one byte, even if it is in 72 points. A picture of the letter "A", with curly-cues, can be a kilobyte all by itself.

Moving Pictures

You all know how a cartoon, video or movie works. There are 32 pictures, each slightly different from the one before, for each second of action. So, a one-second cartoon or movie could be 32 times as big as a plain picture. A one-minute animated clip could be 1,920 times as big as a single picture. File compression (a bit of magic too esoteric for this page) does miracles, but pictures with animation are still bigger than regular pictures.

File Size Etiquette

To review, plain text takes up the least amount of storage. Word documents take up more. Pictures take up much more. Moving pictures take up much, much more. Some people have cable modems and unlimited storage space. Some don't. If you send a cute little 5 MB animated file to your cousin, who has a dial-up connection and a 5MB limit in his E-mail, he won't be able to receive any more e-mail until he opens his mail and waits twenty minutes for it to lumber across the wire from the Internet to his PC. Everyone who sends him e-mail until then will get a message that their mail failed to reach him because his mailbox was full.

So, if you are going to send something big, ask first. You might also consider sending a transcription (of a will, tombstone, census page or love letter) instead of a picture of it. You can always send it again as a picture.

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This page updated: June 21, 2014