How big is a kilobyte? How big is a gigabyte? Relative to each other, how big are any two files? Here’s some information which can help you get your head around file sizes and comparing them.

File size is commonly measured in bytes. In a plain text file, typically each byte is equal to one letter. One page of text can hold a few thousand letters, so a plain text file that has one page of text might be about 4,000 bytes in size. Since 1,000 bytes equals 1 kilobyte, we could express that as 4 kilobytes (KB).

Plain text files can only contain letters, numbers and punctuation marks. In other words, they can’t contain any formatting. Formatting is a general term that includes such things as bold text, colored text or fonts (typefaces). The information needed to store formatting details in a file takes up space and thus increases a file’s size. If we took that one-page plain text file and converted it into a Microsoft Word document, underlined some text, converted some of the text to the Palatino font and other text to Times New Roman, this formatted one-page document might grow to be around 100 KB in size.

If you then placed a jpeg image into that one-page Word file, it’s size would increase further. The size of jpeg images can vary widely from tens of kilobytes to hundreds, thousands or even tens of thousands. Let’s imagine that we insert a jpeg that is about 2 inches x 3 inches in size and is a low resolution. Such a jpeg might be 100 KB in size. Now our Word file could be about 200 KB in size.

At this point, you hopefully remember that 1 letter is typically 1 byte in size and have a sense for how file size grows. You likely have a rough idea of how large a word processing file can be. Let’s move on.

In addition to kilo- there are other metric prefixes which you’ll commonly see in front of byte. Here’s a list of some of these prefixes and their size relationship to each other:

8 bits = 1 Byte = 1 letter

Bytes
1 kilobyte = 1000 bytes
1 megabyte  = 1000 kilobytes (KB)
1 gigabyte = 1000 megabytes (MB)
1 terabyte  = 1000 gigabytes (GB)
1 petabyte = 1000 terabytes (TB)
1 exabyte (EB) = 1000 petabytes (PB)

While file sizes can vary widely, here are some gross generalizations to further help get your head around the sizes of different types of files.

1 page of text might be around 50 KB
1 email message of two paragraphs might be 10 KB
1 jpeg photo could be 500 KB
60 minutes of spoken words could be 25 MB = 25,000 KB
a 10 minute long mp3 song could be  20 MB = 20,000 KB
1,000 minutes of an audiobook could be = 1.4 GB = 1,400 MB = 1,400,000 KB
2 hour long movie could be = 5 GB = 5,000 MB = 5,000,000 KB

These days a computer’s hard drive is typically measured in terms of gigabytes or possibly even terabytes. So how many Word files, jpeg photos, mp3 songs and movies could be stored on your hard drive?

If your hard drive’s storage capacity is 100 GB then you could have room for the following. Let’s assume that Mac OS X requires about 5 GB of storage space and that your applications (Word, Safari, iPhoto, etc) take another 20 GB then that leaves us with 75 GB to store our own files. Again, let me use some gross estimates and generalizations. This will let you see some rough comparisons and that photos and music files take up a lot more space than individual Word files or a typical email message.

5,000 photos could use 10 GB
5,000 mp3 songs could use 30 GB
5,000 email messages with their attachments could use 100 MB  or .1 GB
5,000 Word, Excel and PDF documents could use 600 MB or .6 GB

Added together, these photos, songs, email messages and other files are about 42 GB. Since we have 75 GB of available space on our 100 GB hard drive we know that we’d still have a healthy amount of free space even with this many items. Roughly 30 GB of space would still be free. To fill up this much space you might need another 15,000 photos or another 5,000 songs or another million or so email messages.

With this information you should have a general sense of how large files of different types should be and how their sizes compare. If somebody told you that you need to clean up your hard drive to free up storage space, you’d know that you should focus on files that take up 100 megabytes or gigabytes of space. You’d know not to worry about files which take up kilobytes of space since it would take thousands and thousands of these smaller files to equal the space of a larger file. Thus, when you need to clean up, you typically shouldn’t waste your time deleting old emails. Instead, review your pictures, music and video files to see if you can get rid of some of those.