Just How Big IS My Disk?
SHA Newsletter: Vol 1. No. 16

In the last newsletter, we talked about clock speed and how it reflects the performance of your computer. A 1 gigahertz (1 GHz) processor executes one billion instructions per second, if each instruction takes one clock cycle.

That's easy to follow: 1 Hz means one clock cycle per second, and 1GHz means 1 billion clock cycles per second. Makes sense, doesn't it? So if a byte is the basic unit of storage on a hard disk, what is a gigabyte? How about 1 billion bytes?

If you guessed that, you'd be wrong. Let's find out why. The answer lies in tradition and in the way computers count.

Nearly all computers in recent memory are binary machines. This means that they manipulate data as binary numbers. In the binary number system, the only valid digits are zero or one. In our decimal number system, we use powers of ten: ones, tens, hundreds, thousands, and so forth. In the binary number system, we use powers of two: ones, twos, fours, eights, and so forth. So computers interpret numbers differently than you and I do.

A Thousand Isn't a Thousand

When we use the prefix kilo-, we generally mean "one thousand of something". 1 kilohertz is 1000 Hertz, 1 kilometer is 1000 meters, and so forth. 1000 is a power of ten, so its easy to calculate. But 1000 isn't easy for a computer to work with, because 1000 isn't a power of two. The closest we can come to 1000 is the value 1024. That's a power of two, which is easy for a computer to figure.

By tradition, computer engineers call 1024 bytes a kilobyte. It's not exactly correct, but it's a convention that is commonly accepted. It came about because 1024 is the power of 2 that is closest to 1000. They use the abbreviation B for bytes. So

1KB = 1 kilobyte = 1024 bytes (2^10 bytes), closest to 1000
1MB = 1 megabyte = 1,048,576 bytes (2^20 bytes), closest to 1 million
1GB = 1 gigabyte = 1,073,741,824 bytes (2^30 bytes), closest to 1 billion

The numbers are hard for us to work with, but easy for computers to handle. This difference is because computers use binary numbers, but we use decimal numbers.

Why You Should Care

This affects how you buy disk drives, either at retail or inside a new computer. The disk vendor will often tell you that a gigabyte is 1 billion bytes. However, your computer will beg to differ. The computer understands 1 gigabyte to be 1,073,741,824 bytes -- 2 raised to the power of 20. That is the closest the computer can come to 1 billion. As a result of this, the computer needs 73,741,824 more bytes to make up a gigabyte. That error adds up.

Let's use a specific example. A new disk that holds 100 billion bytes is often advertised as being a 100 GB disk. But from the computer's point of view, this disk is too small by

100 X (1,073,741,824 - 1,000,000,000) = 7,374,182,400 bytes

Your computer will consider this new disk a 93GB disk, not 100GB.

On top of this, not all operating systems will be able to use all of the space on the disk. A fraction of the space is taken for bookkeeping needed to keep your files in order. And more space is frequently taken by computer makers who set up disk partitions for maintenance, diagnostics, and recovering the operating system. They do this instead of giving you software CD's and diagnostic disks in the box. Finally, differences in how disks are built (the "geometry" of the disk drive) some space might be unreachable in any event. That space will simply go to waste. As a result, anywhere from 10% to 20% of the disk space might never be available for you to use.

Why Do Makers Do This?

My guess is that using decimal instead of binary numbers to advertise disk size is intended to make the numbers easier for customers to understand. It also means that they can put bigger numbers in their ads. After all 100GB sounds better than 93GB.

Isn't that False Advertising?

Whether this is deceptive depends on your point of view. Most manufacturers will tell you, in the fine print, how many bytes are in a gigabyte. They haven't misled you. This is a bit of an arcane topic -- not the kind of thing you'd splash in an advertisement. Modern hard disks are so big that most users will never fill them up.

Taking space for recovery partitions, diagnostic programs, and the like is a slightly different issue. Vendors would do well to disclose how much space they reserve on your hard disk when you buy it. From what I have seen, they don't. So when you buy a new hard disk, there's no practical way to know how much free space you will actually have. Of course having ready access to backup software and diagnostic tools can be a great timesaver, especially if you use a laptop and take it on the road a lot.

As with so many computer topics, the more you know, the more confident you can be around your computer.

Please note: Any trademarks and trade names of others mentioned in this message are the property of their owners, and not Stoney Hill Associates, LLC. We respect the intellectual property of others. The information provided is believed to be reliable, but we cannot guarantee that the procedures and information given here will work correctly for your specific situation.

If you would like help with a computer or software problem you face, contact us. Send an email to request@stoneyhillassociates.com.

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