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Few hard drive topics generate as much confusion as how to figure the data capacity of the disk. But it's really not complicated; it's a matter of terminology.

The Numbers Game

The basic issue is that what a hard drive manufacturer calls "a gigabyte" is not the same as what your computer calls "a gigabyte". So you buy a hard drive, and when you get it home your computer says it's less than you bought. And once it's formatted, it looks even smaller! What's up with that???

Okay. We're going to clear up the confusion for you.

Let's take a few disk sizes as an example, we'll use "80 GB", "120 GB", "160 GB", and "200 GB" (GB=gigabyte). The raw (unformatted) and the usable (formatted) storage capacities of these "Sizes" are:


Raw (Unformatted) Capacity

Formatted Capacity *






GB (approx.*)







74.4 GB







111.7 GB







149.0 GB







186.2 GB


At the

On Your Computer


*Formatted capacities are approximate; different operating systems and formatting options may result in slightly more or less.

Size vs. Raw Capacity

The first thing you notice is that the "Size" in GB (first column) doesn't match the "Raw Capacity" in GB (second column) — the Size looks like somebody took the "Raw Capacity" in Bytes (fifth column), and divided by a billion. Yup, that's what they did.

This is because hard drive manufacturers (including our suppliers) define a GB as "1 billion bytes" (1000 x 1000 x 1000), whereas computer software (MacOS and Windows) usually defines a GB as 1024 x 1024 x 1024 (1024 is a "kilobyte" to the software).

This difference (1000 vs. 1024) makes the disk look like it's bigger on the store shelf vs. in the computer. Even computer makers like Dell and Apple play the same game: on the computer case it says " 80GB Hard Drive", while the computer software says it's only 74.5GB!

But the disk didn't shrink after you bought it — it's just how the GB units are defined in one place vs. the other. Hang tight, keep reading, and we'll explain.

Raw vs. Formatted Capacity

The second thing you notice is that the "Formatted Capacity" in GB is slightly less than the "Raw Capacity" in GB. This is because when the computer formats the disk, it has to use some of the space for the "file system" structure, which organizes your data. The filesystem is like a filing cabinet — it organizes the documents that are in it, and it takes up a little more space than the documents would by themselves.

The Macintosh HFS+ filesystem typically uses about 35-50MB (around 0.5% of an 80GB hard disk). The Windows NTFS filesystem typically uses about 60-90MB (around 1% of an 80GB hard disk). So the filesystem subtracts a tiny bit from the overall available disk space. But the bigger factor is that difference between 1024 and 1000.

(There's yet another factor, the overhead loss on each file. A given amount of data, stored as lots of little files, will take up more of your diskspace than the same amount of data stored as a few big files. But we won't get into that here; 1000 vs. 1024 is enough to deal with.)

It's True: Size DOES Matter

The reason computers use capacities based on 1024 is that data storage devices (disks and RAM memory) come in sizes that are multiples of 1024 bytes. The value 1024 (2 to the 10th power) was given the name "kilobyte" as a shorthand long ago. 1024 x 1024 became "megabyte", and 1024 x 1024 x 1024 became "gigabyte" — for the purpose of describing data storage capacities.

But the disk manufacturers use sizes based on 1000 (1GB = 1 billion bytes) because they think it's simpler. And — not coincidentally — their disk size sounds bigger that way. A "gigabyte" based on a billion is smaller than one based on 1024x1024x1024, so the same disk has more of the (smaller) gigabytes. And on the box label, "more gigabytes" = "bigger", even though it's really the same size.

Funny thing is, years ago, the manufacturers used disk capacity based on 1024, like the computers do. But then one manufacturer started advertising their disk size based on 1000 — and suddenly they could say their disk was a few percent bigger than before. It wasn't any bigger, it just seemed that way. Naturally, all the other manufacturers followed suit, lest they be at a marketing disadvantage because their disk didn't look as big as the other guy's. It was actually pretty silly. But you know how worried some guys are about their disk size.

Who's Right?

In a sense, everybody is "right". The answer depends on what you're talking about:

  • For everything except data storage, kilo=thousand, mega=million, giga=billion.
  • For data storage on your computer, kilo=1024, mega=1024x1024, giga=1024x1024x1024.

Except that disk drive manufacturers use the "non-storage" numbers to describe their storage products, because a) they think it's simpler, and b) it makes their disk look bigger.

Okay, that wasn't too bad.

Hopefully now you're cleared up on why the computer says you have "less than you bought". But if not, or if you want some background to help clarify thing, read on.

Powers of 10

Normal everyday decimal counting is based on the powers of 10:

10, 100, 1000, 10000, 100000, 1000000, and so on...

Kilo — That's a Thousand, Right?

Right — in common usage, "kilo" means 1000, "mega" means 1,000,000, and "giga" means 1,000,000,000. In the US these are referred to as "thousand", "million", and "billion". These are decimal (powers-of-10) numbers, just like your fingers, assuming you've got all ten of 'em:

  • kilo = 1,000
  • mega = 1,000,000
  • giga = 1,000,000,000

Most Things Are Counted in Decimal

Decimal quantities are used for counting nearly everything. Decimal counting is also used for computers, for instance CPU clock speed, which is specified in MHz (1 megahertz = 1,000,000 cycles/second) or GHz (1 gigahertz = 1,000,000,000 cycles/second). So a "2 GHz CPU" is running at 2,000,000,000 cycles per second.

BUT Computers Count Data Storage in Binary

Computer data storage quantities are not based on decimal. They are based on binary, because that's how computer storage is actually made and used, same as the computations done by the CPU are done in binary ("ones and zeros").

Binary numbers are the "powers-of-2":

2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, and so on...

So when you have a device to store computer data, like RAM or hard disk, its size will be a power-of-2 binary number, not a power-of-10 decimal number.

A Couple of Kilos of Bananas

The binary power-of-2 numbers include 1024 — which by coincidence is close to 1000. Long ago for convenience, computer folks started saying "kilo" to mean 1024 — but only when they were talking about data storage. This was fine as long as everybody knew the context was data storage. A kilobyte (KB) of data was 1024 bytes. A megabyte (MB) was (1024 times 1024) = 1,048,576 bytes.

Not surprisingly, computer software gets written with this definition. "kilo" = 1024, not 1000, bytes.

As storage sizes grew, this shorthand had to be extended also. But what would a "megabyte" (MB) of data storage be?

1,000,000 (a million)?   or 1,048,576 (1024 x 1024)?

The computer folks said "1 MB is 1,048,576 bytes", because literally that's how many bytes are there. Everybody was okay with that for a while. But some years back the storage marketers and sales people went bananas:

"Nobody understands that number. To us, mega means a million, so we're gonna say that a megabyte is a million bytes. Besides, that will make our disk seem bigger, 'cause it'll have more megabytes."

Uh, oh. There it is, the source of the confusion — and the source of worry about where those missing bytes went. But that's not the end of it — because these days we have huge amounts storage. So what's a "gigabyte" (GB) of data storage gonna be?

1,000,000,000 (a billion)?   or 1,073,741,824 (1024 x 1024 x 1024)?

You guessed it. The computer folks said "1 GB is 1,073,741,824 bytes", because literally that's how many bytes are there. But storage marketers and sales people said:

"NO! A gigabyte is a billion bytes. It makes our disk look even bigger."

And so hard disks are sold on the basis that a GB is a billion bytes, even though that means the size "in the store" is different from the size "in the computer" by over 7%. On a 200GB hard drive, that difference is almost 15GB! That would be a decent sized hard drive all by itself.

How Come Glyph Sells Drives Using the Decimal (Sales/Marketing) Sizes?

We have to — it's the nature of the business. Every hard disk on the market today, whether external or internal, is described in the store in terms of the decimal size. But once it's in the computer, the software describes it in binary size. Oh well. We can't change the drive manufacturers' preference for decimal, and we can't change the computer software manufacturer's preference for binary.

What we can do is try to help you understand what's going on.

So don't worry because your "120GB" drive only appears to have 111GB of formatted space on it. You can thank the computer engineers and the hard drive sales/marketing folks for setting up a brain-teaser!

We're just doing our best to make it all make sense. We hope this helped.

The Folks at Glyph

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