Helping people with computers... one answer at a time.
As it turns out, common terms used to measure disk space can be ambiguous. As a result, a megabyte to you might not be the same as a megabyte to your computer.
I just purchased a new computer with a 1TB (terabyte) hard drive. Windows is telling me it has 976,760,000KB of disk space. I don't get it. Shouldn't it be 1,000,000,000KB? Is there anything I can do to recover the other space? If I go to Windows Explorer and click on the hard drive symbol, it will show me the space on the hard drive. It said the total available space is 931 GB! What happened to the other 69 gigabytes?
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They were never there.
Believe it or not, there's no real, agreed upon definition of what a gigabytes is.
Let me clarify: there are definitions. Plural. And which one gets used depends on ... well, it depends on how you think.
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Thinking like a computer
The fundamental problem stems from the fact that computers think in powers of two (1, 2, 4, 8, 16 and so on) while people think in terms of powers of 10 (1, 10, 100, 1000 and so on).
So to a computer, a kilobyte is 1024 bytes (two to the 10th power). A megabyte is 1,048,576 (1024 times 1024, or two to the 20th). And a gigabyte is 1,073,741,824 (1024 times 1024 times 1024, or two to the 30th).
To a computer, a terabyte would be 1,099,511,627,776 bytes (1024 * 1024 * 1024 * 1024, or two to the 40th).
Thinking like a human
Naturally, people don't think like that.
We think of a kilobyte as "around" 1,000 bytes. Close. Close enough for most conversations. But when we think of a megabyte as "around" 1,000,000 bytes, and a gigabyte as "around" 1,000,000,000 bytes, or a terabyte as "around" 1,000,000,000,000 bytes, we're getting less and less accurate at each step along the way.
To a computer, 1,000,000,000,000 bytes is really ... 931 gigabytes.
And sure enough, my own "one terabyte" drive shows exactly that:
Thinking like a salesman
If you're going to sell a hard drive that holds 1,000,000,000,000 bytes, and you have the opportunity to call it one terabyte (in human terms) or 931 gigabytes (in computer terms), which would you choose?
"1TB" sure sounds like you're getting a bunch more for your money than "931GB", doesn't it?
So, you're getting a terabyte drive, alright. Except that you're not, if you're thinking like a computer.
Thinking like a linguist
Of course, the problem that we've seen is that the terms kilobyte, megabyte, and so on are all ambiguous1. The same term can mean different things depending on context or even just depending on a whim.
Enter the "kibibyte" (and the mebibyte, gibibyte, tebibyte and so on).
These terms are defined to mean only the "think like a computer" value. A kibibyte is exactly 1024 bytes, always. A mebibyte is 1,048,576, always. A gibiyte is 1,073,741,824 bytes, always.
And a tebibyte is exactly 1,099,511,627,776 bytes. Always.
These terms even have a slightly different abbreviation: while KB refers to a kilobyte, KiB refers to a kibibyte.
And in contexts where these terms are used, "kilo", "mega" and so on are all assumed to take on their correct "human" values, based on powers of 10.
Which means that the salesmen weren't even lying when they sold you a terabyte drive that has only 931GiB on it. If anything, it's Windows (and other operating systems) failure to use the new more accurate terms that leads to the confusion.
Because 931GB is not 931GiB.
And, to be fair, I can't blame the OS designers for not wanting to throw even more typically unheard-of computer terms at us.
Thinking like an operating system
Even though what I just described is the major reason for the discrepancy in total disk capacity, there are other things that contribute to even less space actually being available or free.
On a completely empty hard disk, the operating system will reserve some amount of space for its own use. For example, the top level directory structure, even if empty, takes some space. Security information, the recycle bin, and other information is placed on the hard disk before you ever create your first file. Exactly how much will vary, depending on how the disk is formatted.
And of course, if this is your system drive, the operating system may also place certain hidden files that can get quite large, including your swap and hibernation files.
(This is an update to an article originally published March 13, 2005.)
1: Technically, they are not at all ambiguous. The prefixes kilo, mega, and so on actually specifically mean factors based on powers of 10, not powers of two. In other words, they are specifically human-based numbers. A kilometer is unambiguously exactly 1,000 meters, for example. It's the computer industry that's caused the confusion by using the terms inaccurately for numbers based on powers of two. For years.
Article C2304 - October 16, 2012 « »
Leo A. Notenboom has been playing with computers since he
was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed.
After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers
to common computer and technical questions. More about Leo.
October 20, 2012 2:59 PM
Leo gives a nice explanation, but does it really matter anyway? Unless you are going to write one file that is exactly 500 Mb, you are never going to use all 500 Mb of a 500 Mb drive, even if you had all 524,288,000 bytes.
Have you ever noticed the file sizes when you click on the properties of a file? This 12,130 byte file on my computer is actually using 12,288 bytes (Leo's got another article about why this is, so I won't go into it here).
There is always some lost space. There always will be. It's a fact we have to live with. I once bought a 512 Mb hard drive for more than the cost of the external 3 Tb drive I just bought. Hard drive prices have come down a lot, so I just don't see the point of quibbling over where the extra bytes are.
That however, does not excuse the manufacturers from being less than ethical (in my books). They're not driving down the highway; they're not selling me apples. They're selling computer equipment and should use the computer terminology, not the metric terminology.
Now I wish the
October 20, 2012 6:10 PM
I thought that all hard drives have the technology to swap bad sectors in and out of operation, therefore reducing read/write errors. Could that not exploain the "missing bytes"?
October 21, 2012 4:59 AM
Someone sued the monitor manufactures, resulting in 15 inch monitor(13.9 viewable). So Lets sue the hard drive manufactures. The worst offenders are the flash drives. My first drive 2GB(IBM) was actually bigger than what I paid for. I believe the lost capacity is due to the format sectors taking up space in addition . My 320 GB drive is showing up as 292GB. I am missing 21 GB. Also NTFS may eat up more space than FAT, but that is another question here.
October 29, 2012 3:30 AM
I was to go for 500 GB portable Hard Disk till the time I could generate fund for a Tablet or at least for a Laptop. After reading this Leo's letter, enligtened to go for 1TB unit to have the required capacity. Thanks for the timely guidance.
April 12, 2013 8:34 PM
By z way did you put in mind that the operating system uses some of the hard drives space? Like 20-40gb.