# Hard Drive Size on the box vs actual: are there different measuring conventions?

On my new HD box it says 1GB = 1 billion bytes.  I thought there were 1024k in a kb, 1024kb in a mb, and 1024 mb in a gb.  So a gb should be 1,073,741,824 bytes not 1,000,000,000.  Which is right?  Are there two ways of measuring the same thing?  Maybe this explains why my 250gb HD only shows as 243gb capacity in disk management.  Wonder why they are allowed to exaggerate the size so much;  7gb is quite a significant amount of "rounding".
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Commented:
found the perfect explaination:

First, Windows detects drive space as a power of 2 function. So, to Windows, 1KB = 2^10 (2 to the power of 10) = 1,024 bytes.

Then:

1MB = 2^20 = 1,048,576 bytes
1GB = 2^30 = 1,073,741,824 bytes
1TB = 2^40 = 1,099,511,627,776 bytes

etc.....

However, this makes drive look a little small in Windows. Hard drive manufacturers don't like that. They want the biggest number on the box as possible. So, they use the conventional base-10 method of counting. To them, 1KB = 1,000 bytes.

Then:

1MB = 1,000,000 bytes
1GB = 1,000,000,000 bytes
1TB = 1,000,000,000,000 bytes

Now let's do some math. Your hard drive says it has 120GB. It's detected as having exactly 120,023,252,992 bytes. Now, that's base-10. Let's convert it.

120,023,252,992 / (divided by) 1,073,741,824 (Window's # for 1GB).

We get a nice round 111.78GB.

The second drive is 122,904,969,216 / 1,073,741,824, and we get 114.464GB

Source:
http://computing.net/windowsxp/wwwboard/forum/89813.html
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EMEA Server EngineerCommented:
Hi mark876543,

You're right, a read GB is 1,073,741,824 bytes, but harddisk manufactors always advertize with a GB being 1,000,000,000 bytes.
So in this case, you bought a 250GB disk, which is in fact only a 243GB disk.

Greetings,

LucF
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Commented:
> why they are allowed to exaggerate the size so much

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Commented:

Our brains are so used to the decimal system that 1024 is the easiest and closest we can get to 1000 by using binary.

According to the actual definition stretching back to the metric system, A Kilobyte should equal 1000 bytes.  In computers, it's more effecient to let this equal 1024.  Otherwise we'd be wasting space.

In that sense 250 Gigabytes litteraly means 250 billion bytes, but computers count Gigabytes in increments of 1024, instead of 1000.
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