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# Drive Size – Advertised vs. Actual

Wednesday, July 2nd, 2014 by | Filed Under: Hardware & Peripherals

Ron writes:

How about giving the full story about storage drive size? Mention that HD makers report in powers of 10 rather than powers of 2. And do the math to show how much we are getting “ripped off” now that we are getting into TB and higher local storage.

When we were dealing with KB and MB the difference was no big deal. But now that we are regularly dealing with GB and even multiple TB we are being significantly mislead.

Frankly, I think it is time for computing to take the step to report in powers of 10. We have enough “spare” computing power to convert powers of 2 to 10′s. It only makes sense since “people” think in powers of 10.

Whether or not people are getting ripped off is up for them to decide, but I can break down why storage drives are labeled this way.

When you buy a hard drive, it will be listed as, for example,  100 GB, 500 GB, or 1 TB, a nice even number. However, when you get it home and look at the actual size available, you will notice a difference. A terabyte drive, for instance, will show 931 GB. Here’s why:

When the hard drive is manufactured and advertised, 1 terabyte = 1,000,000,000,000 bytes. BUT  1 TB  actually equals 1,024 gigabytes or 1,099,511,627,776 bytes. However, we are used to working with the decimal system, rather than a binary system, so it is common place to start with the decimal equivalent to the amount.

The difference stems from the idea that 10³ (or “kilo”) is close to 210(binary form), but not exactly equal. Interestingly, someone did create terms for the exact sizes, including kibi, mebi, and gibi. However, these have never been successfully adopted. This results in a difference in the amount of actual space on the hard drive. The manufacturers are providing a full terabyte of drive in our conventional terms, but when converted to actual usage, you do lose out on some of the size.

– Audra

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### 5 Responses to “Drive Size – Advertised vs. Actual”

1. Jim B says:

I see getting ‘extra’ not ripped off. Compare these numbers:
1,000,000,000,000 bytes = 1 terabyte
1,099,511,627,776 bytes = 1,024 gigabytes or 1TB
the way I see it, I’ve gotten 99,511,627,776 for free. In other words, 1% more for free, or 99Gb, or 99 thousand 1-gig memroy stick equivalents for free

2. Jim B says:

correction:
I see getting ‘extra’ not ripped off. Compare these numbers:
1,000,000,000,000 bytes = 1 terabyte
1,099,511,627,776 bytes = 1,024 gigabytes or 1TB
the way I see it, I’ve gotten 99,511,627,776 for free. In other words, 1% more for free, or 99Gb, or 99 thousand 1-Meg memroy stick equivalents for free

3. Jim B says:

correction correction:
I see getting ‘extra’ not ripped off. Compare these numbers:
1,000,000,000,000 bytes = 1 terabyte
1,099,511,627,776 bytes = 1,024 gigabytes or 1TB
the way I see it, I’ve gotten 99,511,627,776 for free. In other words, 1% more for free, or 99Gb, or 99 1-gig memroy stick equivalents for free

4. Bill Leach says:

Good explanation. An important concept though is that you can not normally store a terabytes worth of data on a terabyte drive. The drive manufacture rate the drive in terms of the total capacity of the drive. Various file systems have overhead which must fit within that total capacity.

In addition most file system will only allocate storage space on the drive in sectors and or blocks.

Even a 1 bite item for storage will take a minimum of 1 sector or block. If a sector that means that the single byte stored is taking the space required to store 512 byte (or more for newer operating systems). Most file system allocate a minimum of a block which is usually some multiple of sectors.

In addition to all of that, the file(s) containing directory (or folder if you prefer) information also take room on the drive.

And if all of that was not enough, there are usually multiple copies of critical disk structure information stored on the drive (for recovery from what otherwise might be a ‘disaster’), and spare blocks that can be allocated to the file system itself should bad blocks be detected.

The difference between that ways that drive size is expressed are pretty minor compared to the effects of all the above!

• audra says:

Bill,

Excellent point. I did consider adding something about how some sectors are reserved for the file system and bad sectors, but it did not seem to find a place in this article. Thanks for adding to the article.

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