Hard Drive Capacity Always Less Than Stated - Why?

[rayinkrabi]

Hi all, can anyone explain why the capacity of a hard disc always shows up as less than stated, i.e my 1 TB Acer actually shows as 931 GB and my friends 2 TB shows 1.81 TB, not a big problem of course but am curious as to the reason.

[NanLaew]

The following is from the third item from a google search for "Hard Drive Capacity Always Less Than Stated"

Introduction

At some point in time, most users have come across a situation where a drive or disk being used run out of space even though there is thought to be plenty of space. A lot of times, this is the rude awakening for the consumer that the device they are attempting to store data on is not as large as it was advertised. In this article, we take a look at how manufacturers rate the capacity of storage devices such as hard drives, floppies, flash media and compact disks and their actual size.

Bits, Bytes and Prefixes.

All computer data is stored in a binary format as either a one or zero. Eight of these bits together for the most commonly referred to item in computing, the byte. The various amounts of storage capacity are referred by a prefix to represent a specific amount, similar to the metric prefixes. Since all computers are based on binary math, these prefixes represent base 2 amounts. Each level is an increment of 2 to the 10th power or 1,024. The common prefixes are as follows:

• Kilobyte (KB) = 1,024 Bytes
  
• MegaByte (MB) = 1,024 Kilobytes or 1,048,576 Bytes
  
• Gigabyte (GB) = 1,024 Megabytes or 1,073,741,824 Bytes
  
• Terabyte (TB) = 1,024 Gigabytes or 1,099,511,627,776 Bytes
  

This is very important information because when a computer operating system or program reports the available space on a drive, it is going to report the overall total of available bytes or reference them by one of the prefixes. So, an OS reporting a total space of 70.4 GB actually has around 75,591,424,409 Bytes of storage space.

Advertised vs. Actual

Since consumers don't think in base 2 mathematics, manufacturers decided to rate most drive capacities based on the standard base 10 numbers we are all familiar with. Therefore, one Megabyte equals one million bytes while one Gigabyte equals one billion bytes. This isn't too much of a problem with fairly small numbers such as a Kilobyte, but each level of increase in the prefix also increased the total discrepancy of the actual space compared to the advertised space.

Here is a quick reference to show the amount that the actual values differ compared to the advertised for each common referenced value:

• Megabyte Difference = 48,576 Bytes
  
• Gigabyte Difference = 73,741,824 Bytes
  
• Terabyte Difference = 99,511,627,776 Bytes
  

Based on this, for each Gigabyte that a drive manufacturer claims, they are over reporting the amount of disk space by 73,741,824 Bytes or roughly 70.3 MB of disk space. So, if a manufacturer advertises an 80 GB (80 billion bytes) hard drive, the actual disk space is around 74.5 GB of space, roughly 7% less than what they advertise.

Now, this isn't true for all the drives and storage media on the market. This is where consumers have to be careful. Most hard drives are reported based on the advertised values where a Gigabyte is one billion bytes. On the other hand, most flash media storage is based around the actual memory amounts. So a 512MB memory card has exactly 512 MB of data capacity, but this leads to the next area of reported space.

Formatted vs. Unformatted

In order for any type of storage device to be functional, there must be some method for the computer to know which bits stored on it relate to the specific files. This is where formatting of a drive comes in. The types of drive formats can vary depending on the computer but some of the more common ones are FAT16, FAT32 and NTFS. In each of these formatting schemes, a portion of the storage space is allocated so that the data on the drive can be catalogued enabling the computer or other device to properly read and write the data to the drive.

This means that when a drive is formatted, the functional storage space of the drive will be less than its unformatted capacity. The amount by which the space is reduced will vary depending upon the type of formatting used for the drive and also the amount and size of the various files on the system. Since it does vary, it is impossible for the manufacturers to quote the formatted size. This problem is most frequently encountered with flash media storage over larger capacity hard drives.

Conclusions

Hopefully this look at how both manufacturers and computer devices see storage devices has been helpful to explain why the values will differ. It is important when purchase a computer, hard drive or even flash memory to know how to read the specifications properly. Typically manufacturers will have a footnote in the device specifications to show how it is rated. This can help the consumer to make a much more informed decision.

[richardt1808]

Amazing what we can find when we Google, isn't it !

[MJCM]

NanLaew's post should be pinned !!

[bobl]

You've opened a huge can of worms with that question.

I'll give you the short answer first. Drive manufacturers use the (base 10), decimal form of kilo, mega, giga & tera - i.e. 1,000, 1e6, 1e9 & 1e12.

Computer people (when they're referring to anything digital) use the (base 2), binary form for kilo, mega, giga & tera i.e. 1,024 (2^10), 2^20, 2^30 & 2^40.

Drive manufacturers take advantage of this difference, and advertise their drives as "100Gb" meaning 100,000,000,000 bytes. Your operating system divides this by 2^30 for the purposes of displaying the size of the drive, and shows you 93Gb. Since file sizes have always been displayed in base 2, the figure displayed by the operating system (93Gb) IS the more appropriate figure to use (imho). Some operating systems have been tweaked to display the numbers "correctly" (I hesitate to use the word correctly) as 100Gb for a 100Gb drive - but it is a 100Gb drive in name only - even though your OS says it's a 100Gb drive you can still only physically fit 93Gb of (sane people's) data onto it.

Many years ago, television and screen manufacturers got a slap on the wrist for advertising their sizes in a misleading way (quoting the physical size of the screen, which wasn't necessarily the viewable area) and were disallowed for advertising thus. Drive manufacturers, for some reason, are still getting away with it. In fact, even solid state drive manufacturers jumped on the bandwagon. While a 1Gb stick of memory is 2^30 bytes, a 1Gb flash drive won't necessarily hold 2^30 bytes.

This brings us to the reason it's a can of worms. There is an extremely vocal minority of fruitcakes who start foaming at the mouth over this issue. They maintain that the prefixes kilo (etc) CANNOT be used to mean something different than 1000. HOW CAN YOU USE the same word (or prefix) to mean two different things, they will rant. Presumably they are worried that if they go to the butcher for a kilo of mince, the butcher will be confused whether they mean 1000 or 1024 grams and the universe will implode.

These pedants have also tried to force new prefixes on us - reserving "kilo" as a decimal prefix and replacing it with "kibi" as a binary prefix. So you will often find these nutcases lurking in forums like these, waiting to pounce on people like yourself asking a fairly innocent and relevant question, to try to persuade you to always use the term gibibytes when you mean 2^30 bytes and, accept the fact that gigabyte means 1e9 bytes.

You can probably guess my opinion of these people from my tone...

[astral]

As Nanlaew says, you have been short changed.............

[rayinkrabi]

Nice one Nan & Bob, good replies, thx..I guess I should have googled it myself first but fancied the idea of a forum chat on the issue.

As a side issue and a warning to anyone buying a new drive in an unsealed box (as I did in tesco last year)..I was searching for some family pics which I'd deleted error so decided to use a recovery software on all my drives..it picked up lots of porn on my brand new Acer!

[meand]

All I know is if these same marketing people were allowed time machines, we'd be buying a dozen donuts nowadays and getting 10. In fact, this whole discussion is starting to remind me of the famous amp that went to 11.

[impulse]

.it picked up lots of porn on my brand new Acer!

Lucky they didn't charge you extra...

[Daffy D]

As a side issue and a warning to anyone buying a new drive in an unsealed box (as I did in tesco last year)..I was searching for some family pics which I'd deleted error so decided to use a recovery software on all my drives..it picked up lots of porn on my brand new Acer!

Hard disk programs like http://www.hdsentinel.com/ (not free) will monitor your drives and warn of any potential problems and also show the "Power On" time of a drive so if you buy a "New" drive you can check how old it actually is.

[NanLaew]

Nice one Nan & Bob, good replies, thx..I guess I should have googled it myself first but fancied the idea of a forum chat on the issue.

As a side issue and a warning to anyone buying a new drive in an unsealed box (as I did in tesco last year)..I was searching for some family pics which I'd deleted error so decided to use a recovery software on all my drives..it picked up lots of porn on my brand new Acer!

I did recall there was talk of a new system architecture that would mean that users would get exactly what it said on the box with regard to HD sizes as well as other changes that would replace BIOS as we know it with something that would totally streamline how computers configure hardware. As far as I know HD's and BIOS are still the same as ever. I think it was a lack of universal agreement on these new standards that has delayed any implementation.

[meand]

That is the "tax" on hard drive space. Varies state to state. Right when you plug into the net that extra memory goes to the govt and is used to keep detailed files on you.

Edited June 7, 2012 by meand

[endure]

If you asked the average punter what base 2 meant they'd gaze at you with a blank expression on their face. That's why there's a difference.

[rayinkrabi]

If you asked the average punter what base 2 meant they'd gaze at you with a blank expression on their face. That's why there's a difference.

True but no excuse in this day n' age as virtually any query can be answered / explained online..good to chat about it tho :-)

"Since binary is a base-2 system, each digit represents an increasing power of 2, with the rightmost digit representing 2⁰, the next representing 2¹, then 2², and so on. To determine the decimal representation of a binary number simply take the sum of the products of the binary digits and the powers of 2 which they represent. For example, the binary number:

100101

is converted to decimal form by:

[(1) × 2⁵] + [(0) × 2⁴] + [(0) × 2³] + [(1) × 2²] + [(0) × 2¹] + [(1) × 2⁰] =

[1 × 32] + [0 × 16] + [0 × 8] + [1 × 4] + [0 × 2] + [1 × 1] = 37

To create higher numbers, additional digits are simply added to the left side of the binary representation." (thanks to Wikipedia)

[endure]

I know how it works. I did it in college. The fact remains that non-nerds who buy hard disks don't know what base 2 (or base 16 for that matter) is. They don't even know that they go through life counting in base 10.

[gk10002000]

And even if you sort out the capacity correctly, every hard drive comes with software on it that you don't see but you use. Controller software, etc. So if you buy a 500 gig drive, it probably already has several gigabytes taken up before you even put any specific files on it.