Real and advertised hard drive sizes - why you don't get all the space you pay for.

[ArcticFox]

I see a lot of threads asking why someone's new 80GB drive only shows up as 74.5GB, etc, and why a newly purchased 250GB drive only shows up in Windows as 137GB. Thanks to duffyone, I present the following, pulled from Western Digital's website:

Decimal vs. Binary:
For simplicity and consistency, hard drive manufacturers define a megabyte as 1,000,000 bytes and a gigabyte as 1,000,000,000 bytes. This is a decimal (base 10) measurement and is the industry standard. However, certain system BIOSs, FDISK and Windows define a megabyte as 1,048,576 bytes and a gigabyte as 1,073,741,824 bytes. Mac systems also use these values. These are binary (base 2) measurements.

To Determine Decimal Capacity:
A decimal capacity is determined by dividing the total number of bytes, by the number of bytes per gigabyte (1,000,000,000 using base 10).

To Determine Binary Capacity:
A binary capacity is determined by dividing the total number of bytes, by the number of bytes per gigabyte (1,073,741,824 using base 2).
This is why different utilities will report different capacities for the same drive. The number of bytes is the same, but a different number of bytes is used to make a megabyte and a gigabyte. This is similar to the difference between 0 degrees Celsius and 32 degrees Fahrenheit. It is the same temperature, but will be reported differently depending on the scale you are using.


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Various Drive Sizes and their Binary and Decimal Capacities


Drive Size in GB Approximate Total Bytes Decimal Capacity
(bytes/1,000,000,000)
Approximate Binary Capacity (bytes/1,073,724,841)
10 GB 10,000,000,000 10 GB 9.31 GB
20 GB 20,000,000,000 20 GB 18.63 GB
30 GB 30,000,000,000 30 GB 27.94 GB
36 GB 36,000,000,000 36 GB 33.53 GB
40 GB 40,000,000,000 40 GB 37.25 GB
60 GB 60,000,000,000 60 GB 55.88 GB
74 GB 74,000,000,000 74 GB 68.91 GB
80 GB 80,000,000,000 80 GB 74.51 GB
100 GB 100,000,000,000 100 GB 93.13 GB
120 GB 120,000,000,000 120 GB 111.76 GB
160 GB 160,000,000,000 160 GB 149.01 GB
180 GB 180,000,000,000 180 GB 167.64 GB
200 GB 200,000,000,000 200 GB 186.26 GB
250 GB 250,000,000,000 250 GB 232.83 GB
300 GB 300,000,000,000 300 GB 279.40 GB
320 GB 320,000,000,000 320 GB 298.02 GB

Now, for those running into the 137GB limit, or any other size limit for that matter, you either...
A) Don't have Service Pack 1 installed (assuming you run XP)
or
B) Your drive controller is old enough where it doesn't support large drives. I ran into this problem with an old eMachines that had a 466Mhz Celeron. I bought an 80GB ATA/133 Maxtor and then realized my board didn't support something "that large". I could have bought a PCI IDE controller however, but decided to build a new system instead.

I'd like for this to be a sticky, any mods up for it?
Also, if I made a mistake, or you would like to add to it, speak up!

[Peter M]

GB = decimal SI unit, 1,000,000,000 bytes
GiB = binary SI unit, 1,073,741,824 bytes

Nothing wrong with the advertized drive sizes - mucho wrong with all software that displays GiB and writes a "GB" suffix behind it.

On the other side of the story, I've yet to hear someone complain they bought a "1 GB" memory DIMM and got 1,074 GB ...

[PC TECH250]

"When a Byte is not a Byte" is an article that goes into further explaination of how to read
actual drive sizes found at: http://compreviews.about.com/od/stor...ualHDSizes.htm

[Peter M]

That article, like so many others, has the units of measurement wrong. See my previous post.

SI units are internationally standardized.

[PC TECH250]

That's where you have to correct the article's author. But for all too many users the simple
terms generally suffice without question. Different units of measurement obviously reflect a
totally different result when using the same multiplier.

[Peter M]

The point - and the "simple term" you're asking for is:

ADVERTIZED harddisk sizes are CORRECT.

... because "GB" is the decimal unit. Buyers of an "80 GB" drive may expect to get 80,000,000,000 bytes.

The linked article misses the point entirely, no point in trying to correct it. So does the section quoted from WD in this thread's first post.

Linux btw gets the GB vs. GiB thing right. Most other software, including BIOSes, OSes and utilities, doesn't.

So don't rant at the harddisk manufacturers, rather look to it that the software is being corrected to put the right units to the numbers.

[PC TECH250]

Actually the "simple" layman's terms can be read at: http://en.wikipedia.org/wiki/Gigabytes

[Peter M]

... which is just a more verbose version of what I said above. Thanks for proving my point.

[PC TECH250]

Glad to be of help!

[mujjuman]

very good thread. this should let people know why their drives show less space than advertised. i hate it when people complain and say something like "i bought this huge 200GB drive and i only get 186GB, whats wrong?"

for those of you getting 137GB for a drive that is much larger, its most probably because your motherboard doesnt support it, or your OS doesnt support it.