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prasad's avatar

Why don't we get full space that is specified on drives?

Asked by prasad (3841points) October 29th, 2009

My external drive is of 160 GB, but it shows only 148 GB of available space. And also, some space is lost in hard drives and ram. Why is it so?

During formatting, while creating partitions, they leave out some space. Is this got something to do?

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16 Answers

sandystrachan's avatar

I asked this a while back , can’t remember what i was told . But would you buy a 148gig hard drive was something along the lines of what i was told

virtualist's avatar

You may also have a ‘hidden’ partition which is part of the ‘restore’ function for your operating system.

patg7590's avatar


sandystrachan's avatar

It does piss me off tho when you buy something stated as 80Gig ps3 and dont get the full space, they should add extra space to cover the OS and whatnot so we get the use of a full HDD.

Response moderated
skorned's avatar

Because some space goes into overheads. More technically, the hard drive needs to maintain an index of all the files it contains, and this index takes up space which is then not available to you to store files. The Hard drive company isn’t cheating you, its just some space gets used up for overheads…

skorned's avatar

@patg7590 , that conversion actually usually causes you to have more space than expected. Since the hard drive might say 1 GB, which is 1000 MB, while in reality it will be 1024 MB. But yes, this conversion actually has caused some confusion in the latest Mac OS X, because people expect different amounts of space.

@virtualist is right, and the system may also use some hard drive space as RAM in the form of a pagefile and virtual memory, although it is unlikely to use an external hard drive for that.

@sandy, it is kinda hard for them to predict how much real space you’ll have availalble, coz the overhead u can’t use depends on a number of things like the partition system you have (NTFS or FAT etc)...

virtualist's avatar

@skorned ... no , e.g. on my Dell, there is a Dell installed partition of 10GB for the ‘restore’ function.

Agreed, that should not be true if @prasad has just purchased an external drive.

patg7590's avatar

my post is the longest thus I win.

MissAnthrope's avatar

I don’t see it as cheating you. You buy the 160 gig drive, that’s what you get. It’s just that other things take up space on the drive. The manufacturer has a vague idea of how you’ll use it, but uses vary.

It is a bummer, I agree, but that’s just how stuff works. :P

El_Cadejo's avatar

i dont get why they dont make HDs a bit larger than they say they are so that way when it comes done to it after conversion and what not, its actually 160GB or whatever.

ragingloli's avatar

no when on the package it says 1GB, you get 1000Mbyte. Windows however uses the 1024 scale and divides the 1000Mbyte by 1024, so you only get 0,9765625 GB

grumpyfish's avatar

The drive overhead is a very small portion of this. It really is the conversion between base 10 and base 2 as @patg7590 said, which is why you get 148G reported on a 160G drive.

@skorned You don’t get more space than advertised, you get exactly the same amount as advertised, just in a different base.

patg7590's avatar

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.


Various Drive Sizes and their Binary and Decimal Capacities

Drive Size in GB Approximate Total Bytes Decimal Capacity
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


I in no way meant to charade that I wrote all of that myself

prasad's avatar

So, we can’t do much about it; we’ll have to manage within the available space.

@uberbatman I’d like if they make such drives. That is, for 160 GB drive, they can actually manufacture 160 GB + whatever overheads they require. That way people like me, who are not much computer geeks, won’t get confused.

Thanks @All!

jerv's avatar

I reclaimed a chunk of my 160GB drive by wiping out the recovery partition (after making a backup image, of course). That took me from ~142GB to ~158GB.

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