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I have 4GB Pendrive, but I observed that it actually shows only 3.7GB as a total free memory when the pendrive is completely blank. In the product specification, it is always specified as 4GB, 8GB etc. but actually it is less than that. Anybody please explain me the reason behind it?

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marked as duplicate by Ƭᴇcʜιᴇ007, Tog, random Feb 15 at 5:06

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

    
Here is an article detailing this as well. –  Alex McKenzie Feb 13 at 17:14
    
Short version: 1) The manufacturers fiddle around with the math, 2) The file system itself needs space to keep track of files. –  Ƭᴇcʜιᴇ007 Feb 13 at 17:36
    
After all, whats the reason that manufacturer design it in decimal metric unit instead of binary? Why can't it use binary metric only since 'bit' is the fundamental unit of memory and not a decimal. –  user1612078 Feb 13 at 17:39

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In addition to the size difference the of a KByte in how the memory is accounted for (1 KB=1000 Bytes vs. 1 KB=1024 Bytes) that JSanchez mentioned, there is a more significant factor.

The apparent "loss" that you observed is about 7.75% ( 4.00 - ( 4.00 * 0.0775 ) ). The effect of the different sized KBytes accounts for a difference of about 2.5%.

The other factor is in accounting for the difference between "formatted" vs. "unformatted" capacity.

When a storage device is formatted, a portion of the storage space is reserved for the "File Allocation Table", and the "Root Directory". The free space of the device (in your case, 3.7GB) is the amount of available storage space that is left after these spaces are reserved.

The amount of space that is taken up by the "File Allocation Table", and the "Root Directory", depends on the "Cluster size" (actually depends on the number of clusters), and the formatting type such as FAT, FAT32, NTFS, etc. In your case, this accounts for the remaining difference of about 5.25%.

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This is due to the way manufacturers advertise space, and the way the OS calculates this space.

Manufacturers use "decimal multiples" (1000 bytes) whereas, in Windows case, the OS uses "binary multiples" (1024 bytes). So, if the manufacturer says a device has 100MB of storage, that would be 100,000,000 / 1000 = 100,000 bytes. Windows, on the other hand, would see this as 100,000,000 / 1024 = 97,656 bytes. I believe OS X uses "decimal multiples" Not so sure about Linux.

See the HDD page at Wikipedia, specially the Units section for a more in-depth explanation. Also found more information regarding which OSes use decimal vs. binary multiples on Wikipedia.

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I observed it on ubuntu as well, it shows the same. Does it depends on the filesystem format with which the usb drive is formatted? Generally it is formatted with FAT32(the one compatible with windows & linux)? So if I use OS X filesystem as a format for usb, in that case it should show me the size in decimal multiples. Isn't it? –  user1612078 Feb 13 at 17:26
    
Found this bit of information regarding which multiple some OSes use: en.wikipedia.org/wiki/Binary_prefix#Operating_systems –  JSanchez Feb 13 at 17:52
    
After all, whats the reason that manufacturer design it in decimal metric unit instead of binary? Why can't it use binary metric only since 'bit' is the fundamental unit of memory and not a decimal. –  user1612078 Feb 13 at 18:13

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