suppose you have a 4 gb pendrive then why does the pendrive shows only 3.73 gb available free space to use
Answers
4gb pendrive shows 3.73gb because the rest is used by the software
No need to go crazy on this! You are really asking several questions here:
1.) Does 1 GB = 1000 MB or =1024? This is *entirely* up to the manufacturer, which usually means marketing will win out.
The strict scientific meaning of prefixes like kilo, mega, giga, milli, etc is as short forms for powers of 10. That means for scientists concerned with *prefixes* 1 Mega-blah = 1000 Kilo-blahs = 1,000,000 blahs. These are just short forms for 10^x, where x is an integer multiple of three (exception: we also have prefixes for 10 = deka, 100 = centa, 1/10 = deci, and 1/100 = centi).
When computer engineers started throwing around large numbers, it was more convenient in *some* circumstances to think in powers of 2 rather than powers of 10, so megabytes was use to mean 2^20 bytes which is only approximately 10^6. Some (but not all) engineers tried to make a careful distinction by using the term "MB" rather than "megabyte", but this has long since been lost.
Also, the power-of-two usage is not consistent, so CPU clocks and communication speeds are measured with powers of 10 (4 GHz = 4,000,000,000 or 4*10^9 cycles per second, *not* 2^32), while memory sizes (disk, RAM, ROM, etc) are measured with powers of 2.
2.) What does the "capacity" measurement provided by the manufacturer or by my operating system for a disk, flash drive, CD, etc really mean?
There are plenty of technical standards controlling how these devices are formatted, accessed, etc for interoperability, but there are no strict standards for how information is to be reported to the end user. This generally means each reporting party will provide numbers that are to their own advantage.
The O/S reports capacity after formatting to show how much data can be put on the device, and may report in KB or in some other unit, and may use the power-of-two rule or something else. My Windows system, for instance, reports formatted capacity of disks in GB, meaning 2^30 bytes. But other OSs and other devices may be different.
The manufacturer of a thumb drive will generally report *approximate* capacity because these devices include error correcting algorithms, driver code, spare blocks, etc that will reduce the available capacity by some significant fraction. Similarly, disk drive manufacturers want to tout the highest number they can manage, so your actual post-format capacity measurement will almost always be less.
3.) Why does your thumb drive claim 4GB, but show as 3.73GB in Vista, and show as 4,00,44,95,360 bytes?
Don't confuse what Microsoft shows you for reality. The drive maker is putting *approximately* 2^32 or 4*10^30 bytes unformatted capacity on your drive. These differ by about 7.3%, which is less than many other approximations used in your computer.How much RAM do you really have? How fast is your machine? What is the diagonal measurement of your screen? Or, worst of all, how long does your battery *really* last?