On a computer, a file's size is the amount of space it takes up on the hard drive. The most basic unit of file size, a byte, is a string of eight binary digits, or bits, in binary code. File size generally depends on the type of file; text documents are generally smaller than graphic-heavy documents or applications. Large files indicate more information stored in binary; the computer can also use decimal, or base 10, like humans use; or hexadecimal, base 16.
Information in a computer, at its most basic level, is represented by electrical impulses traveling along a circuit at a set voltage. Different sequences of these impulses represent different types of information. Computers acknowledge only two states of charge for a circuit: on, represented by a 1; or off, represented by a 0. In binary code, each digit, 0 or 1, is known as a bit, and eight of these bits make up a byte, the most basic unit of file size.
File size can vary depending on what type of file the computer is storing; for example, a text document in Notepad could be kilobytes, or thousands of bytes. An installation program could be millions of bytes, or megabytes. Although a kilobyte is commonly thought to be equal to 1,000 bytes and a megabyte is equal to 1,000 kilobytes, this is actually not the case. As computers use a base 2 system, all measurements must be powers of two; hence, the actual conversion factor is 1,024.
Computer hard drives store these bytes as a series of magnetic imprints on the surface of a disk and the disk head reads them to transmit information to the processor and output devices, similar to how a laser reads an optical disk drive and transmits data. Hard disks contain a set number of sectors, each with a subsection of clusters which in turn contain the magnetically-charged portions of the disk. The file size is the amount of physical space taken on the hard drive; file systems are organizational methods formatted onto the drive to determine how a hard drive stores and accesses its data.
A file size determines how much hard drive space is reserved by a given file. Even after the file is deleted, its information remains on the drive and can be recovered. Deleting a file only removes the 'read-only' aspect of the drive which prevents the drive from writing over the file.
There are two main classes of file systems in use: File Allocation Table (FAT) and New Technology File System (NTFS). FAT stores data in clusters of a set size and wraps the data over to the next cluster. These systems are useful for smaller drives, because the maximum partition size is 4 gigabytes. NTFS is the standard for every operating system since Windows NT. It is used for larger drives.