Since the early 2000s most consumer hard drive capacities are grouped in certain size classes measured in gigabytes. The exact capacity of a given drive is usually some number above or below the class designation. Although most manufacturers of hard disk drives and flash-memory disk devices define 1 gigabyte as 1,000,000,000 bytes, the computer operating systems used by most users usually calculate size in gigabytes by dividing the total capacity in bytes (whether it is disk capacity, file size, or system RAM) by 1,073,741,824. This distinction is a cause of confusion, as a hard disk with a manufacturer-rated capacity of 400 gigabytes may be reported by the operating system as only 372 GB large, depending on the type of report.
The difference between units based on SI and binary prefixes increases exponentially — in other words, the SI kilobyte value is nearly 98% of the kibibyte, but a megabyte is under 96% of a mebibyte, and a gigabyte is just over 93% of a gibibyte value. This means that an actual 500 GB hard disk drive appears as only "465 GB" large. As storage sizes increase and larger units are used, this difference will become more pronounced.
Some consumers feel short-changed when they discover the difference, and claim that manufacturers of drives and data transfer devices are using the decimal measurements in an intentionally misleading way to inflate their numbers. Several legal disputes have been waged over the confusion.
Due to its physical design, computer memory is addressed in base 2, thus, memory size can always be factored by a power of two (for instance 384 MiB = 3×2/27 bytes). It is thus convenient use binary units for non-disk memory devices at the hardware level (for example, in using DIMM memory boards). Most software application have no particular need to use memory in binary multiples and operation systems often use other granularities when allocating it. Other computer measurements, like storage hardware size, data transfer rates, clock speeds, operations per second, etc., do not have an inherent base, and are usually presented in decimal units.
To further complicate matters, flash memory chips, which are often used is disk-like devices, are organized in multiples of 2, like random access memory (RAM), but retail flash memory products have available capacities specified by multiples of 10. Removable flash storage products contain file systems that make the devices behave like hard disks instead of RAM, yet it is called 'memory'. In operating systems like Windows Vista, flash memory can indeed be treated like RAM.
The basis of the problem is that the correct definition of the SI units is not well know by the public,[citation needed] and some legal settlements include directions for manufacturers to use clearer information, e.g., by stating a hard disk's size in both GB and GiB units. However, JEDEC memory standards still use the IEEE 100 nomenclatures.
