Storage density is a measure of the information units that can be stored per unit area or length. Also known as bit density, this measure relates to semiconductor memories, magnetic, optomagnetic and optical storage media and is specified in bit/ inch( bpi) or bit/sqinch. Due to a constant development of the magnetizable or the photosensitive layer as well as the recording techniques, these values are subject to constant change. They apply to all storage media: semiconductor memories, floppy disks, hard disks, magnetic tapes, compact discs( CD) and DVDs.
- In the case of semiconductor memories, the maximum storage density for 0.1-micron technology is `10^13 bit/(cm)^3`. There are DDR2 Synchronous Dynamic RAMs( SDRAM) with `2^30` memory cells.
- In the early years around 1958, hard disks had storage densities of about `2 (kbit)/(inch)^2` (square inch); today's magnetic disks, on the other hand, have storage densities of about `5 (Gbit)/(inch)^2` up to `20 (Gbit)/(inch)^2`, with peak values of 500 Gbit/sqinch. With Perpendicular Magnetic Recording( PMR) the storage density was continuously increased and with Shingled Magnetic Recording( SMR) storage densities of `1 (Tbit)/(Inch)^2` are reached. The next generation is Heat Assisted Magnetic Recording( HAMR).
- For magnetic tapes, where the storage density in the 1970s was still around 2,000 bits/sqinch and by the end of the 1990s was already 1 Mbit/sqinch, storage densities of 100 Mbit/sqinch were achieved around 2010. A few years later, due to reduced magnetic particles and Advanced Metal Evaporated ( AME) technology, there were already magnetic tapes with a storage density of over 100 GB per square inch.
- Optical storage media have also undergone rapid development. The DVD has 27 times the storage capacity of a compact disc (CD) when used double-sided, which corresponds to a storage density of approximately 1 GB/sq. inch.
- With Ultra Density Optical( UDO), the Professional Disc for Data( PDD) and the Blu-Ray disc, optical storage media with the highest density, the storage density is between 7.4 GB/sq. inch and 30 GB/sq. inch.