Not too long ago, I wrote an article about the true storage capacity of a DVD disc. I think that Blu-ray discs also deserve a post on this topic, because it's important to get the information out there as more people start adopting Blu-ray media for their storage needs.
There are two main types of recordable Blu-ray discs: single-layer and dual-layer. Single-layer discs have a rated capacity of 25 gigabytes while dual-layer discs have a capacity of 50 gigabytes. As we already know, the actual capacity of a recordable disc varies depending on the scale used to measure it.
A disc with a capacity of 25 gigabytes is equivalent to 23.28 gibibytes, depending on whether you are using a base-2 system or a base-10 system. Additionally, some of that space is reserved as overhead for the file system as well as error-correction information. This results in a true recordable capacity of 23.28 gibibytes instead of the 25GB stated on the package.
The same situation is true of dual-layer Blu-ray discs. These are normally rated with a 50GB capacity, but in practice you can record about 46.57GB of data on a disc. Again, this is due to differences in the way the disc capacity is measured plus a small percentage of space reserved for file system information.
It remains to be seen whether this trend will continue with newer types of optical media such as Blu-ray XL.
The reason that Blu-ray discs are able to store such vast quantities of information is because of the density with which the information is recorded on to the disc. Data is stored in a linear path around the disc using a series of pits and lands, much like a vinyl record. The difference is, the pits and lands on a recordable Blu-ray disc are packed together at a microscopic level compared to a record. Even CD and DVD discs seem spacious by comparison.
A Blu-ray disc uses a recording laser with a size of just 405 nanometers, which allows for many more tracks of data to be recorded closer together. By comparison, DVDs use a larger 650 nanometer laser and CDs use a larger still laser that is 780 nanometers in size.
From a data storage perspective, the amount of data that a Blu-ray disc can hold is mind-boggling. For the price of a blank disc, there's really no cheaper way to store your information.