Writable CD Capacities

Standard CD-R discs are available in 63-, 74-, and 80-minute lengths, which respectively store about 550, 650, and 700 MB of data in ISO-9660 format. For example, a nominal 74-minute CD-R disc stores (74 min × 60 sec/min × 150 KB/sec) = 681,984,000 bytes, 666,000 KB, or 650.390625 MB. CD data formats use 2 KB sectors, which means that a standard 74-minute disc contains (666,000 KB / 2 KB) = 333,000 sectors. Most blanks actually contain more than the required number of sectors. For example, a nominal 74-minute blank may contain 344,250 sectors, which translates to 76.5 minutes. This “extra” space permits overburning (writing more data to a disc than its nominal capacity) if the CD writer and software both support overburning.

Warning

Think twice before you try overburning. If your CD writer doesn’t support it, attempting to overburn may physically damage the drive beyond repair and void the warranty. Many CD-ROM drives, even those that read ordinary CD-R discs, cannot read overburned CDs. Our advice about overburning is simple: unless there is no other way to do the job, don’t use it.

Remember that when you write data to a CD recorder, the 2 KB logical sectors are actually written to 2,352 byte physical sectors, with the remaining space used for ECC code. That means that a standard 74-minute CD-R blank actually stores (333,000 sectors × 2,352 bytes/sector) = 783,216,000 bytes or about 747 MB. Accordingly, you may find 74-minute discs with identical true capacities ...

Get PC Hardware in a Nutshell, 3rd Edition now with the O’Reilly learning platform.

O’Reilly members experience live online training, plus books, videos, and digital content from nearly 200 publishers.