HOW MUCH DATA WILL FIT ON A CD-ROM???

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I posed my CD-ROM capacity question to a number of people, receiving an astonishingly 
wide range of replies. The most complete replies were in agreement on several facts.

Recordable CDs come in 63 minute, 74 minute and 'high density' 80 minute sizes. The reason CDs are measured in minutes instead of bytes or megabytes is both historical and logical.   The history part is that CD technology was originally created for audio -- with tracks and discs measured in time rather than as data.  The logical part is the result of the fact that a byte is not always the same size -- different systems measure them differently, so saying a CD holds X megabytes can be very inaccurate. However most people who do recording come to know how many bytes or megabytes for their system will fit on each size CD.

Generally on PC-compatible systems, each second of a CD-ROM contains 75 sectors, each of which can hold 2048 bytes of Mode 1 user data.  

The capacities in bytes for the 'normal' CD sizes would be...
63 min x 60 sec x 75 sectors x 2048 = 580608000 = 567000 KBytes = 553 MBytes.
74 min x 60 sec x 75 sectors x 2048 = 681984000 = 666000 KBytes = 650 MBytes.

However, don't expect to fit files that total that exact capacity on a disc, since there is some overhead for the directory and partially used sectors. This varies with the number and size of files and the complexity of the directory structure.

For example, two seconds is lost at the start and end of the image for padding areas.  The first useable block starts at 0m 2s 16sectors, but the next couple of sectors will be used for path tables and directory structure.  You will aprox use one sector for every 40 files of directory and path table.

The warning on sector overheads can be very serious.  As everything is stored in 2K sectors, a one byte file will take 2K of storage space - thus one million one byte files (the client argues they should only total one meg, right?!) will not fit on a CD!!

For real digit heads, according to my calculations, an 'average' capacity estimate could be made by the following formula for a 74 minute disk... (681984000 bytes total)-(614400 bytes for padding)-(NUMBEROFFILES&DIRS x 1075 bytes).  And a very 'safe' and extreamly conservative capacity estimate could be made by the following formula for a 74 minute disk... (681984000 bytes total)-(614400 bytes for padding)-(NUMBEROFFILES&DIRS x 2200 bytes). Each of these formulas will yield the size, in bytes, that your data and program files may occupy on your CD-ROM, after allowances for the overhead described above.

Thanks to the many experts who offered their input.
Submitted by CIS 71412,3700