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The actual diameter size of the compact disc (CD) is actually completely arbitrary, but has an interesting history to it. The size of the disc and the hole in the center, as well as the length of playing time (originally 74 minutes, now commonly 80 minutes or as many as 82 minutes) was determined as part of the design of the disc specifications put together in a joint venture between Sony and Philips Electronics in the late 1970's and published in a series of three books in 1980. The diameters are specified in what is known as the "Red Book", due to its red cover.

The 120mm (5-inch U.S. & U.K.) size was chosen entirely because of the desire for playing length. An early decision was made that the CD should be small enough to allow Sony and Phillips to build portable players to follow-up Sony's extrodinarily popular "Walkman" portable tape cassette line. The original diameter, given the amount of data that could be stored, was specified at 115mm, which would hold just over an hour of audio.

Not verified as fact, this size was over-ridden, depending on what story you hear, by request of the wife of Akio Morita, the president of Sony, or Mr. Morita himself, or the then Sony vice-president, and extended the capacity to 74 minutes to accommodate a complete performance of Beethoven's "9th Symphony", although chief CD engineer/design Kees Immink of Philips denies this, and there is some likelihood this is an Urban Legend, since some performanceas of the 9th Symphony exceed 74 minutes. Nonetheless, to accomodate 74 minutes of music, a diameter of 120mm was required.

The 14mm diameter of the hole in the middle was also arbitrary. A CD player is not "linear speed", in other words, as the disc is read from the middle near the hub out towards the outer edge (opposite of a vinyl record album), the speed of the motor must varying to speed up as the disc is read further out from the hub. The designers took into account the best performing but least expensive variable speed motors to help come up with the size of the hub. It was also found that 14mm was the smallest size that could be used without causing the disc to "wobble" and become unstable and unreadable. So the size has absolutely nothing to do with the Dutch "dubbetje'" coin or other Urban Legends, but practical engineering.

Overall, the size was dictated by the desire to have a small enough format to be able to build portable players, and then by engineering constraints on how much audio data (length) was needed, and what variable-speed motors were capable of in the late 1970's when the spec was being developed.

As an interesting aside, the sales of CDs exceeded that of vinyl records in terms of units sold in 1988, only 6 years after CDs were made available in the U.S. (in 1982.)

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