Mac sales plummeted in the fourth quarter of 2012, falling 22% from the same period the year before, with computer sales accounting for a record low percentage of Apple's total revenue of $54.5 billion.

The decline in Mac sales was substantially greater than the global personal computer industry average, which research firm IDC recently pegged at minus 6.4% year-over-year.

Apple CEO Tim Cook attributed the downturn in Mac fortunes to a slew of factors, including short supplies of the new iMac and cannibalization of its computers by the iPad.

Apple sold slightly less than 4.1 million Macs during the three months ending Dec. 31, down from both the 5.2 million it sold in the fourth quarter of 2011 and the 4.9 million sold in 2012's third quarter.

Last October, Apple had warned that Mac sales would suffer in the fourth quarter because of a late arrival of new iMacs introduced that month. Today, those desktop machines remain in short supply, with shipping delays ranging from two to three weeks for the 21.5-in. model, Apple's most popular, to three to four weeks for the pricier 27-in. system.

Apple did not start selling the smaller iMac until Nov. 30, over a month after its unveiling in a major Mac refresh that also introduced a new 13-in. MacBook Pro laptop with a higher-resolution Retina screen.

The 27-in. iMac did not go on sale until mid-December, and has been in even shorter supply than its sibling.

Because Apple stopped selling the previous generation iMacs in late October, it essentially left the desktop line swinging in the wind during most of the quarter.

Definitive sales numbers for the iMac in the quarter were unavailable, as Apple broke with precedent and lumped both desktops and notebooks into a single "Mac" figure rather than listing them separately.

But in a question-and-answer session during Wednesday's earning call, CEO Cook said that iMac sales were down 700,000 units over the same period last year because of the supply snafu. Although he didn't explain what caused the shortages -- speculation has highlighted the display and the case, both of which rely on new manufacturing methods -- Cook chided analysts for not remembering his October warning.

"I said that we would have significant constraints on iMacs, but I recognize to some folks this may be a surprise," said Cook.

The 700,000 in lost unit sales cost Apple nearly a billion dollars in revenue, using the average selling price (ASP) of the company's desktops during 2012's third quarter.

Yet the missing iMac sales couldn't account for all of the Mac shortfall, something Cook himself acknowledged.

He cited the fact that 2011's fourth quarter included 14 weeks, while 2012's had thirteen, and again admitted that Macs, like PCs, have been affected by the trend of consumers diverting dollars from personal computers to other devices -- tablets in particular.

Mac sales dropped 22% in the fourth quarter of 2012 to just 4.1M systems, beset by slumping demand and shortages of the new iMac. (Data: Apple.)

"We have always said there is some cannibalization there. I am sure there was some cannibalization of Macs there," Cook said of sales going to iPads rather than Macs. He also pointed to the weak demand for PCs in general as a reason for the Mac decline.

Ezra Gottheil, an analyst with Technology Business Research, agreed on all counts. "The Mac is affected by the same things as [Windows] PCs," Gottheil said. "Everyone is suffering from the personal computer blues."

Some customers, Gottheil said, bought an iPad rather than a Mac to do light-weight work like browsing the Web and email, and so are keeping their current Macs longer than they would have in the absence of other choices.

Gottheil, however, thought that sales would pick up once Apple solves its iMac supply problems, essentially pushing sales that would otherwise have occurred in the fourth quarter of 2012 into 2013. "I don't see these people saying, 'Well, since Apple doesn't have a desktop all-in-one, I'll try Windows,'" Gottheil said.

Cook said that although Apple would "significantly increase the supply" of iMacs during the quarter that ends March 31, he would not guarantee that the company could reach a supply-demand equilibrium in the period.

Because of the decrease in Mac sales year-over-year, along with record sales of the iPhone and iPad, the Mac's contribution to Apple's revenue reached a new record low of 10.1% last quarter. The previous low for the Mac was the first quarter of 2012, when computers accounted for 12.9% of Apple's revenue.

Not surprisingly, Cook tried to make lemonade out of the lemons delivered by the poor Mac sales figures.

"I see cannibalization as a huge opportunity for us," said Cook on Wednesday. "One, our base philosophy is to never fear cannibalization. If we do, somebody else will just cannibalize it, and so we never fear it. We know that iPad will cannibalize some Macs [so] that doesn't worry us."

Cook also called the situation "the mother of all opportunities," referring to the much larger number of Windows PCs sold than Macs, and the idea that if the company's iPads cannibalize personal computers, they're much more likely to eat into Windows PC sales than into the firm's own systems.

"I have said for two or three, actually three years now, I believe, that I believe the tablet market will be larger than the PC market at some point," said Cook. "And I still believe that. And you can see by the growth in tablets and the pressure on PCs that those lines are beginning to converge."

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed. His email address is [email protected].

See more by Gregg Keizer on

Read more about macintosh in Computerworld's Macintosh Topic Center.

See also:

Samsung tops smartphone and overall handsets market in Q4

Microsoft revenue up, aided by WIndows unit sales, though profit declines

iPhone, iPad sales up, Macs fall as Apple sees record sales