Apple shares gained more than two per cent on Monday, valuing the company at $623 billion US and good enough to make it the most expensive public company of all time.

Stock in Cupertino, Calif.-based Apple closed at $665.15 Monday, up $17.04 or 2.63 per cent on the day.

The Big Five

The five largest public companies in the world are:

  1. Apple ($623 billion US)
  2. Exxon ($405 billion)
  3. Microsoft ($257 billion)
  4. PetroChina ($233 billion)
  5. IBM ($229 billion)

That per-share price values the company at $622 billion overall. It makes Apple worth more than the previous title holder, Microsoft, which peaked at $618.9 billion the day before New Year's Eve 1999, near the height of the technology bubble.

Microsoft is currently worth $257 billion.

It is important to note the title does not factor in inflation. If America's inflation rate is taken into account, in today's dollars Microsoft was worth about $850 billion on the eve of the new millenium.

A case could also be made that state-controlled Chinese oil company PetroChina was briefly more valuable than either company, as its 2007 IPO on the Shanghai stock exchange briefly valued the company at more than $1 trillion. But that only happened because China's stringent rules on foreign ownership drove the hard-to-get shares to exorbitant highs.

The company also lists on the New York Stock Exchange, where they are currently worth $233 billion and have never eclipsed the $500 billion level.

Apple's stock has hit new highs recently because of optimism around what is believed to be the impending launch of the iPhone 5, and possibly a smaller, cheaper iPad.

Apple has been the most valuable company in the world since November, when it passed oil giant Exxon. Apple is currently worth 53 per cent more than Exxon, but hadn't touched the old Microsoft record until Monday.

With files from The Associated Press