Film execs are always obsessed with box office numbers, but this time of year, the key phrase is not “opening weekend” but “Christmas multiples.”

While bizzers claim box office is a 52-week proposition, the math does not remain steadfast year-round.

During summer — the other most lucrative B.O. period — a film’s multiple (i.e., its eventual domestic cume) is usually two or three times its opening weekend (of course there are exceptions: “Inception” and “Toy Story 3” both had higher multiples).

But the multiple varies more radically at year’s end, which creates more optimism and skepticism within the industry.

“The thing about this play period is how the Christmas multiples take over,” said Disney distribution head Chuck Viane. Though some “Tron’s” $44 million bow as underwhelming, Viane said, “It’s at the number where it needs to be.”

Though films like “Tron” and Warner Bros.’ “Yogi Bear” may start off somewhat slower than studios had hoped (Disney would have breathed easier with a launch north of $50 million for “Tron”), studios count on high multiples this time of year for both family and adult-skewing titles.

Sometimes studios cite “playability” as a stalling tactic, hoping a film will take off after a slow opening. But especially during the holiday season, high multiples are an important factor in parsing the B.O. equation. That realization has increased this season due to “Avatar.” The Fox pic was the gold standard, launching this time last year with $77 million and going on to cume 10 times its opening weekend figure.

And while most films don’t get to a tenfold multiple, Warners would be happy if “Yogi Bear” had a multiple similar to that of Fox’s first “Alvin and the Chipmunks,” which ended with nearly five times its $44 million opening, cuming $217 million in 2007. This year, Disney’s “Tangled,” which bowed on Thanksgiving weekend with $49 million, is headed for a hefty multiple, having already grossed $128 million.

Toons and other family pics often display the longest legs. After a comparatively modest $43 million March bow, “How to Train Your Dragon” played well into summer; cume is $218 million.

Pre-holiday dates are not traditionally huge moviegoing weekends, with auds often too distracted with shopping and travelling. But after things settle down, holiday-timed expansions can pay off big, since the over-25 crowd sometimes waits until after Christmas to catch up on new releases. Unlike adult summer pics with multiples of two to three, holiday titles can sometimes see multiples of five or six.

So “Tron” has the potential to keep broadening beyond its fanboy appeal, while awards-fodder titles like Paramount-Relativity Media’s “The Fighter” and Fox Searchlight’s “Black Swan” seem to be heading into fruitful territory.

“This is definitely the kind of movie that plays six to seven times multiples,” Par vice chair Rob Moore said, referring to “The Fighter.”

Revised opening figures Monday came in higher for “Tron” at $44 million, while “Yogi Bear” was down slightly with $16.4 million.