The NBA Playoffs Are Months Away. But Math Says Only Three Teams Can Win the Title.

The “40-20 rule,” attributed to legendary Chicago Bulls and Los Angeles Lakers coach Phil Jackson, is a formula for assessing a team’s championship bona fides before the postseason has even begun. The test is simple. To be considered a true contender, a team must win 40 games before it loses 20.

In Jackson’s case, the rule never failed. All 11 championship teams he coached in Chicago and Los Angeles met the mark. (And the 2004 Lakers, who lost in the Finals in an upset against the Pistons, did not.) Even the 1970s Knicks, with whom Jackson won two titles as a player, passed the test in each case, going 40-11 in the 1969-70 season and 40-12 in 1972-73.

This year, as the calendar approaches the 60-game point, only three teams meet the criteria. They are the defending-champion Boston Celtics, who have shaken off a midseason swoon to win 10 of their last 11 games; the Oklahoma City Thunder, with the NBA’s best defense; and the Cleveland Cavaliers, who overhauled their team with new head coach Kenny Atkinson’s turbocharged attack.

Casual NBA fans often gripe that the regular season, with an 82-game slate that allows more than half the league’s teams into the playoffs, doesn’t matter anymore. But looked at through the lens of the “40-20” rule, history suggests something different: that regular-season dominance is the easiest way to forecast playoff success. Since the league expanded its playoffs to the current 16-team format, in the 1983-84 season, 40 teams have been crowned NBA champions, excluding the lockout-shortened 1998-99 year.

In all, 36 of those—90%—met the “40-20” criteria.

If you’re a fan of one of the other 27 teams, don’t write off the season just yet, though. Because like any good rule, this one has some exceptions. Among the teams that didn’t hit the mark in the regular season and still went on to lift the Larry O’Brien trophy in the summer are the 1995 Houston Rockets and 2004 Detroit Pistons.

Both teams traded for a star player in the middle of the season. In Houston’s case, it was a deal for guard Clyde Drexler. In Detroit’s case, forward Rasheed Wallace rode to the rescue.

If those scenarios sound a little familiar, it’s because there’s a team that sits at 35-21 this season and still harbors championship hopes. The Lakers are an altogether different team than the one started the season, having traded this month for superstar guard Luka Doncic. They have won 13 of their last 16 games.

Now they just have one thing left to do: beat history.

Write to Robert O’Connell at robert.oconnell@wsj.com