*Washington Post* sports columnist Michael Wilbon says he can't understand the formula the National Football League uses to rate quarterbacks. Explainer to the rescue! How does the NFL calculate its passer ratings?

The NFL's system uses four metrics: completion percentage, yards per attempt, percentage of touchdowns thrown per attempt, and percentage of interceptions per attempt. The four factors are weighted equally.

A score between zero and 2.375 is calculated for each metric. A score of 1.0 is supposed to be average. A completely average quarterback would complete 50 percent of his passes, average 7 yards per attempt, throw 5 percent of his passes for touchdowns, and throw an interception 5.5 percent of the time.

Here's how each metric is calculated (remember that no score can be lower than zero or higher than 2.375, no matter how well or how poorly the QB throws):

**1. Completion percentage:** Subtract 30 from the percentage of passes that are thrown for completions, then multiply by .05.

**2. Yards per attempt:** Subtract yards per passing attempt by three, then multiply by .25.

**3. Touchdown percentage:** Multiply the percentage of touchdown passes per passing attempt by .2.

**4. Interception percentage:** Multiply the percentage of interceptions per passing attempt by .25, then subtract that number from 2.375.

The scores for each category are added together. That sum is divided by six and multiplied by 100, which converts it into a rating on a scale from zero to 158.3. A putatively average QB would receive a rating of 66.7 (1 + 1 + 1 + 1 = 4, and 4/6 * 100 = 66.7).

If you don't have the patience for the math, you can enter the numbers into this ratings calculator.

No matter how well a QB plays, his score can never exceed 158.3, even if every pass is a 99-yard touchdown strike. By the same token, a QB's rating can never drop below zero, even if every pass he throws gets picked off.

The NFL emphasizes that its ratings system assesses how well a quarterback passes, not how well a quarterback plays. It doesn't account for rushing yardage or rushing touchdowns, nor does it account for variables such as leadership, play calling, or how good the team's wide receivers or offensive linemen are. But as this article points out, the NFL's ratings system is also limited in how well it measures passing. The system doesn't value third-down conversions, avoidance of sacks, or late-game heroics. (It actually penalizes a quarterback who throws an incompletion to avoid a sack.) John Elway's career passer rating is 79.9, good for 33^{rd} overall, below lesser contemporary QBs such as Neil Lomax and Danny White.

One other problem with the NFL's system: In the modern NFL, the average rating is higher than 66.7. That's because the scores are calculated based on what the NFL thought would be average statistics for a quarterback in 1973, when the system was introduced. But in 1978, the NFL changed its rules to open up the passing game, by prohibiting defenders from bumping receivers downfield and by allowing offensive linemen to extend their arms to pass-block. As a result, modern-day QBs dominate the list of all-time passers. (Though old Cleveland Browns great Otto Graham still ranks third. Click here to view the list kept by the Pro Football Hall of Fame. You'll need Adobe Acrobat to view it.) Last year's average passer rating was 78.1, roughly equivalent to the career rating for Johnny Unitas.

*Explainer thanks the Pro Football Hall of Fame, this NFL.com page, and this article.*