How Automatic Calculators Determine a Quarterback Rating

The Math Behind the Rating

Close up of football center preparing to snap football
Tetra Images - Erik Isakson / Getty Images

Before there were online quarterback calculators that would compute a quarterback's rating, the NFL used a quarterback's statistics and simple calculations to determine the rating.

To learn how to manually calculate a quarterback's rating, you will need the same: the quarterback's current stats and a little basic arithmetic.

Passer Rating Not Quarterback Rating

The NFL rates passers for statistical purposes against a fixed performance standard based on statistical achievements of all qualified professional passers since 1960.

It is important to note that the system is used to rate all passers, not only quarterbacks. The statistics do not reflect a player's leadership, play-calling and other intangible factors that go into making a successful professional quarterback

History of the Rating System

The current rating system was adopted by the NFL in 1973. It replaced one that rated passers in relation to their position in a total group based on various criteria. The new system removed inequities that existed in the former methods and provided a means of comparing passing performances from one season to the next.

Before the development of the current passer rating system in football, the NFL had difficulties determining a passing leader. In the mid-1930s, it was the quarterback with the most passing yardage. From 1938 to 1940, it was the quarterback with the highest completion percentage. In 1941, a system was created that ranked the league's quarterbacks relative to their peers' performance.

Until 1973, the criteria used to determine a passing leader changed several times, but the ranking systems used made it impossible to determine a quarterback's rank until all the other quarterbacks were done playing that week or to compare quarterback performances across multiple seasons.

The Math Behind the Rating

There are four categories that are used as a basis for compiling a rating: the percentage of completions per attempt, the average yards gained per attempt, the percentage of touchdown passes per attempt and the percentage of interceptions per attempt.

The four categories must be calculated first, and then, combined, those categories make up the passer rating. 

Let's use an example of  Steve Young's record-setting season in 1994 with the San Francisco 49ers when he completed 324 of 461 passes for 3,969 yards, 35 touchdowns, and 10 interceptions.

Percentage of Completions324 of 461 is 70.28 percent. Subtract 30 from the completion percentage (40.28) and multiply the result by 0.05. The result is a point rating of 2.014. 
Note: If the result is less than zero (Comp. Pct. less than 30.0), award zero points. If the results are greater than 2.375 (Comp. Pct. greater than 77.5), award 2.375.
Average Yards Gained Per Attempt3,969 yards divided by 461 attempts is 8.61. Subtract three yards from yards-per-attempt (5.61) and multiply the result by 0.25. The result is 1.403. 
Note: If the result is less than zero (yards per attempt less than 3.0), award zero points. If the result is greater than 2.375 (yards per attempt greater than 12.5), award 2.375 points.
Percentage of Touchdown Passes

Percentage of Touchdown Passes — 35 touchdowns in 461 attempts is 7.59 percent. Multiply the touchdown percentage by 0.2. The result is 1.518. 
Note: If the result is greater than 2.375 (touchdown percentage greater than 11.875), award 2.375.

Percentage of Interceptions

Percentage of Interceptions — 10 interceptions in 461 attempts is 2.17 percent. Multiply the interception percentage by 0.25 (0.542) and subtract the number from 2.375. The result is 1.833. 
Note: If the result is less than zero (interception percentage greater than 9.5), award zero points.

The sum of the four steps is (2.014 + 1.403 + 1.518 + 1.833) 6.768. The sum is then divided by six (1.128) and multiplied by 100. In this case, the result is 112.8.  That was Steve Young's stellar rating in 1994.

Given this formula, 158.3 is the maximum possible rating, which is considered a perfect passing rating.