|Riffenburgh, Beau. The Official NFL Encyclopedia. New York, 1997: 540.||"It's weight was set at 14 to 15 ounces in 1912."||0.43–0.40 kg|
|NFL. Make the Right Call: The NFL's Own interpretations and guidelines plus 100s of official rulings on game situations. Chicago. 1999: 10.||"short circumference, 21 to 21¼ inches; weight, 14 to 15 ounces."||0.43–0.40 kg|
|Arlott, John, The Oxford Companion to World Sports and Games. 1975: 319.||"with an oval leather ball, inflated to 12½ to 13½ p.s.i and weighing 14–15 ounces."||0.43–0.40 kg|
|Poole, S. Fun Facts. 24 December 1996.||"A football weighs between 14 and 15 ounces."||0.43–0.40 kg|
Textbooks and teachers often present information in a way that suggests that what they say is fact. In the case of the mass of a football, every source I found had the same numerical value for the mass of a football. According to the Oxford Companion to World Sports and Games, the official National Football League Encyclopedia, the NFL "make the right call" book and the football "fun facts"sheet the mass of a football must be between 14 and 15 ounces (0.40 to 0.43 kg) for it to be used in a NFL game.
Football evolved from rugby which evolved from soccer. When the game of football (American) was established all that was official was the shape of the ball, it looked like "an elongated pumpkin"which was later called a prolate spheroid. In 1912 the weight of a football was established to be 14 to 15 ounces (0.40 to 0.43 kg)
Before each professional football game all twenty-four balls (if the game is played in a domed stadium) or thirty-six balls (if the game is played in an open-air stadium) must be inspected by the referee. This is done by sight, making sure that each ball is a Wilson ball signed by the NFL commissioner. The ball size and shape are also tested by placing each ball in a mold which each referee has.
For the most part football is the one sport I have never been able to understand.
Kendel Bell -- 2000