Why does an American football game take three hours for 60 minutes of play?

Because the clock stops constantly, and because a great deal of the broadcast is not play at all.

A game is four quarters of 15 minutes, 60 minutes of game clock. The elapsed time is typically around three hours and ten minutes. Studies analysing broadcasts have repeatedly found that the ball is actually in play for roughly 11 to 18 minutes of that.

The rest divides between several things.

The clock stops often. It stops on an incomplete pass, when a ball carrier goes out of bounds, on a change of possession, on a score, on penalties, and on timeouts. American football's clock rules are unusual in stopping for so many routine events.

The play clock between snaps is 40 seconds. Teams use most of it huddling, substituting and reading the defence. Since there are around 150 plays in a game, that alone accounts for a large share of the time.

Commercial breaks. A broadcast includes roughly 20 breaks, and they are scheduled at natural stoppages — after scores, at the end of quarters, on timeouts. Around an hour of a typical broadcast is advertising.

Replays and reviews add further stoppages.

The sport is structured around discrete plays with a reset between each, unlike football or basketball where play flows continuously. That structure suits strategy, substitution and broadcast advertising, and it is not an accident that the clock rules and the commercial format fit together so neatly.

The NFL has trialled changes to reduce dead time, including a running clock after certain plays.

College games run longer still, partly because the clock stops after every first down.