Only 1 infant may be seated in the lap of each ticketed accompanying adult and the infant must be included in the reservation. Additional infants under 2 years old must be ticketed and occupy an infant safety seat or in a separate aircraft seat.

The answer depends on whether you are talking about aircraft capacity or weight and balance. For the purposes of airline seating capacity, a lap child (an infant under age 2) does not count toward the number of "fixed seats" available on the plane, allowing an airline to carry more humans than they have seats for. However, for Weight and Balance calculations, every lap child absolutely counts as a passenger. Pilots and dispatchers must include the "average weight" of an infant in their pre-flight calculations to ensure the plane is within its safe center-of-gravity limits. Furthermore, in terms of safety and emergency equipment, a lap child counts as a soul on board; for instance, the number of oxygen masks in a row (usually 4 masks for 3 seats) determines where a lap child can sit, as there must be a mask for every "person," including those on a lap. While they fly for free (or a small tax) on domestic flights, you must still register them with the airline so they are accounted for on the official "Passenger Manifest" for security and emergency purposes.