The size of a tunnel is measured most commonly by the diameter of the flight chamber. The larger the diameter, the more people can fly simultaneously. The most common size is 14 ft (4.3 m), which is required by pro skydivers and for competitions. However, recently smaller sized tunnels are becoming increasingly popular, as they are more efficient for training beginners and more affordable for the general public. But generally, wind tunnels from 2m up to 9.8m in diameter can be found.