The free-fall time in air was studied using the new dimensionless number GH (here mentioned as the Galilei-Huygens number to commemorate the achievements of Galileo Galilei and Christian Huygens in physics of free fall), a combination of the falling body mass and effective cross-section area, air density, and air drag coefficient. This number equals zero in vacuum and can be interpreted as the ratio of the air drag resistance force, calculated for the final velocity of the freefall in vacuum from the same height, to the gravity force. The free-fall time in air is shown to be a function of two parameters: the free-fall time in vacuum and dimensionless parameter GH . In most practical cases ( ), this function can be closely approximated as the product of the free-fall time in vacuum and a linear function of the parameter GH. To illustrate the accuracy and simplicity of the approximate equation, the free-fall time was calculated for various spherical bodies (ping-pong and tennis balls, hailstones, basketball, and track-and-field men’s shot) if dropped off the Leaning Tower of Pisa. The results obtained are straightforward and traceable and can be of educational value and interest for physics teachers and students.