The louver free area is the minimum area through which air can pass and it varies with louver size. Free area is determined by calculating the percentage of the total opening that is unobstructed by blades and frame. Louvers in this search are organized from the highest to lowest free area independent of model type. Example calculation: Let’s take for instance the case of a female aged 32 with a height of 175cm. Her estimated peak flow value would be 467.4 L/min. Or in the case of a male aged 27 with a height of 186 cm. His predicted peak flow value would be 639.948 L/min. But if he already has a measured peak flow of 568 L/min, the percentage is 88.76.