According to meteorologist Jeff Haby, the formula to convert millibars to inches is y = (29.92 * x) / 1013.25. Here, "x" equals the current measurement in millibars, and "y" equals the equivalent measurement in inches of mercury.
Jeff Haby further says that the average air pressure at sea level is 29.92 inches of mercury, or 1013.25 millibars. Both types of measurements are commonly used in meteorological work, according to Aviation Weather, so it is useful to convert between the two. As millibars are a measure of force per unit area, measurements in millibars are commonly used to describe pressure in applications other than meteorology.