Two-phase flow in single mini tubes and parallel mini channels find a lot of practical applications. Although it is possible to measure pressure drop and void fraction, using methods like visualization, quantifying the mass split of the two phases is a hard task. This is especially the case when dealing with parallel channels where there is a need to correct the maldistribution between the channels, which is a common issue. In the present study, a semi analytical model based on the separated flow model is proposed to estimate the mass split between air and water, one of the most commonly handled situations, for slug flow regime in horizontal mini channels, with diameters ranging from 1.0 to 2.0 mm, where the competing magnitudes of inertia and surface forces makes the flow distribution complicated.