Frost growth decreases the energy performance of air-source heat pump systems during heating periods. When frost forms on the fins of the outdoor evaporators it obstructs the air flow and increases the thermal resistance of the heat transfer process. Several studies investigated hydrophilic and hydrophobic surface coatings applied to heat exchanger fin structures for inhibiting frost growth. However, these studies are mainly experimental and the empirical correlations are often limited to specific coatings, geometries, and laboratory test conditions. In addition, most of the data focuses on frost growth after the first layer of frost is already established while surface coatings primarily affect the formation of this first layer of frost deposited on the fins.Frost formation consists of three primary phases: condensation and droplet growth, crystal growth, and growth of a homogeneous, porous frost layer. Surface contact angle mainly affects the droplet growth and crystal growth phases, which most existing frost models do not completely account for. In order to capture the characteristics of the initial frost layer growth, this paper proposes a physics-based frost model that combines models for droplet condensation and growth, crystal growth, and frost layer growth phases into one new comprehensive model. The model was validated against experimental data for cold flat plates from four independent sources in the literature, matching experimental trends for all data sets except one, which had natural convection conditions. The present model was used to study the effects of plate surface temperature and humid air relative humidity on frost thickness and density characteristics.