Soil grains when treated with dimethyldichlorosilane (DMDCS) create a thin soft coating layer. The presence of such a layer changes the grain surface wettability giving desirable hydraulic properties which has great potential in engineering practice. The thickness of such a layer has been identified to be an influencing parameter for the mechanical properties. But its estimation is not straightforward as the soil grain contains a relatively rough surface which is non-negligible compared to the thin soft coating. In this paper, we proposed a method that is based on the surface topography to estimate the soft coating thickness. A high-resolution microscope equipped with a white light interferometer was employed to examine the soil grain surface. It was found that the coating thickness is largely dependent on the surface topography of soil grains. The present method, applicable for particles with a very thin coating layer that is comparable to the surface roughness, such as for natural sands, could be used to further study the effect of coatings, of synthetic or natural origin, on the hydro-mechanical behaviour of coated soil samples. (C) 2020 Elsevier B.V. All rights reserved.