Hair density is the most important structural parameter contributing to insulation performance of mammalian pelage, and often is measured in ecophysiological, thermal biological, and evolutionary studies. To date, hair density has been measured using invasive methods on research objects; however, such methods remain challenging despite efforts to increase their ease of use. In this paper, we develop a new method to estimate hair density without skin sampling. We expressed hair density as the inverse of the number of hairs per unit area, that is, the surface area occupied by a single hair (A_h). This area could be further estimated by measuring distances between nearest neighboring hairs (L_n) and calculating the areas of triangles (A) defined by three randomly selected nearest neighboring hairs and representing half of A_h. Empirical tests using 11 skin samples from specimens of six small mammal species showed this to be a simple, lightly invasive, but accurate and widely applicable method.