Sliding of drops over solid surface is a ubiquitous process. Surprisingly, it is still impossible to predict the velocity and hence the position of drops after sliding down a few centimeters on a tilted surface. The reason is that numerous dissipation channels cause drop friction. How and how much dissipation channels contribute to the drop friction on different surfaces is, however, unclear. With tilted plate experiments we find that friction forces follow a simple, universal empirical equation. Only one material-specific parameter is necessary to describe drop motion. We term this dimensionless parameter “friction coefficient”. In contrast to static wetting of sessile drops, which is fully described by the advancing and receding contact angles, dynamic wetting is determined by this friction coefficient. In this sense, the friction coefficient is an independent parameter characterizing polymer surfaces. Sliding drops are omnipresent in many applications including spray coating, heat transfer, water management on vehicles, and microfluidics. Therefore, a universal quantitative law is essential to describe drop’s sliding behaviour. Furthermore, contact angles are established parameters to characterize solid surfaces. We anticipate that the friction coefficient will take a similar role when it comes to dynamics of wetting. Liquid drops sliding on tilted surfaces are an everyday phenomenon and are important for many industrial applications. Still, it is impossible to predict drop’s sliding velocity. To make a step forward in quantitative understanding, we measured the velocity U, width w, length, advancing θα, and receding contact angle θr of liquid drops sliding down inclined flat surfaces made of different materials. We find the friction force acting on sliding drops of polar and non-polar liquids with viscosities (η) ranging from 10-3 to 1 Pa∙s can empirically be described by Ff(U)=F0+μwηU for a velocity range up to 0.7 m/s. The dimensionless friction coefficient μ defined here varies between 20 to 200. It is an independent material parameter, specific for a liquid/surface combination. While static wetting is fully described by θα and θr, for dynamic wetting the friction coefficient is additionally necessary.