Liquid drops sliding on tilted surfaces is an everyday phenomenon and is important for many industrial applications. Still, it is impossible to predict the drop’s sliding velocity. To make a step forward in quantitative understanding, we measured the velocity (U), contact width (w), contact length (L), advancing (θa
), 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
+ βwηU for a velocity range up to 0.7 ms−1
. The dimensionless friction coefficient (β) defined here varies from 20 to 200. It is a material parameter, specific for a liquid/surface combination. While static wetting is fully described by θa
, for dynamic wetting the friction coefficient is additionally necessary.