A method for ascertaining flow of a fluid, which is a gas mixture, through a circularly cylindrical measuring tube having a straight, measuring tube, longitudinal axis and an inner diameter DI, wherein at least one component of the gas mixture is a hydrocarbon. The steps comprise: ascertaining a first average flow velocity vL by means of travel-time difference measurement of acoustic signals along a signal path; ascertaining a modified Reynolds number Remod according to the formula Remod=(vL*DI)/vkin, wherein the kinematic viscosity vkin of the fluid is known; and ascertaining a second average flow velocity vA by means of a known function vA=f(Remod) as a function of the modified Reynolds number Remod, wherein the method step of ascertaining the modified Reynolds number Remod precedes the method step of ascertaining the kinematic viscosity vkin of the fluid.