A method for determining the flow rate of a fluid, which is a gas mixture, wherein at least one component of the gas mixture is a hydrocarbon, through a circular cylindrical measuring tube with a straight measuring tube longitudinal axis and an internal diameter D
I , characterized by the following method steps: determining a first average flow velocity v
L by measuring the propagation time difference of acoustic signals along a signal path; determining a modified Reynolds number Re
mod according to the formula Re
mod = (v
L *D
I )/v
kin , wherein v
kin , the kinematic viscosity of the fluid, is known; determining a second average flow velocity v
A using a known function v
A = f(Re
mod ) on the basis of the modified Reynolds number Re
mod , wherein the method step of determining the modified Reynolds number Re
mod is preceded by the method step of determining the kinematic viscosity v
kin of the fluid.