An apparatus and a method for determining the permeability of a circulating band, in particular a water-absorbing dewatering band in a papermaking machine. The apparatus is equipped with a nozzle from which a measuring fluid flows onto or through the band. Fluctuations in the flow of the measuring fluid and in the nozzle exit pressure can distort the measurement of the permeability. The apparatus has a device for determining the speed of the band and/or a device for determining the nozzle exit speed and the nozzle exit pressure of the measuring fluid. These three measured variables are measured simultaneously, so that a fluctuation in one of these measured variables cannot impair the accuracy of the permeability measurement.