Abstract: Disclosed is a method for determining fibre porosity in a fibre suspension. A sample is taken from a fibre suspension and fibre porosity determination is performed by determining the ratio of fibre-internal water to fibre-external bound water as an online measurement using a NMR spectroscope in successive steps of: generating a magnetic field for exciting protons of water contained in the sample; exciting water molecules of fibre suspension contained in the sample with a frequency pulse provided by a coil; measuring a return signal of the frequency pulse that returns from water molecules to the coil; determining the proton relaxation time and the amplitude of the return signal from the return signal; and determining fibre porosity of the fibre suspension based on the amplitude and the relaxation time of the exponential return signal. Also disclosed is a device for determining fibre porosity of a fibre suspension and a control system connected to a fibre web machine.