A method for determining a characteristic of a fibre suspension, wherein a sample is taken from a fibre suspension, wherein the characteristic determination from the sample is performed 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 (v) of fibre suspension contained in the sample with radiofrequency pulses provided by a coil, - measuring backward signals of the radiofrequency pulses that return from water molecules (v) to the coil, - determining a proton's relaxation time and an amplitude of the backward signal from each backward signal, characterized in that the method further includes a step of determining fibre porosity of the fibre suspension as the characteristic by determining the ratio of fibre-internal water to fibre-external bound water based on the amplitudes and the relaxation times of the exponential backward signals using the double exponential signal model Exp = A I *exp(R 2I *t) + A E *exp(R 2E *t) + D for determining the relaxation time, where D is a third empirical constant and the fibre porosity is determined as a Water Retention Value in successive steps of: - calculating, based on the amplitudes A I and A E, a relative amount of fibre-internal water A I_rel and a relative amount of external water A E_rel, - calculating a proportion of water in fibre bound inside fibre p IB based on the relaxation speed of completely free water R 2F, the relaxation speed of completely bound water R 2B and the relaxation speed of fibre-internal bound water R 2I, - calculating a proportion of water in fibre bound outside fibre p EB based on the relaxation speed of completely free water R 2F, the relaxation speed of completely bound water R 2B and the relaxation speed of fibre-external bound water R 2E, - calculating an amount of bound water in fibre W IB based on the relative amount of fibre-internal water A I_rel, the proportion of water bound inside fibre p IB and consistency c, - calculating an amount of bound water outside fibre W EB based on the relative amount of fibre-external water A E_rel, the proportion of water bound outside fibre p EB and the consistency c, - calculating the WRV as a sum of the amount of bound water in fibre W IB and the amount of bound water outside fibre W IB. - generating a magnetic field for exciting protons of water contained in the sample, - exciting water molecules (v) of fibre suspension contained in the sample with radiofrequency pulses provided by a coil, - measuring backward signals of the radiofrequency pulses that return from water molecules (v) to the coil, - determining a proton's relaxation time and an amplitude of the backward signal from each backward signal, - calculating, based on the amplitudes A I and A E, a relative amount of fibre-internal water A I_rel and a relative amount of external water A E_rel, - calculating a proportion of water in fibre bound inside fibre p IB based on the relaxation speed of completely free water R 2F, the relaxation speed of completely bound water R 2B and the relaxation speed of fibre-internal bound water R 2I, - calculating a proportion of water in fibre bound outside fibre p EB based on the relaxation speed of completely free water R 2F, the relaxation speed of completely bound water R 2B and the relaxation speed of fibre-external bound water R 2E, - calculating an amount of bound water in fibre W IB based on the relative amount of fibre-internal water A I_rel, the proportion of water bound inside fibre p IB and consistency c, - calculating an amount of bound water outside fibre W EB based on the relative amount of fibre-external water A E_rel, the proportion of water bound outside fibre p EB and the consistency c, - calculating the WRV as a sum of the amount of bound water in fibre W IB and the amount of bound water outside fibre W IB.