A method calculates the boundary layer level between fluids, in particular the boundary layer level between oil and water in connection with the use of a segment-based boundary layer meter. The calculations are based on the use of measurement signals from a number of segments, n, and the boundary layer level, L, of the water is calculated as follows: L = ρ _ - ρ 0 ρ v - ρ 0 ⁢ ( L u - L l ) + L l where: Lu Upper water range area Ll Lower water range area ρn Density of segment no. n from the profile meter {circumflex over (ρ)}n Corrected density for segment no. n ρ Calculated mean density ρv Mean density of water from the profile meter ρo Mean density of oil from the profile meter N Total number of segments included.