Ultrasonic meters are calibrated for flowrate measurement of pure oil flows. However, the multiphase separation is not enough to totally separate water and oil and the indicated oil flow rate is larger than its true value when water is mixed with oil in the flow, as usual in pipeline flows. A methodology was developed to measure the water content of water-oil mixtures automatically, using the already measured speed of sound by installed intrusive or non-intrusive (clamp on type) ultrasonic meters for flowrate measurement. That measure together with the previously determined speed of sound and density of pure water and oil as a function of temperature, thus avoiding additional meters. This dissertation shows a reduction of the uncertainty of measurement when the meter indicated water-oil mixture speed of sound is directly related to the traceable measurements of water content and temperature of previously prepared water-oil mixtures, what turns out to be the meter calibration for water content measurement. Water content uncertainty value of 0.25 per cent, 95.45 per cent level of confidence, is obtained when curve fitting de data in the 0 per cent - 2.5 per cent range, that covers the fiscal measurement range (less than 1 per cent).