A method for measuring density of a fluid that includes (i) flowing the fluid through a substantially horizontal conduit 1; (ii) measuring a pressure difference W P1 between two vertically-spaced measurement positions in the conduit 1; and (iii) using the pressure difference W P1 to determine the density of the fluid.; More particularly, the method may provide for measuring a flow rate of a gas-liquid fluid mixture by (i) flowing the fluid through a substantially horizontal conduit 1 having a constriction region 3; (ii) measuring a first pressure difference W P1 between two vertically-spaced measurement positions in the conduit1 ; (iii) measuring a second pressure difference W P2 between two horizontally-spaced measurement positions in the conduit 1, the first horizontally-spaced measurement position being at the constriction region 3 and the second horizontally-spaced measurement position being upstream or downstream of the constriction region 3; and (iv) using the first and the second pressure differences to determine a flow rate of the gas-liquid fluid mixture.; The method might further comprise measuring a third pressure difference W P3 between two further vertically-spaced measurement positions in the conduit 3 spaced upstream or downstream of the vertically-space measurement positions where the first pressure difference W P1 is measured, enabling slip between the gas and liquid phases to be accounted for. A further embodiment of the method concerns conditioning the fluid mixture to exhibit swirling flow 5 which separates the liquid from the gas at the measurement positions. A section of a conduit is provided with upper and lower passages, directly above one another and extending horizontally.