A method and apparatus for determining a parameter of fluid flow within a piping system is provided. The method includes the steps of: determining a measured velocity of the fluid flow using a fluid flow meter; determining a measured difference in pressure value within the fluid flow using a differential pressure meter; selecting an estimated Reynolds number of the fluid flow; determining a calculated difference in pressure value using a model which relates the calculated difference in pressure value to the measured velocity and the estimated Reynolds number of the fluid flow; determining a degree of inaccuracy of the estimated Reynolds number using the measured difference in pressure and the calculated difference in pressure; and iteratively adjusting the estimated Reynolds number, determining the calculated difference in pressure, and determining the degree of inaccuracy using the adjusted Reynolds number, until the degree of inaccuracy is within an acceptable range.