The characterization of fuel injector orifices is important for engine performance engineers, combustion engineers, and as input into 3-D spray and combustion simulations. Typical steady flow measurements done by fuel injector manufacturers are not indicative of their flow at engine injection pressures. Orifice characterization is commonly done on a rate bench or for further detail by using momentum rate measurements. The common Bernoulli equation assumptions made for momentum rate measurements are not always proper. This work compares the common approach with a more detailed approach for calculating the nozzle flow coefficients and sac pressure. The detailed approach includes friction and contraction pressure losses. A discussion on sac velocity is included. A non-iterative calculation of the Darcy friction factor is reviewed and an uncertainty analysis is also provided.