The Christiansen's friction factor, Fc, is usually used to compute head loss due to friction in laterals of pressurized irrigation systems. In this research, this factor was evaluated considering variable discharge for outlets located along the laterals. For this purpose, two geometrical progressions were established between the outlets’ discharges. The first progression was appointed between the discharge of the first outlet and the outlet which is assumed to possess the average pressure. The next progression was developed between the discharge of this outlet and the most downstream outlet which is assumed to operate at least discharge. The proposed F-factor is a function of number of outlets, the permitted variation of pressure between the first and the last outlet, and also the formula being used to calculate friction head loss along the lateral. The results showed that Christiansen’s method accurately estimates total friction head loss along the laterals such that maximum relative error caused by assuming equal discharges in outlets does not exceed 3%.