The parameters of the transmission lines are used in several monitoring and control tools of the power system control center. An accurate knowledge of the power system model (including transmission lines impedances) impacts positively the situational awareness of the power system operators. It also improves the protection and corrective measures that may be applied in case of a disturbance. Nevertheless, line parameters that are stored in the control center database are typically subjected to errors, and therefore it is necessary to regularly refine them. The presence of a phasor measurement unit (PMU) at both ends of the transmission line allows the calculation of the transmission line parameters using synchronized current and voltage phasor measurements. Although the use of PMU measurements is a straightforward way to calculate the transmission line parameters, the uncertainty of the synchronized phasor measurements should be incorporated to the calculated transmission line parameter values. In this paper, an analytical expression for the bounds of the calculated transmission line parameters from PMU measurements is derived, considering both instrument transformers and PMU errors. The calculated bounds of uncertainty are evaluated through Monte Carlo simulations, while the importance of knowing the bounds of the calculated line parameters is underlined through the derivation of power–voltage curves considering the lower and upper bounds of the parameters. The simulation results are obtained using the IEEE 14-bus system.