In both deviated and vertical wells, coiled tubing takes a helical shape. The conventional approach to describing coiled tubing constrained inside a wellbore relies upon the relationship between the axial force and the helical pitch. However, in the field and in experiments, it is hard to measure accurately the helical pitch for very long tubing. Instead, the tubing operators collect data on the force and shortening of coiled tubing in the axial direction of wells. This paper transforms the classical expression for coiled-tubing deformation into an alternative form that relates the axial force to the axial shortening of the tubing. The new model is consistent with experimental data from several previous studies. Moreover, through this new model, a simple technique is developed to predict the lockup length of the coiled tubing resulting from friction. This leads to an improved prediction of lockup length for coiled tubing, allowing more accurate modeling of downhole depth.