•A modified Darter and Barenberg’s equation to predict crack widths of thin FRC overlays.•Previously constructed full-scale thin FRC overlays subjected to environmental loadings.•A simple two-dimensional finite element model for the cracking and debonding of thin FRC overlays.•Validation of predicted crack widths through the comparison with measured crack widths from field studies. This study aims to predict the early-age crack width of thin fiber reinforced concrete (FRC) overlays through the modified prediction equation and finite element modeling. The equation which was originally developed to predict the joint opening width of plain concrete pavements, was modified in this study to either consider the fiber aspect ratio or the residual strength ratio of FRC. The predicted crack widths were compared and validated against the measured crack widths of full-scale FRC overlays constructed in the field. Thin FRC overlays in the field had varied joint spacing from 1.12 to 3.35m and were subjected to only environmental loading, rather than traffic loading. A finite element model was generated using fracture energy to account for the fiber-bridging effect and tensile bond strength for interface bonding with the underling structures. The results from a sensitivity analysis indicated that the crack width, vertical lift-off, and debonding length all decreased as the fracture energy of the FRC increased, or as the interfacial tensile bond increased. The proposed equation was found to predict the field crack width within 0.19mm or 26% difference.