In design practice, the transverse bending analysis of box-girder bridges is commonly done by modeling the cross section as a frame of unit width with imaginary supports at the web locations. The transverse bending moments obtained from simple frame analysis (SFA) is sometimes increased by a small percentage to accommodate the errors in modeling. In this paper, a large number of simply supported box-girder bridges have been analyzed by both SFA and three-dimensional finite element analysis for different load conditions and wheel contact areas, and the errors in SFA have been studied and quantified. The error is found to vary widely at the web-top flange junction as well as under the load (maximum sagging moment), depending on the eccentricity of loading, the wheel contact dimensions and the web-flange thickness ratio. Accordingly, a set of correction factors to the results of SFA have been proposed, which is expected to be of significant use in design practice. The use of the correction factors is demonstrated by means of two illustrative examples. The scope of the study is limited to the simplest case of a single-cell concrete box-girder bridge (simply supported with end diaphragms) without overhanging flanges. ©ASCE.