ABSTRACTIn this study, a comprehensive experimental study was performed to find an equation for the estimation of flow discharge over a sharp‐crested side weir in a trapezoidal channel. A physical model with side slopes of 1.5, 1, 0.5 and 0 was used and sharp‐crested side weirs with different lengths and heights were tested under different flow conditions. In this study, 212 laboratory tests were performed with different side‐wall slopes under subcritical flow conditions since the Froude number = 0.08–0.80. As a result, using statistical analysis, a linear relationship is proposed here for estimation of the side‐weir discharge coefficient in a trapezoidal channel. The new equation is a function of the upstream Froude number, weir height and side‐wall slope. The new equation enables estimation of flow discharge over the side weir with consideration of different side slopes. Furthermore, previously proposed discharge relationships for estimation of the discharge coefficient over sharp‐crested side weirs were compared with the new equation and experimental data from this study and this comparison is also reported in this paper. Copyright © 2014 John Wiley & Sons, Ltd.