Abstract Physical testing and theoretical methods to determine the critical local flange buckling load in concentric pultruded columns are presented. Knowledge of this critical load for wide flange profiles is required in a new universal column design procedure. Because there are too few test results for other profile sizes only the standard 203×203×9.53 mm (8×8×3/8 in.) profile is considered. Reasons are given to explain why the 14 data points give buckling loads between 307 and 406 kN. To obtain a test result corresponding to the true physical situation the minimum column length should be four half-wavelengths of the local instability. The true critical load is about 360 kN. Presented are nine closed form equations that can be used to predict the critical load for the local instability. To use these equations effectively, it is shown that the elastic constants of the flange material must be those of the profile being analysed. Numerical methods, and their closed form equations, that include the rotational stiffness at the web–flange connection are shown to be reasonably accurate. However, without direct access to the software they are difficult to apply, and these equations are therefore unsuitable for application in the preparation of universal design guidance. A simpler equation that predicts the buckling load to within 4% of 360 kN is therefore recommended.