Online measurement of diameters and concentricities of optical fibre layers, and the coating layer in particular, is one of the challenges in fibre manufacturing. Currently available instruments can measure concentricity and diameter of layers offline, and are not suitable for precise monitoring or control of the manufacturing process in real time. In this work, we use two laser beams, positioned orthogonally to illuminate the fibre from two sides, and calculate deviations from the expected geometry by analysing the scattering pattern. To measure the diffraction pattern we use two 8K linear array detectors, with the scattered light incident directly on the sensors. Each detector is capturing approximately 90° angular range directly behind the fibre. The two measurement channels are positioned at different heights. The scattered pattern is modelled mathematically with finite-element and Fourier-modal methods, with various diameter and concentricity deviations. The sensitivities of the changes in the scattering pattern are identified in respect to these deviations. Since calculations are computationally intensive, the sensitivities are pre-calculated in advance, and the realtime measurement is based on pattern recognition. The symmetry of the pattern is used to differentiate between diameter and concentricity variations. We performed online measurements with the prototype instrument in production conditions, and show that this method is sensitive enough to measure deviations of under 1 µm in diameter and concentricity of the coating layer.