The Base Metal Thickness (BMT) measures the steel substrate prior to any metallic coating or paint being added to the substrate. The Total Coated Thickness (TCT) measures the steel substrate as well as the metallic coating, resin coating and/ or paint film. If a Tata BlueScope Steel AZ150 coated product is labeled as 0.42mm BMT means that it will have TCT of 0.47mm. If the BMT is measured at 0.48mm means that the product will have a TCT of 0.53mm. For design calculations or structural design one should use BMT since the actual strength of the material is from its base steel. The metallic coating layer provides the corrosion resistance against natural weathering.