Tensile strength isn't really calculated; it's measured. It is defined as the point at which a sample of material fails when subjected to tensile stress. The unit of measure of tensile strength (and stress) is the Newton per square meter (N/m2), also known as the Pascal (Pa). Tensile strengths of various building materials, such as steel, are measure in millions of Pa.

when asking how it is measured, a description of how it is measured would be helpful.