Engineering stress and true stress are two ways of measuring stress in a material during deformation, particularly in tensile testing. The key difference lies in how the cross-sectional area of the specimen is considered in the calculation.
- Engineering Stress (Nominal Stress) is calculated by dividing the applied force by the original cross-sectional area of the material:
engineering stress = force / original area or σₑ = F / A₀
where σₑ is the engineering stress, F is the applied force, and A₀ is the original cross-sectional area. Engineering stress assumes that the cross-section remains constant throughout the test, making it simpler to calculate. It is widely used in design calculations but becomes less accurate once significant plastic deformation occurs. - True Stress accounts for the actual cross-sectional area of the material as it deforms:
true stress = force / instantaneous area or σₜ = F / A
where σₜ is the true stress, and A is the actual (instantaneous) cross-sectional area at that moment. Since materials typically shrink in cross-section when stretched, true stress is always higher than engineering stress after yielding.

At small strains, engineering stress and true stress are nearly identical, but as plastic deformation progresses, true stress values become significantly higher because they account for the decreasing cross-sectional area. Despite the technical accuracy of true stress, engineering stress is more commonly used in design and analysis because it simplifies calculations and aligns with standardized material properties, such as yield strength and ultimate tensile strength, which are typically based on the original cross-section. Using true stress values in design would lead to drastically undersized components since it does not reflect the actual conditions under which a structure or part will first begin to yield or fail. Therefore, while true stress is valuable for detailed material studies, forming processes, and failure analysis, engineering stress remains the standard in most practical engineering applications.