The uniaxial true stress-logarithmic strain curve for a thick metal section can be determined from the load-diameter reduction record of a round tensile specimen. The correction of the true stress for necking can be performed by using the well-known Bridgman equation. For thin sections, it is more practical to use specimens with rectangular cross-section. However, there is no established method to determine the complete true stress-logarithmic strain relation from a rectangular specimen. In this paper, the authors perform an extensive three-dimensional numerical study on the diffuse necking behaviour of tensile specimens made of isotropic materials with rectangular cross-section. As a result, an approximate relation is established between the area reduction of the minimum cross-section and the measured thickness reduction. It is found that the area reduction can be normalized by the uniaxial strain at maximum load which represents the material hardening.