The aim of this work was to develop a methodology and a mechanical tester to determine the macro- and micromechanical behaviours of elastomers under biaxial tension by spherical indentation. Combining the Yang and Begley models, in addition to the experimental data obtained from our mechanical tester, a new methodology to understand the mechanical response of elastomers is presented; this methodology includes elastic modulus, elongation ratios and full-field micro-strain patterns of polymeric membranes undergoing biaxial tension. It was demonstrated that the micro-strain patterns, obtained through 3D digital image correlation, were heterogeneous and higher than those estimated for region contact by numerical models. After an extensive analysis, a general analytical expression that considers the complete set of load versus central displacement experimental data was found to calculate the elastic modulus of elastomers with better accuracy than traditional models. Our device and methodology could become an excellent tool to evaluate the macro- and micromechanical behaviours of soft polymers, cross-linked hydrogels and even biological tissues that exhibit rubber-like mechanical behaviour.