Specific Absorption Rate (SAR) measurement systems are used to evaluate exposure to fields radiated by wireless devices placed close to people's bodies. One of the fundamental elements of these systems is an isotropic electric field sensor that is small relative to the radiated wavelength. The obtained SAR value must be compared with the basic restriction levels or exposure limits established by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) or similar international bodies. This work presents the design and implementation of a novel isotropic electric field sensor made with low-cost materials and processes. The response of the developed sensor was compared with the response of a commercial probe in order to perform calibration. The calibration methodology proposed in this work allows transforming voltage measured with the developed low-cost probe to electric field values measured with a calibrated commercial probe in order to calculate SAR values. The results show that the developed sensor can be used in SAR measurement processes.