Process and device for measuring weld seam geometry in making lap, strip, wire or deposited metal seams in weldable materials which are joined by means of a hot-air nozzle or wedge or a heating wedge and then joined together by means of pressing components at a predetermined jointing pressure. The distance between the pressing components is contactlessly detected during jointing by sensors and the jointing travel is found as a measure of the quality of the weld seam from the difference between the total thickness of the unwelded material and the spacing of the pressing components. With pressure rollers used as the pressing components, magnets are rigidly secured to their bearing components and arranged in such a way that a magnetic field is generated between them. In said magnetic field there is a Hall effect sensor which generates an electric signal in proportion to the strength of the magnetic field and transmits it to an evaluation unit for further processing. The magnets and the Hall effect sensor are preferably arranged near the faces of the pressing components, with the auxiliary magnet and the Hall effect sensor cast in a sensor block. The process and device can be used for all materials, weld seams and hot air, hot air wedge or hot wedge welding equipment.