Measuring instrument for orthodontic appliances comprising a body with a handle and a rectilinear head oriented orthogonally to the handle, wherein from a distal edge of the head protrudes an appendix, wherein said appendix is orthogonal to the head, that is, the appendix is oriented parallel to the handle, and is formed by a base having a predetermined width from which emerges a pin having a length which is also predetermined； the head is provided with a millimeter scale on at least one side of the appendix； the width of the base is a value selected among a plurality of predetermined values and is equal to the distance between the bodies of a palatal expander in closed configuration.
