Ultrasonic guided waves play an essential role in determining the severity of defects in plate-pipe-like structures. One of the parameters that influence the decision to replace a defective structure is the thickness remaining in the structure. Hence, a precise measurement of the remaining thickness is essential in many industries. The objective of this paper is the quantification of defects in small thickness structures. The cut-off property of the guide wave mode is applied to quantify the lowest remnant thickness of defects in small thickness structures. Implementing the cut-off approach in small thickness structures is challenging because of the small wavelength requirements. A linear array transducer, capable of selectively exciting the transducer elements, is used for this application to transmit and receive wave modes. The study focuses on the A1 Lamb wave mode and its cut-off behaviors. The remnant thickness reflects the A1 mode if the frequency of the wave mode is lower than the cut-off frequency of the defect. The reflection of the A1 mode from the defect is analyzed at multiple frequencies utilising the wavelength constrained excitation. The frequency below which the A1 mode appears indicated the remnant thickness of the defect. Furthermore, an advanced excitation approach using the broadband A1 mode identifies the remnant thickness values from a single experiment. In addition, rather than specifying a range of remnant thickness, it precisely quantified its value. This approach was capable of identifying remnant thickness within a maximum error of 2.3% among the set of examples selected. © 2022 Author(s).