The British Institute of Non-Destructive Testing's NDT Reference App is based on the Institute's best-selling book: 'Mathematics and Formulae in NDT', edited by the late Dr Ron Halmshaw. Prepared by members of the BINDT Technical Committee, this app is planned to help NDT practitioners and students to understand and use the formulae which are employed in NDT. It contains basic useful formulae used in radiography and radiology, ultrasonics, magnetic particle testing and eddy current testing. The ultrasonics section includes some values for ultrasonic velocities for a number of materials. Additional images by storyset on Freepik (https://www.freepik.com)