The depth of short cracks (70–200 μm surface length) has been measured with an acoustic microscope by utilizing the nondestructive time-of-flight diffraction technique (TOFD). The depth measurements were first carried out in the transparent polymer polystyrene, thus allowing a comparison between the acoustical values and direct optical measurements: the agreement in the results was better than 95%. The depth of a 70 μm long crack in an aluminium alloy was then measured, demonstrating the application of the technique to metals.