In this article an attempt has been made to quantitatively assess the extent of delamination in composite laminates, using time-of-flight of fundamental Lamb wave modes, without recourse to baseline data from a healthy structure. An expression has been derived to determine the delamination size, from group velocities of primary Lamb modes in the sub-laminates and time-of-flight of transmitted Ao signal and mode converted A o signal, which is generated when Ao mode propagates through a delamination. The effectiveness of the expression, when group velocities of primary Lamb modes in the main laminate were used, has been verified through numerical simulations carried out on a quasi-isotropic glass/epoxy laminate with various delamination interfaces. The effectiveness of the expression has also been verified experimentally, on two GFRP cross-ply laminates of [0/90/0] lay-up with 40 mm and 50 mm delamination sizes, using air coupled ultrasonic transducers. The predicted delamination sizes were found to be in good agreement with the actual delamination sizes. Using the proposed technique absolute identification of delamination is possible. A supplementary equation for determining the minimum length of delamination has also been derived and presented in the article. © The Author(s), 2010.