The current investigation introduces the concepts of the equivalent crack length approach for thin and thick adhesive joints. Its applicability is assessed for adhesively bonded composite and steel joints with a bondline thickness ranging from 0.4 mm to 10 mm. To achieve this objective, the equivalent crack length method is formulated utilizing a beam on elastic foundation model. A series of experiments were performed utilizing various loading rates and geometries, and the energy release rate of the thick adhesive joints was determined through beam on elastic foundation model using the crack length values obtained both experimentally and through the equivalent crack length technique. In general, the energy release rate calculated using the equivalent crack length approach and crack length measured experimentally yield comparable results for all tested specimens. For side-grooved specimens with steady crack propagation, the average calculation error of the energy release rate obtained from the experimentally measured crack length experimentally and the equivalent crack length approach is less than seven and six percent for low and high loading rates, respectively. The proposed equivalent crack length method facilitates the experimental fracture characterization of adhesive joints since it eliminates the need for tedious crack length measurements during the test.