<p>Entanglement in fibrous materials strongly affects their mechanical performance, yet quantitative experimental characterisation of entanglement has proved elusive. The widely used hook-drop test is known to be inadequate to evaluate entanglement. Here, we directly demonstrate its shortcomings using a combination of micro-tomography and high-speed photography. These observations motivated us to propose a simpler and more reliable method—a pin insertion test, which directly relates entanglement to its mechanical effects. Application of the basic principles of fracture mechanics to this test allows a direct quantification of the length scale of entanglement. Our ideas not only improve the assessment of entanglements in fibre composites, but also open pathways for investigating and quantifying entanglement in a large class of fibrous materials.</p>