For weld-in couplings, you should measure the flange plate diameter at its widest point, bore diameter and wall thickness. For couplings with a hose tail, in addition to the diameter of the flange plate and bore, the diameter of the hose tail measured on the outside, at its widest point, is very important. This measurement helps to identify to which standard the coupling was made, and reduces the risk of error by choosing a wrong coupling.