Elongation is a reciprocal indication of material stiffness. It is defined as the increase in length as a percentage of the original length which, for the ultimate case, is the elongation at the point of breaking. Elongation is also used to define an elastomer that is a material capable of 100% elongation. Permissible elongation (the percentage stretch which can be applied without permanent damage or permanent set) determines the amount by which a ring seal can be stretched to fit in place.

The term modulus is also used in connection with elongation and is generally taken to refer to the modulus (in tension) or the stress produced in the material at a predetermined elongation, for example, 100 % elongation. This can be used as a measure of quality control. Modulus can also refer to stress per specified distortion in shear and compression (modulus in shear and modulus in compression, respectively). A change of modulus of a material indicates a change in material characteristics, a loss of modulus, for example, indicates a degradation of the product.

As a general safety rule elastomers seals should not permanently stretched more than about 5% as otherwise the resulting residual stresses can cause early deterioration further accelerated by any rise in temperature. This applies particularly to the more generally used elastomers such as Nitrile and natural rubbers. Some elastomers, notably ethylene-propylene, can accommodate a relatively high amount of permanent stretch with no adverse effects.