The softening point of Littleton is the temperature at which the viscosity of the glass is equal to 10^7.6 poise, and represents the point at which the glass begins to lose its shape under the effect of its own weight.

The determination of the softening point is carried out following the indications of the ASTM- C 338-93 standard, measuring the elongation under its own weight of a fiber of standard size obtained from the glass sample.