The feel of softness of any flexible foam is quantitatively defined by its indentation hardness. Indentation hardness of foam is determined by pressing a circular indentor at a specified speed against a block of foam and finding the force needed to compress it to a specified percentage of its initial thickness.
The Indentation Hardness Tester For Polyurethane Foam consists of a circular indentor which can be pressed against the foam block under test, a motorized arrangement to move the indentor up or down, and a load measuring and indicating arrangement to indicate the load being exerted by the indentor on the foam block at any instant.
JIS K 6400 – 1997 : Test Methods for Flexible Polyurethane Foam
Clause 6 : Hardness.