A measure of the indentation resistance of elastomeric or rubber materials based on the depth of penetration of a ball indentor. An initial contact force is applied to an indentor and the penetration is set to zero. The force is increased to a specified total load and the depth of penetration is measured. The IRHD value is related to the depth of indentor penetration.
For highly elastic materials, IRHD and Shore A are comparable. An IRHD value of 0 represents a material with an Young's Modulus of zero and an IRHD value of 100 represents a material with an infinite Young's Modulus. The relationship between the depth of penetration (hardness) and the IRHD number is based on: F/E=0.0038r0.65*D1.35
where:
F is the indentation force in Newtons,
E is the Young's Modulus in MPa,
r is the indentor ball radius in mm, and
D is the depth of penetration in mm.
Test 
Ball Diameter (mm) 
Contact Force (N) 
Total Force (N) 
Thickness (mm) 
IRHD Range 
N (Normal) 
2.5 
0.3 
5.7 
4 
30 to 95 
H (HighHardness) 
1.0 
0.3 
5.7 
4 
85 to 100 
L (LowHardness) 
5.0 
0.3 
5.7 
6 
10 to 35 
M (Micro) 
0.395 
0.0083 
0.153 
4 
30 to 95 
