Shore hardness measures how far a spring-loaded indenter sinks into rubber or plastic, on a scale from 0 to 100. The scale letter matters as much as the number, because each durometer type uses a different indenter and spring. Shore A, for soft rubber and elastomers, uses a blunt truncated cone and a spring force of 8.05 N at full scale. Shore D, for hard rubber and plastics, uses a sharp cone and 44.45 N. Shore OO, for foam and gel, uses a ball and 1.111 N.
Shore durometer types
Indenter shape and spring force for each durometer type in ASTM D2240. Full scale is a reading of 100, where the indenter does not penetrate at all.
| Shore type |
Indenter |
Spring force at full scale (N) |
Spring force (gf) |
| A | Truncated cone, 35 degrees | 8.05 | 821 |
| B | Sharp cone, 30 degrees | 8.05 | 821 |
| C | Truncated cone, 35 degrees | 44.45 | 4533 |
| D | Sharp cone, 30 degrees | 44.45 | 4533 |
| DO | Sphere, 2.38 mm | 44.45 | 4533 |
| E | Sphere | 8.05 | 821 |
| M | Sharp cone, 30 degrees | - | 78 |
| O | Sphere, 2.38 mm | 8.05 | 821 |
| OO | Sphere | 1.111 | 113.3 |
| OOO | Sphere, 12.70 mm | 1.111 | 113.3 |
| OOO-S | Disc | 1.932 | 197 |
Shore D presses 5.5 times harder than Shore A and through a sharper point, so the two scales overlap only at their ends. A reading above about 90 on Shore A, or below about 20 on Shore D, is at the edge of what that durometer can resolve, and the material should be measured on the other scale.
Which Shore scale to use
Use Shore A for flexible rubbers and elastomers: O-rings, seals, hose, tyres and soft polyurethane. Use Shore D for hard rubber, rigid polyurethane and plastics such as nylon, acetal and PVC. Use Shore OO or O for very soft materials such as sponge rubber, foam and gel. Type M is a small-scale version for thin sections and small O-rings that a standard durometer foot cannot sit on.
Most sealing elastomers are specified on Shore A, with harder compounds chosen where a seal has to resist being squeezed into a gap at high pressure. See O-ring material selection for how hardness is chosen alongside the compound.
Typical Shore hardness of everyday items
These figures are for orientation only. They come from published comparison charts, which do not always agree, so a range is shown wherever the sources differ. They are not specification values.
| Item |
Scale |
Published value |
Sources compared |
| Rubber band | Shore A | 20 to 30 | 4 |
| Chewing gum | Shore A | 20 | 3 |
| Pencil eraser | Shore A | 35 to 55 | 4 |
| Inner tube | Shore A | 40 to 65 | 2 |
| Door seal | Shore A | 55 | 3 |
| Tyre tread (car) | Shore A | 60 to 75 | 5 |
| Leather belt | Shore A | 65 to 80 | 2 |
| Soft skateboard wheels | Shore A | 78 | 2 |
| Shoe heel | Shore A | 80 | 2 |
| Tap washer | Shore A | 85 to 95 | 3 |
| Hard skateboard wheels | Shore A | 98 | 2 |
| Book cover | Shore D | 40 to 55 | 2 |
| Hard hat | Shore D | 75 to 80 | 2 |
How a Shore reading is taken
The durometer is pressed flat against the sample until its foot is in full contact and the reading is taken after a fixed time, because rubber creeps under the indenter and the number falls as it does. The sample has to be thick enough that the surface underneath does not affect the reading, and flat enough for the foot to sit square. Thin or curved parts read high. Quote the scale, the value and the reading time together.
Shore values cannot be converted exactly to Rockwell or to IRHD. For hardness scales used on metals see the Rockwell hardness scales chart and the hardness conversion chart.
References
- ASTM D2240 - Standard test method for rubber property, durometer hardness
- ISO 48-4 - Rubber, determination of hardness, indentation hardness by durometer method
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