Understanding Durometer: What Shore Hardness Really Means
Every rubber part has a hardness, and that hardness has a number. It is called durometer, and once you can read it, a lot of confusing product descriptions suddenly make sense. A “60A” rubber foot and a “90A” caster wheel are not just vaguely different — they are precisely different, and the number tells you how.
What durometer measures
Durometer measures a material’s resistance to indentation. A spring-loaded needle, called an indenter, is pressed into the surface under a defined force, and the depth it sinks is converted to a number from 0 to 100. The less the needle sinks, the harder the material and the higher the number. It is a surface-hardness test, standardised so that a reading taken in one workshop means the same thing in another.
The name comes from the instrument, the durometer gauge, developed in the early twentieth century. There are several scales for different material ranges, but two cover almost everything you will meet in rubber hardware: Shore A and Shore D.
Shore A: the scale for flexible rubber
Shore A is the everyday scale for flexible rubbers and soft plastics. It runs from gel-soft materials near the bottom to hard rubber near the top:
- 10–30 Shore A — very soft: gel insoles, rubber bands, soft silicone.
- 40–50 Shore A — soft and pliable: rubber stamps, door seals, swim goggles.
- 55–70 Shore A — medium: tyre tread, most rubber feet and bumpers, shoe soles.
- 80–90 Shore A — hard: skateboard wheels, casters, firm bumpers.
Most protective and anti-slip parts live in the medium band, because that is where a material is firm enough to carry a load and keep its shape, yet soft enough to grip and absorb a knock.
Shore D: the scale for hard rubber and plastics
When a material gets too hard for Shore A to resolve, the scale runs out of room near 100 and testing moves to Shore D. Shore D uses a sharper indenter and a stronger spring, and it covers hard rubbers and rigid plastics: a hard hat is around 75–80 Shore D, a hard caster wheel might be 60–70 Shore D, and a golf-ball cover sits up near the top.
Why the scales overlap, and why conversion is only approximate
Shore A and Shore D overlap at the hard end. A very hard rubber around 95–100 Shore A is roughly 45–58 Shore D, so the same part can carry two numbers depending on which gauge measured it. Below that overlap, a Shore D reading is not meaningful — the indenter barely engages soft rubber — so a soft foot simply has no useful D value.
Published charts let you convert between the scales, but treat any conversion as a ballpark. The two tests use different geometry, so the relationship is not exact and different sources disagree by a few points. When hardness genuinely matters, measure the actual part on the scale you care about. Our durometer hardness guide will classify a reading and give an approximate cross-scale equivalent, with that caveat built in.
How to choose a hardness
Picking a durometer is about the job, not about “better” or “worse”:
- For grip and cushioning — go softer. A lower Shore A conforms to the surface and absorbs shock, which is why non-slip pads and vibration mounts are soft.
- For load and wear — go firmer. A higher Shore A holds its shape under weight and resists abrasion, which is why wheels and heavy-duty feet are hard.
- For sealing — go medium-soft. A seal has to squeeze into gaps and spring back, so it favours the middle of the range.
The one number worth checking
If you remember nothing else, remember that durometer is a single number that captures a real, testable property. When a supplier lists a Shore A value, they are telling you something specific about how the part will feel, grip, carry, and wear. Reading that number, instead of trusting words like “soft” or “heavy-duty,” is the difference between guessing and choosing.