O-Ring Durometer - Why is Shore A Hardness Import?

When we think about the various objects and tools that surround us daily, we often overlook the importance of their material properties. Just how soft or hard is that rubber mat? What makes one silicone baking mold different from another? The answer often lies in something called "Shore A Hardness." Dive with us into the world of Shore A Hardness, and you'll never look at everyday materials the same way again!

What is Shore A Hardness?

Shore A Hardness, commonly referred to simply as "Shore hardness," is a measure of the hardness of elastomers, rubbers, and other soft plastics. Developed by Albert F. Shore in the early 20th century, the Shore A Hardness scale provides a numeric value representing the resistance of a material to indentation. Think of it as a way to determine how "squishy" or "rigid" something is.

Why is Shore A Hardness Important?

  1. Functionality & Durability: The hardness of a material directly affects its functionality. For example, industrial belts made of rubber need to be of a specific hardness to provide optimal performance without wearing out quickly.

  2. Comfort & Flexibility: Items such as shoe insoles have to strike a balance between being soft enough for comfort and hard enough to offer support.

  3. Safety Concerns: The hardness of materials can influence safety, especially in industrial settings. A rubber component that's too soft might collapse under pressure, whereas one that's too hard might not seal properly.

  4. Customization: With a knowledge of Shore A Hardness, manufacturers can tailor products to specific needs, ensuring maximum performance and lifespan.

O-Ring Durometer Chart

Understanding the durometer (or hardness) of an O-ring is crucial for ensuring seals are effective. Here's a simple O-Ring Durometer Chart to give you an idea:

Shore A Hardness Typical Materials Applications
20-30 Soft Silicone, Flexible Rubber Decorative molds, Cushioning pads
40-50 Medium Silicone, Soft Neoprene Door seals, Swimming goggles
60-70 Nitrile, Standard Buna-N O-rings Automotive seals, Gaskets
80-90 Hard Rubber, Rigid Plastics Industrial wheels, Rollers
95-100 Very Hard Plastics Machine components, Hard wheels


The hardness of rubber o-ring compounds are measured by the Shore A durometer; the higher the durometer, the harder the compound.  70-durometer hardness should be used whenever possible as it offers the best combination of properties for most o-ring applications.

Durometer Shore A scale is a type of measurement for rubber material hardness. The rubber durometer chart below gives you an idea of the rubber hardness that you want for your application. Generally, most rubber materials fall under the rubber durometer scale of Shore A. If you need a rubber or O-ring durometer that feels like a running shoe sole, review our rubber hardness chart below, then pick Shore 70A. A rubber durometer of Shore 70A is the most commonly chosen material hardness for all applications.

  • Duro 20A = Rubber Band
  • Duro 40A = Pencil Eraser
  • Duro 60A = Car Tire Tread
  • Duro 70A* = Running Shoe Sole
  • Duro 80A = Leather Belt
  • Duro 100A = Shopping Cart Wheel

*Most common durometer for O-Rings

Softer compounds stretch easier and seal better on rough surfaces.  Harder compounds offer greater abrasion resistance and resistance to extrusion.  Extrusion must always be considered where high pressure is used.  The proper hardness may be selected from this chart by matching the fluid pressure with the maximum extrusion gap.

  • 60 Shore A is softer than 70.
  • 70 Shore A is the standard.
  • 90 Shore A is very stiff.


O-ring Duro

The hardness of an elastomer is measured based on the depth of indentation by a standard size and shape impacting gauge. The hardness is obtained by comparing the difference between a small initial force and a much larger final force. The International Rubber Hardness Degrees (IRHD) scale has a range of 0 to 100, corresponding to elastic modulus of 0 (0) and infinite (100), respectively. The measurement is made by indenting a rigid ball into the rubber specimen.

The Shore A scale is the most prevalent in the United States. The readings range from 30 to 95 points. Harder elastomers use a pointed conical indentor with the Shore D scale. The results of the Shore A scale and the IRHD scale are approximately equal over the same range of resiliency. In elastomers with unusually high rates of stress relaxation or deformation hysteresis, the difference in dwell time in the two readings may cause different results. Also, the results of any hardness test depend on the elastomer thickness. Specified thickness should be used when conducting these tests.

Due to the mechanical limits of the test instruments, hardness measurements of elastomers are rarely expressed more precisely than 5 points.

The surface indentation or hardness usually does not bear any relation to the ability of an elastomeric part to function properly. Hardness is a measure of an elastomer's response to a small surface stress. Stiffness and compressive modulus measure the response to large stresses of the entire elastomeric part.

Shore A Hardness Extrusion Gap Pressure
(Typical psi)
20-30 200-400 Soft molds, Cushioning pads
40-50 500-700 Flexible tubes, Door seals
60-70 800-1200 Automotive parts, Standard seals
80-90 1300-1700 Hardened tubes, Industrial components
95-100 1800-2200 Very rigid components, Machine parts

Extrusion Gap Pressure Info

Hardness (Shore A) Pressure (PSI)
  Below 500 500 - 1000 1000 - 1500 1500 - 2000 2000 - 3000
70 0.016 0.010 0.006 0.004 0.002
90 0.028 0.024 0.020 0.016 0.010


In Conclusion

Shore A Hardness is an unsung hero in our daily lives. From the shoes we wear to the machinery that powers our world, understanding this vital material property can make all the difference in product quality, safety, and performance. So, the next time you step on a rubber mat or pick up a silicone spatula, take a moment to appreciate the science of Shore A Hardness at work!

Information within is believed to be accurate and reliable. However, The O-Ring Store, LLC makes no warranty, expressed or implied, that parts supplied in this material will perform satisfactorily in specific applications. It’s the customer’s responsibility to evaluate the material prior to use.

Copyright © 2024 The O-Ring Store, LLC
1847 Wilma Dr, Clarkston WA 99403
Phone 208-413-6377 - Fax 208-413-6719