The Shore Hardness Scale: A Complete Guide

Shore hardness ratings show up on datasheets for rubbers, elastomers and polymers all the time. Without a solid grasp of what they mean and which scale they refer to, though, they can be surprisingly easy to misinterpret.

The Shore hardness scale measures a material's resistance to permanent indentation. The higher the Shore hardness rating, the harder the material. That part is straightforward, but the complication is that Shore hardness is not one scale but a family of them, each covering a different range of materials and hardness levels.

This guide walks through everything you need to understand Shore hardness with confidence. It explains how the test works, what each scale covers, how to interpret the overlap between scales and how to select the right Shore hardness for your application.

 

Contents:

• What is Shore Hardness?

• The Shore Hardness Scales Explained

• How Shore Hardness is Measured

• Scale Overlap and Shore A to Shore D Conversion

• Shore Hardness of Common Engineering Materials

• How Shore Hardness Affects Component Performance

• Shore Hardness vs Other Hardness Scales

• Choosing the Right Shore Hardness for Your Application

• Key Takeaways

• FAQs

 

What is Shore Hardness?

 Imagine, for example, reading a data sheet and seeing both “Shore 70A” and “Shore 60D” used on it. What do they actually mean? Is one material harder than the other and, if so, which?

Shore hardness is a measure of a material’s ability to resist permanent indentation. To test it, a calibrated instrument called a Shore durometer presses a standardised indenter into the material's surface under a defined load, then measures how far it penetrates. The result is a score from 0 to 100. 0 means the indenter passed straight through, 100 means no penetration at all. The indenter itself varies between the different Shore hardness scales, which are denoted by a letter such as Shore A or Shore D. While the different indenters work on the same principle, there are fundamental differences between them and a different one is required to measure for a rating each different scale. We’ll run through what the specific indenter for each scale is in more depth later.

It’s worth knowing also that the test is only for non-metallic materials like thermoplastics, gels, rubbers, elastomers and polymers. It isn’t used for metals, which instead use the Rockwell, Brinell or Vickers hardness tests.

The score given is significant. A material with a rating of Shore 20A would deform significantly under the pressure of a single fingertip, whereas one with a Shore 90A rating would barely dent at all under the same pressure and feel extremely hard to the touch.

While this all sounds easy to understand so far, there are a few caveats. Shore Hardness is an empirical measurement, meaning the score is only relative to other materials on the same scale. There are actually more than just the one scale as part of the Shore hardness scale, so coming back to the earlier examples of Shore 70A and Shore 60D, the two measurements are not directly comparable. We’ll touch on why and what they denote later.

The Shore hardness scale was initially developed by an American, Albert Ferdinand Shore, who created the first durometer in 1915. The scale is named after him but has since been formalised as ASTM D2240 in the USA and ISO 868 in Europe and the rest of the world. Fundamentally, the methodology behind the scale and the nature of the durometer remain the same as Shore’s invention, so next we’ll cover how a durometer is used.

The Shore Hardness Scales Explained

 Calling it the Shore hardness scale is perhaps a bit of a misnomer, as there are actually several different scales. For this article we’ll discuss the three main scales that are used: Shore A, Shore 00 and Shore D. They are identified by letter, for example a material that scores 40 on the Shore A scale would have a rating of Shore 40A, which shows anyone who checks a material’s rating which scale it was determined using.

Shore A

The most widely used of the Shore hardness scales and the one engineers are most likely to come into contact with. The Shore A scale is the default for soft to medium-hard elastomers like natural rubber, neoprene and polyurethane. 

The Shore A scale runs from 0 to 100. Materials that score less than 20A tend to be extremely soft, gel-like substances with little to no structural resistance. Those that score from 20A to 40A are soft, flexible rubbers that deform easily and have high elongation. Between 40A and 60A the resistance becomes more meaningful, though the material remains clearly flexible. From 80A to 95A the material is hard and approaching semi-rigid. Above 95A the scale loses reliability and Shore D becomes more appropriate.

The Shore A indenter has a flat circular face 0.79 mm in diameter, with conical sides angled at 35° from the vertical. The flat tip distributes force over a small area, making the instrument sensitive to softer materials.

For quick reference, here's the full breakdown of categories and example materials across the Shore A scale:

 

Note: hardness values are typical ranges for compound families. Actual values vary by formulation, filler content and processing. Always verify against the specific material datasheet. Materials above Shore A 95 are generally tested and reported on the Shore D scale.

Shore 00

Shore 00 is designed for very soft materials that fall below the reliable measurement range of Shore A: soft gels, foams, sponge rubbers and highly compliant elastomers. One thing to note about notation: Shore 00 ratings are written with the number before the score, so a reading of 40 appears as Shore 00 40 rather than Shore 40 00.

Below 40 are silicone gels and similar materials used in medical cushioning, prosthetics and vibration-absorbing pads: things very close to fluid in consistency. Above 80 the scale overlaps with the low end of Shore A. Shore 00 materials are not suited to structural or sealing roles; their practical use in engineering is mainly in soft robotics, flexible electronics and specialised medical applications.

The Shore 00 indenter has a hemispherical tip with a 2.38 mm radius. The larger, rounder geometry prevents it from simply passing through very soft materials that a smaller indenter would.

Here is a breakdown of the material characteristics and example materials from the Shore 00 hardness scale:

Shore D

Shore D is for materials too hard to measure reliably on the Shore A scale: hard rubbers approaching full rigidity, semi-rigid plastics and engineering thermoplastics. Where the Shore A instrument would simply bottom out on these materials, Shore D uses a sharper indenter and applies more than five times the load (4,536 grams-force compared to 822 grams-force) to get a meaningful reading.

From 0D to 30D are hard rubbers and very stiff elastomers, though in practice this range overlaps with Shore A and materials here are often reported on the A scale instead. From 30D to 60D are engineering plastics: nylon, acetal and polypropylene. Between 60D and 80D you'll find hard, rigid plastics like ABS. Above 80D the scale begins to lose precision and Rockwell hardness testing becomes more appropriate.

The Shore D indenter comes to a sharp conical point with a 30° included angle and a tip radius of 0.1 mm. The concentrated geometry allows it to penetrate materials that would give no meaningful reading on the Shore A instrument.

Here’s the at-a-glance breakdown of the Shore D hardness scale as well as the characteristics and materials you can expect to find as you progress through the scale:

Note: Shore D values below approximately 40 overlap with the upper end of the Shore A scale (Shore A 85 to 95). Materials in this overlap zone are conventionally reported on whichever scale the supplier or standard specifies. Above Shore D 90, Rockwell hardness testing is generally more appropriate. Hardness values are typical ranges; always verify against the specific material datasheet.

Scale Overlap and Shore A to Shore D Conversion

The Shore A and Shore D scales don't have a clean boundary between them. In the region of Shore A 85 to 95 and Shore D 30 to 50, both instruments can be used to test the same material and will produce different numerical values.

This surprises some engineers. You might expect the top of Shore A to align with the very bottom of Shore D, but that's not how it works. The two instruments use different indenter geometries and very different applied loads, so the same material reads differently depending on which one you use. A material that measures Shore 95A will typically read around Shore 45D, not Shore 0 or 5D.

Because of this, there's no exact mathematical conversion between the two scales. Published conversion tables are approximations built from empirical data on specific material families and don't hold consistently across all compounds. ASTM D2240 doesn't define an official equivalence between scales. When a direct measurement is possible, always take one rather than converting.

The table below gives approximate equivalences across the overlap zone, for reference only:

How is Shore Hardness Measured?

Shore hardness, as previously mentioned, is measured using a handheld instrument called a durometer. It has two key parts: a spring-loaded indenter and a gauge. When you press the instrument flat against the material's surface, the spring exerts a defined force and the gauge reads the depth of penetration on a 0 to 100 scale.

Two testing conditions matter for accuracy. The specimen needs to be at least 6mm thick: thinner samples can give falsely high readings because the hardness of the backing surface starts to influence the result. The reading should also be taken within one second of applying pressure, because viscoelastic materials continue to deform under load (a phenomenon known as creep) and the value drifts if you wait. Both requirements are set out in ASTM D2240 and ISO 868.

Shore Hardness of Common Engineering Materials

The scale categories above give you a feel for what a Shore hardness value means in general terms. When it comes to specifying components, though, you need to know where specific materials sit.

The table below covers all the rubber and plastic materials available in Accu's range of precision fasteners, O-rings, grommets and other engineering components, along with their typical Shore hardness ranges, the appropriate scale for each and links to the relevant Accu product pages. Metals aren't included as Shore hardness testing doesn't apply to them. For metal components, Rockwell is the appropriate test.

How Shore Hardness Affects Component Performance

Shore hardness isn't just a number on a datasheet. It has direct, practical consequences for how a component behaves in service. Here's what that looks like across the main application areas.

Sealing applications: O-rings and gaskets

Shore hardness impacts how well a seal can conform to a mating surface. Softer materials (Shore A 40 to 60) deform more readily under compression, filling surface irregularities and providing better sealing at low contact pressures. Harder materials (Shore A 70 to 90) require more compression force to achieve the same seal, but offer better extrusion resistance under higher pressure.

Extrusion resistance matters in hydraulic and pneumatic systems because a seal under pressure can deform into the clearance gap between mating components, causing leakage or seal damage. High-pressure applications like these require components with a higher Shore rating, typically Shore A 70 to 90 is the right choice. Most O-rings are offered at 70A as a standard grade, or 90A as a hard grade. 70A is ideal for the majority of applications where static sealing is required, whereas 90A is used for high-pressure applications or where extrusion resistance is paramount.

Vibration isolation and damping

Vibration isolators like rubber mounts, anti-vibration grommets and bushings rely on the elastic properties of the rubber compound. Softer materials (Shore A 30 to 50) have lower dynamic stiffness, providing better isolation of low-frequency vibration. Harder materials (Shore A 60 to 80) are stiffer, carrying higher static loads but transmitting more vibration at low frequencies.

A common engineering rule of thumb: softer mounts for light loads and sensitive electronics; harder mounts for heavy machinery and structural applications.

Wear and abrasion resistance

Within the Shore A range, harder compounds tend to have better abrasion resistance. A Shore A 70 compound will typically outperform a Shore A 40 compound in sliding or abrasive contact. However, this is highly dependent on the base polymer. For example, a soft polyurethane can outperform a hard natural rubber in abrasion resistance despite having a lower Shore hardness.

For polymer bearings, bushings and wear pads, Shore D values are relevant. Harder materials (Shore D 70 to 85 in nylon, acetal or PEEK) are ideal as they resist deformation under load and maintain dimensional accuracy under normal service conditions.

Flexibility and assembly considerations

Softer materials are, by their nature, easier to assemble. A Shore A 40 O-ring is straightforward to stretch over a shaft and seat in a groove, whereas a Shore A 90 O-ring of the same cross-section requires much more force and is at greater risk of damage during installation.

For grommets, cable protectors and push-fit seals that must be installed by hand, Shore A 50 to 70 is generally the practical range. Below 40A, the component may lack sufficient shape retention to seat correctly; above 80A, installation force may be impractical without tools.

Shore Hardness vs Other Hardness Scales

Shore hardness exists alongside several other hardness testing methods, each suited to a different class of material. They measure different things using different instruments and the numbers they produce cannot be compared across systems.

Rockwell hardness is the standard for metals and, at the upper end of the Shore D scale, hard polymers. The most common Rockwell scales in engineering practice are B (for softer metals such as aluminium and brass) and C (for hardened steels). 

For polymers, Rockwell R and Rockwell M are used. As a general rule, materials at Shore D 80 to 90 begin to fall within the measurable range of Rockwell R. Above Shore D 90, Rockwell testing is typically more appropriate than Shore D as the durometer becomes less sensitive to differences between very hard materials.

Vickers (HV) and Brinell (HB) hardness tests are both used for metals. Neither has any relevance to rubber or polymer testing.

IRHD, or International Rubber Hardness Degrees, is an alternative rubber hardness scale defined by ISO 48-4 that engineers may encounter on European supplier datasheets. IRHD values are broadly comparable to Shore A values for most rubber compounds in the mid-range (40 to 80), but the methodology differs: IRHD uses a ball indenter with a dead-weight load applied over a fixed time period, rather than the spring-loaded near-instantaneous reading of a Shore durometer. The two systems will not always give identical values for the same material.

The practical takeaway is simple: always check which hardness scale a datasheet value refers to before using it to specify a component. A hardness value quoted without a scale designation is ambiguous.

Choosing the Right Shore Hardness for Your Application

The table below brings the recommendations from across this guide together into a single reference. Each row maps a common application scenario to a recommended Shore hardness range and the key considerations that apply to it.

Note: hardness values given are typical starting points for specification. Always verify against the specific material datasheet and test at the operating temperature where relevant. For dynamic sealing and vibration isolation applications, Shore hardness is one input alongside geometry, compression set, load-deflection data and media compatibility.

Key Takeaways

Shore hardness shows up on more datasheets than almost any other material property for rubbers and polymers, and it's also one of the more frequently misread. Here's what to carry away from this guide.

• Shore hardness is a family of scales, not a single test. The three scales you'll encounter most often in engineering are Shore 00, Shore A and Shore D, each using a different indenter geometry and applied load.

• No universal conversion between Shore A and Shore D exists. Published conversion tables are approximations derived from empirical data on specific material families. When a direct measurement is possible, take one rather than converting.

• Temperature changes Shore hardness. A material's room-temperature datasheet value may not reflect its behaviour at operating temperature: hardness decreases as temperature rises and increases as it falls.

• When a datasheet quotes a hardness value without specifying the scale, context matters. If the material is a rubber or elastomer, assume Shore A. If it is a semi-rigid or rigid plastic, assume Shore D. If the scale is genuinely ambiguous, contact the supplier before specifying the component.

Further Reading

• Engineering Rubber Guide: Types, Properties and Selection

• A Guide to Plastics

• What Is an O Ring?

• What Are Sealing Screws?

FAQs:

Q: What is the Shore hardness scale?

A: The Shore hardness scale is a standardised method of measuring the resistance of elastomers, rubbers and polymers to permanent indentation. The test is carried out using an instrument called a durometer, which presses a defined indenter into the material surface under a calibrated load. The resulting value is a dimensionless number from 0 to 100: 0 represents complete penetration of the indenter and 100 represents no penetration at all.

Shore hardness is not a single scale but a family of scales. The most commonly used in engineering are Shore A, for soft to medium-hard rubbers and elastomers, and Shore D, for hard rubbers and engineering plastics. 

Q: What is the difference between Shore A and Shore D?

A: Shore A and Shore D use different indenter geometries and different applied loads, which makes each scale appropriate for a different range of materials.

A Shore A value and a Shore D value cannot be compared directly. A material with a Shore A hardness of 70 and a material with a Shore D hardness of 70 are not the same hardness.

Q: What does Shore 70A mean?

A: Shore 70A is a Shore A hardness reading of 70, measured using a Type A durometer in accordance with ASTM D2240 or ISO 868. In practical terms, Shore 70A corresponds to the hardness of a rubber tyre tread: firm, with clear resistance to deformation, but still flexible under sustained hand pressure.

Shore 70A is the most widely specified hardness grade for standard O-rings and general-purpose seals. Most rubber compound suppliers offer their materials at Shore 70A as a standard grade, with Shore 90A as the alternative for high-pressure applications.

Q: What is Shore hardness in rubber?

A: For rubber and elastomer components, Shore A is the standard measurement scale. Common rubber hardness grades are Shore A 40, 50, 60, 70 and 90. The Shore 70A grade is the most common default for O-rings and general sealing applications; Shore 90A is used where higher extrusion resistance is required, typically in systems operating above 10 bar.

Different rubber compounds cover different hardness ranges. NBR is typically available from Shore A 40 to 90. Silicone rubber spans a particularly wide range, from around Shore A 10 for very soft medical-grade compounds through to Shore A 80 for hard engineering grades. FKM (Viton) is most commonly specified at Shore A 70 to 90.

Q: Is Shore A 95 the same as Shore D 45?

A: Approximately, but not exactly. The Shore A scale becomes unreliable above around Shore A 95 because most of the indenter's range has been consumed and small differences in hardness produce very little change in the reading. For this reason, materials in the Shore A 85 to 95 range are often reported on the Shore D scale instead, where the sharper indenter and greater load give more meaningful differentiation.

Shore A 95 is roughly equivalent to Shore D 45, though the precise equivalence depends on the material type and compound formulation. Published conversion tables are approximations derived from empirical data; they should be treated as a guide rather than a precise conversion. 

Q: What is the Shore hardness of silicone rubber?

A: Silicone rubber is available across a very wide range of Shore A hardnesses, typically from Shore A 10 (very soft medical-grade gel silicone) to Shore A 80 (hard silicone rubber sheet or moulded components). Standard engineering silicone for gaskets, grommets and seals typically falls in the Shore A 40 to 70 range.

The wide range reflects the variety of silicone formulations available. When specifying silicone components, always check the Shore hardness of the specific grade rather than assuming a default value.

Q: Does temperature affect Shore hardness?

A: Yes, significantly. Shore hardness decreases as temperature increases and increases as temperature decreases. A silicone O-ring rated Shore A 50 at room temperature (23°C) will test softer at 150°C and harder at minus 40°C.