Shore A Hardness Scale: Complete Guide & Chart

Have you ever wondered why some rubber feels soft and bendy, while other kinds feel stiff and hard? The Shore A Hardness Scale helps us measure these differences. This important tool helps engineers, manufacturers, and everyday users understand how soft or hard rubber and plastic materials are. Let’s dive into everything you need to know about this key testing method.

What is the Shore A Scale?

The Shore A Hardness Scale is a standardized test that measures how soft or hard flexible materials are. Created to test soft elastomers, rubbers, and flexible plastics, this scale gives us numbers from 0 to 100 that tell us about a material’s firmness.

Think about it this way: if you press your thumb into a rubber eraser and it barely moves, it’s probably high on the Shore A scale. If you press into a gel insole and it squishes easily, it’s low on the scale.

The Shore A scale matters because:

It helps link material hardness to how it will perform
It’s vital for quality control in manufacturing
It helps engineers pick the right materials for specific jobs

Shore A vs. Shore D: Key Differences

Though they might sound similar, Shore A and Shore D scales measure different types of materials:

Scale	Materials Tested	Indenter Type	Typical Uses
Shore A	Soft elastomers, gels, rubber	Rounded cone	Rubber bands, silicone products
Shore D	Hard plastics, rigid polymers	Sharp cone	PVC pipes, hard hats, plastics

The key differences include:

Shore A uses a rounded cone indenter with lower spring force
Shore D uses a sharper cone with higher testing force
Shore A 95 roughly equals Shore D 45 (there’s some overlap at the high end)

When you need to test something very soft like foam or a sponge, neither works well – you’d need the Shore OO scale instead.

How Shore A Testing Works

The testing process is pretty straightforward, but it follows strict rules set by organizations like ASTM International through their ASTM D2240 standard and the International Organization for Standardization’s ISO 868 standard.

Testing Equipment

The main tool used is called a durometer. This device has:

A spring-loaded indenter (the part that pushes into the material)
A calibrated gauge that shows the reading
A flat presser foot that touches the material surface

Popular brands include Rex Gauge and other ASTM-compliant testers that cost between $200-$500 for handheld models.

Step-by-Step Process

Prepare the sample – The material needs a smooth, flat surface
Position the durometer – Place it perpendicular to the material
Apply pressure – Push the durometer against the material for exactly 15 seconds
Read the value – Record the number shown on the gauge (0-100)

For accurate readings, the material should be at least 6mm (1/4 inch) thick, and multiple readings should be taken and averaged.

Shore A Applications by Industry

The Shore A scale is used across many industries for different products:

Automotive

The automotive industry relies on rubber CNC machining to create precise rubber parts with specific Shore A hardness. Typical applications include:

Seals and gaskets: Usually Shore A 50-70
Vibration dampeners: Shore A 40-60
Weather stripping: Shore A 55-75

Footwear

Shoe soles: Range from Shore A 30 (soft) to 80 (firm)
Gel insoles: Very soft at Shore A 10-30
Athletic shoe midsoles: Usually Shore A 55-65 for good balance of cushioning and support

Medical Devices

Medical device manufacturers often use precision CNC machining to create parts with exact Shore A hardness values for:

Silicone tubing: Usually Shore A 20-50
Prosthetic liners: Shore A 30-40 for comfort
Dental materials: Shore A 60-90 depending on the application

Consumer Goods

Erasers: Shore A 20-40
Phone cases: Shore A 70-90 for protection with slight flex
Kitchen utensils: Shore A 60-80 for grip and durability

Interpreting Shore A Values

Understanding what the numbers mean helps you choose the right material:

Typical Ranges

Soft (A 10-40): Very flexible materials like soft silicone, gel products
Medium (A 50-80): Car tires, shoe soles, firm rubber products
Hard (A 90+): Hard rubber, approaching rigid plastics

Here’s a helpful breakdown of what different Shore A values feel like:

Shore A Value	Feel	Common Examples
10-30A	Very soft, easily compressed	Gel insoles, soft silicone
40-60A	Medium soft, moderate resistance	Automotive seals, pencil erasers
70-90A	Firm with minimal compression	Phone cases, hard rubber wheels
90+A	Very firm, minimal give	Industrial rollers, hard rubber mallets

Material Selection Guide

When choosing materials based on Shore A:

For high-flexibility needs: Choose lower Shore A values (10-40)
For wear resistance: Higher Shore A values (70+) will last longer
For vibration damping: Mid-range values (40-60) often work best

Shore A hardness comparison showing material deformation

Limitations & Considerations

The Shore A scale isn’t perfect and has some limitations to keep in mind:

Not for All Materials

This scale won’t work well for:

Very soft foams (use Shore OO instead)
Textured surfaces (readings will be inconsistent)
Very thin materials (less than 6mm thick)

Environmental Factors

Testing conditions matter a lot:

Temperature changes can alter readings by about 2A points per 10°C change
Humidity can affect some materials’ hardness
Material age – some rubbers harden over time (called “aging”)

When performing quality control testing on machined rubber parts, these environmental factors must be controlled.

Complementary Tests

Shore A hardness should be used alongside other tests for a complete picture:

Tensile strength tests
Compression set analysis
Rebound testing
Tear resistance measurements

Shore A vs. Other Hardness Scales

Different hardness scales serve different purposes:

Rockwell Scale

Used for metals and very hard materials
Measures permanent indentation
Uses different scales (B, C, etc.) for different hardness ranges

Brinell/Vickers

For metals and ceramics
Uses heavy loads and measures indentation area
More suitable for very hard, non-elastic materials

Shore OO

For ultra-soft materials like foams and sponges
Uses a much more sensitive spring force
Scaled 0-100 like Shore A, but much softer range

Case Studies

Industrial Urethanes

PSI Urethanes creates custom formulations with Shore A values from 60-90 for different uses. Their most popular products are:

Shore A 70 rollers for paper mills that balance grip with durability
Shore A 85 bumpers used in automated warehouse systems

Xometry’s Material Database

Xometry’s comprehensive database shows how Shore A ratings impact 3D printed elastomers and other flexible materials:

Their TPU materials range from Shore A 85 (flexible) to A 95 (semi-rigid)
Customer satisfaction is highest with A 90 materials, which balance durability with just enough flex

Common products with different Shore A hardness ratings

FAQs

Can Shore A values be converted to Shore D?

No direct formula exists, but there’s some overlap. The highest Shore A values (95+) roughly equal the lowest Shore D values (around 45).

How can I test Shore A hardness at home?

You can buy handheld durometers for $200-$500, but for casual testing, comparison to known objects can help (pencil erasers are about Shore A 30-40).

What’s the hardest Shore A material?

The highest Shore A readings approach 100, with rigid urethanes reaching A 95+. Beyond this, materials would be measured on the Shore D scale instead.

How accurate are Shore A measurements?

Professional testers typically have ±2 points accuracy, but can vary by ±5 points between different testers and environmental conditions.

Conclusion

The Shore A Hardness Scale plays a vital role in understanding and selecting the right elastomers and flexible materials for countless applications. From the tires on your car to the case on your phone, Shore A hardness influences how materials perform in the real world.

When working with custom CNC machining services for rubber or plastic parts, understanding Shore A hardness helps ensure your parts have the right balance of flexibility and durability.

By knowing how to interpret these values and recognize their limitations, you can make better decisions about materials for both industrial and everyday uses.