Durometer Scale Explained: Shore A vs Shore D for O-Rings
Updated: Nov 6, 2026
Quick answer: what is the durometer scale?
The durometer scale measures how hard a rubber or plastic is by how far a spring-loaded indenter sinks into it. Readings run from 0 to 100, and higher means harder. Rubber seals use the Shore A scale; harder plastics use Shore D. Most O-rings are 70 Shore A, a medium hardness that balances sealing force, extrusion resistance and friction.
If you buy or design seals, durometer is the second number you look at after size. It changes how easily a seal deforms, how much pressure it can hold, and how it behaves in a groove. This guide covers how the scales work, how hardness is tested, and how to choose a durometer for an O-ring.
Durometer scale at a glance: O-ring hardness bands
The Canyon O-ring store groups compounds into five Shore A bands. You can filter by any of them.
Band | Shore A | Typical use | Example compounds |
|---|---|---|---|
30–50 | Low closure force, fragile or uneven mating parts, low pressure | SI50OR00 silicone | |
51–65 | Rough surfaces, low-pressure static seals, vacuum flanges | CP60BK00 Canrez® FFKM | |
66–75 | General-purpose static and dynamic seals | ||
76–85 | Higher pressure, larger extrusion gaps | 80 Shore A compounds in most families | |
86–95 | High pressure, extrusion and wear resistance, rapid gas decompression |
What does durometer measure?
Durometer measures indentation hardness: how much a material resists a small, blunt point pressed into its surface. The instrument is also called a durometer. A calibrated spring pushes an indenter into the sample, and the gauge reads 0 when the indenter sinks fully and 100 when it does not sink at all.
Hardness is not the same as strength. A 90 Shore A O-ring is not "stronger" than a 70 Shore A one in every way. It is stiffer, so it needs more force to compress and resists being pushed into a gap. Tensile strength, elongation and compression set are separate properties listed on each compound's datasheet.
The test method for durometer hardness is ASTM D2240, with ISO 48-4 as the international equivalent. Both define the instruments, the indenter shapes and how a reading is taken.
Shore A vs Shore D (and Shore 00 and Shore M)
There are several Shore scales. Each uses a different indenter shape and spring force, so each suits a different range of materials.
Scale | Indenter and force | Used for |
|---|---|---|
Shore 00 | Rounded tip, light spring | Gels, foams, very soft rubber |
Shore A | Blunt tip, moderate spring | Rubber O-rings, gaskets and seals |
Shore D | Sharp, pointed tip, stiff spring | Hard rubber, polyurethane, plastics such as PTFE and PEEK |
Shore M | Small, light indenter | Thin or small parts, such as O-ring cross-sections |
The scales overlap but do not convert exactly. As a rough guide, a material near 90–95 Shore A reads somewhere in the low-to-mid 40s on Shore D. Because the indenters differ, no formula turns one scale into the other for every material. If a drawing calls out Shore D for a rubber part, or Shore A for a plastic, check the spec with the designer before quoting.
For most elastomer seals, Shore A is the only scale you need. Shore D comes up with hard polyurethane and plastic parts such as PTFE backup rings.
What 70 Shore A means for an O-ring
70 Shore A is the default hardness for O-rings. Most standard AS568 groove designs assume a 70 durometer seal, which is why it is the most widely stocked hardness. It is soft enough to seal well at low pressure and hard enough to resist extrusion in typical clearances.
In a part number, Canyon puts the hardness right in the compound code. In NB70BK00, "NB" is nitrile, "70" is 70 Shore A and "BK" is black. That compound is rated -35°C to 125°C (-31°F to 257°F). EP70BK01 is a 70 Shore A EPDM rated -55°C to 130°C (-67°F to 266°F).
The number is also a nominal value. Hardness specs usually allow a tolerance, commonly ±5 points, so a "70" compound can read anywhere from about 65 to 75 and still meet spec.
Softer vs harder O-rings: the trade-offs
Changing durometer moves several properties at once. This is what typically changes as you go up or down from 70 Shore A.
Property | Softer (50–65 Shore A) | Harder (80–90 Shore A) |
|---|---|---|
Force to compress | Lower | Higher |
Sealing at low pressure | Better, conforms to rough surfaces | Needs more squeeze or smoother surfaces |
Extrusion resistance | Lower | Higher |
Friction and breakaway | Usually higher | Usually lower |
Wear and abrasion resistance | Lower | Higher |
Installation | Easier to stretch over parts | Stiffer, easier to cut or twist |
Hardness is only one input. Material choice still controls temperature and chemical resistance, and the groove still controls squeeze and gland fill. Our General Groove Design Guide covers both.
How is durometer measured?
A hardness reading depends on how the test is run, not just the material. Three things matter most:
- Sample thickness. Standard Shore A tests need a flat sample at least 6 mm (about 0.24 in) thick. Thinner samples let the indenter feel the surface underneath, which reads harder than the true value.
- Dwell time. Rubber relaxes under the indenter, so a reading taken instantly is higher than one taken after a few seconds. Specs should state when the reading is taken.
- Temperature. Rubber gets harder as it gets colder and softer as it gets warmer. Tests are run at standard room temperature.
Why you can't test most O-rings with a Shore A gauge
A small O-ring's round cross-section is too thin and curved for a Shore A reading. That is why compound hardness is normally measured on flat molded test slabs or buttons, then reported on the datasheet. When the finished O-ring itself must be checked, labs use a Shore M or micro-IRHD tester (ISO 48-2), built for small, curved samples. Readings on an O-ring will not match the slab value exactly, so compare like with like.
If you need to know what an unknown O-ring is made of, hardness alone will not tell you. A bounce tester helps narrow down the material family; see our post on identifying O-ring materials with a bounce tester. Canyon also offers material testing and part identification.
How hardness changes in service
The durometer on a datasheet is the hardness of a new part at room temperature. In service it can drift:
- Cold makes elastomers harder and less flexible, which is why low-temperature sealing needs compounds made for it.
- Fluid swell usually softens a seal. Chemical compatibility testing often reports the hardness change after immersion; see the Chemical Compatibility Table.
- Heat aging can harden or soften a compound, depending on the material.
- Compression set is a separate effect: the seal takes a permanent set and loses sealing force. See Compression Set in Elastomers.
A large hardness change on a used seal is a useful clue during failure analysis. Our O-Ring Damage and Failure guide covers what to look for.
How to choose a durometer for an O-ring
- Start at 70 Shore A. It suits most static and many dynamic seals in standard AS568 grooves.
- Go harder (80–90) for pressure and extrusion. Higher system pressure, larger clearance gaps or rapid gas decompression call for a harder compound, often with a backup ring.
- Go softer (50–65) for low force or rough surfaces. Soft compounds help with low clamp loads, plastic housings, uneven flanges and low-pressure or vacuum seals. Our post on 60 Shore A FFKM covers one example.
- Pick the material first, then the hardness. Temperature and chemistry set the material family. Many families come in several hardnesses: Canrez® FFKM runs from CP60BK00 (60 Shore A) through CP75BK01 (75 Shore A, -15°C to 320°C) to CP90BK01 (90 Shore A, -15°C to 310°C).
- Check the groove. A harder seal needs more force to reach the same squeeze. Run the numbers with the face seal, rod or piston groove calculators, and test before production.
The Elastomer Material Comparison is a good next step when you are weighing families.
FAQ
What does 70 durometer mean? It means the material reads 70 on the Shore A durometer scale. For O-rings that is medium hardness, the standard choice for most static and dynamic seals.
Is a higher durometer harder or softer? Harder. The scale runs from 0 to 100, and a higher number means the indenter sinks in less.
What is the difference between Shore A and Shore D? Shore A uses a blunt indenter for rubber and soft elastomers. Shore D uses a sharp indenter and a stiffer spring for hard rubber and plastics, and the two scales do not convert exactly.
What durometer O-ring should I use? Start with 70 Shore A. Move to 80–90 for high pressure or large extrusion gaps, and to 50–65 for low clamp force, rough surfaces or low-pressure seals.
How do you measure the hardness of an O-ring? Compound hardness is measured on flat test slabs with a Shore A durometer. A finished O-ring is checked with a Shore M or micro-IRHD tester, which is made for small, curved samples.
Does O-ring hardness change over time? Yes. Cold, heat aging and fluid exposure can all change hardness, which is why compatibility tests often report hardness change.
Find O-rings by durometer
Canyon Components stocks millions of O-rings, with live price and stock online. Filter the O-ring store by durometer, material and size. For a hardness or compound you don't see, Request a Quote and our engineering team will help you choose.
