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A durometer reading is only meaningful when it carries its scale letter and its test conditions. This guide explains what a durometer is, how Shore A and Shore D differ, and how to read a hardness figure on a specification for medical tubing, silicone parts or elastomer components. It is general product information, not medical advice.
Short answer: what is durometer?
Durometer is a measure of the relative hardness of soft materials, usually plastic or rubber. A durometer instrument presses a specified indentor into the material under a specified force and for a specified time, and the depth of penetration is converted to a number. Shore A and Shore D are the two scales most often quoted. A is used for softer materials and D for harder ones, and a number is only meaningful with its scale letter (80 Shore A is not 80 Shore D).
Definition and scope
Intertek’s test encyclopedia describes durometer hardness as a way to determine the relative hardness of soft materials, usually plastic or rubber, by measuring the penetration of a specified indentor under specified conditions of force and time. It adds that the value is often used to identify or specify a particular hardness of elastomers, or as a quality-control measure on lots of material (Intertek Testlopedia, checked 2026-10-30).
The test method is standardised as ASTM D2240, and the same kind of test is covered by DIN ISO 7619-1. Wikipedia’s summary of the Shore durometer lists twelve scales in the ASTM method (A, B, C, D, DO, E, M, O, OO, OOO, OOO-S and R), each running from 0 to 100 (Wikipedia, “Shore durometer”, checked 2026-10-30).
How a durometer is used in practice
Intertek describes the test in three steps: the specimen sits on a hard, flat surface; the indentor is pressed into it, parallel to the surface; and the hardness is read within one second of firm contact, or as specified by the customer. Specimens are generally 6.4 mm (1/4 in) thick, and several thin specimens can be piled to reach that thickness, although one specimen is preferred (Intertek Testlopedia, checked 2026-10-30).
Rex Gauge, a durometer manufacturer, gives the reason for the thickness rule: the indentor can travel up to 0.100 in, so a specimen that is too thin lets the indentor sense the surface underneath and gives a false reading. Rex generally recommends samples not less than 1/4 in (6 mm) thick, and, for soft materials, at least 1/4 in (6 mm) of material around the sides of the indentor (Rex Gauge Durometers, “What is a Durometer”, checked 2026-10-30).
The reading time is a place where sources differ. Intertek gives one second (or as the customer specifies), while Wikipedia’s summary of the standard gives a measuring time of 15 seconds. Because sources differ, a specification should state its own reading time.
Types: Shore A vs Shore D
The two common scales differ in indentor shape and spring force. The figures below are from Rex Gauge’s published gauge specifications.
| Scale | Indentor (Rex Gauge) | Spring force (Rex Gauge) | Typical applications (Rex Gauge) |
|---|---|---|---|
| Shore A | Flat cone point, 35° | 822 g | Rubber, elastomers |
| Shore D | Sharp cone point, 30° | 10 lb | Hard rubber, plastics, thermoplastics |
Other scales exist for very soft or very thin materials. Rex lists, for example, spherical indentors for the O, OO and OOO types and a Type M gauge for materials thinner than 1/4 in (as thin as 0.050 in).
What is Shore A hardness?
Shore A is the scale for softer materials such as rubber and elastomers. A higher number means a harder material, on a 0 to 100 scale.
Shore hardness scale and chart: why not to convert
Many pages publish a Shore A to Shore D conversion chart. Rex Gauge’s own comparison chart carries the note that it is “for comparison purposes only”, and Wikipedia’s summary states that there is no simple relationship between a material’s durometer on one scale and another scale, or other hardness tests. For a purchaser or specifier, the practical consequence is to compare only like scale against like scale, measured under like conditions, and to avoid a conversion table for any decision.
Wikipedia’s summary adds that results below 10 or above 90 on a scale are not to be trusted, and that the measurement should be redone with the adjacent scale type (Wikipedia, checked 2026-10-30).
Where it matters for labs, clinics and supply decisions
Medical-supply and laboratory documents quote hardness in different ways. The hardness figures below are already cited in other CASRAI guides, each as stated by the supplier or guide named.
| Material | How hardness is stated | CASRAI guide |
|---|---|---|
| Thermoplastic polyurethane, Texin RxT80A | 80 Shore A | What Is TPU? |
| Silicone earmolds | 25 to 60 Shore A | Earmold Types |
| Pebax MED (Arkema) | A broad range of durometers, with no single value | What Is Pebax? |
Checklist: reading a durometer value on a specification
- Scale letter. The number must be followed by its scale, for example 80 Shore A. The same number on a different scale describes a different material.
- Specimen thickness. Sources describe a specimen of roughly 6 mm (1/4 in) or 6.4 mm. A value measured on a thinner specimen may reflect the surface beneath it.
- Reading time. Intertek reads within one second; Wikipedia’s summary of the standard cites 15 seconds. If two suppliers quote the same number, confirm they read at the same time.
- Usable range. A figure below 10 or above 90 on a scale is considered unreliable, in which case the adjacent scale is used.
- Range versus value. Arkema describes Pebax MED as available in a broad range of durometers; a family description is not a single figure, so the grade matters.
Common misunderstandings
- “Any two Shore numbers can be compared.” The A and D scales use different indentors and spring forces, so numbers on different scales cannot be compared directly.
- “Durometer is a property of the material alone.” The reading depends on the indentor, force, time and specimen, which is why Intertek and Rex specify them.
- “A conversion chart gives exact equivalents.” The manufacturer’s own chart is described as for comparison purposes only.
FAQ
What is durometer?
It is a measure of the relative hardness of soft materials, usually plastic or rubber, found by pressing a specified indentor into the material under specified force and time (Intertek Testlopedia).
What is Shore A hardness?
It is the durometer reading on the Shore A scale, the scale Rex Gauge lists for rubber and elastomers. A higher number means a harder material.
What is Shore hardness?
Shore hardness is the durometer reading expressed on one of the Shore scales, each running 0 to 100. Shore A and Shore D are the common ones.
What is the durometer scale?
ASTM D2240 defines twelve scales (A, B, C, D, DO, E, M, O, OO, OOO, OOO-S and R), per Wikipedia’s summary; A and D are the most commonly used.
Is there a Shore A hardness chart that converts to Shore D?
Charts exist, but Rex Gauge describes its own chart as for comparison purposes only, and there is no simple relationship between scales. Compare like scale against like scale.
Related CASRAI guides on materials and components: polyolefin and autoclave gasket.
Sources
- Rex Gauge Durometers, “What is a Durometer”, https://www.durometer.com/?p=269, checked 2026-10-30.
- Intertek, Testlopedia, “Shore Hardness, ASTM D2240”, https://www.intertek.com/polymers-plastics/testlopedia/shore-hardness-astm-d2240/, checked 2026-10-30.
- Wikipedia, “Shore durometer” (for the standards ASTM D2240 and DIN ISO 7619-1), https://en.wikipedia.org/wiki/Shore_durometer, checked 2026-10-30.
