Shore (durometer) and IRHD are the two main methods for measuring the hardness of rubber, elastomers and plastics. The Shore method presses a spring loaded indenter into the surface and reads a value from 0 to 100, on the Shore A scale for soft materials or the Shore D scale for hard rubber and plastics, with the O and OO scales available for very soft materials. IRHD (International Rubber Hardness Degrees) applies a defined force with a dead load and measures the penetration. Because polymers are soft, time and temperature dependent and easily deformed, correct technique matters as much as the instrument. The governing standards are ISO 868 and ASTM D2240 for Shore, and the ISO 48 series and ASTM D1415 for IRHD. For the wider context see the Hardness Testing Academy.

What is Shore hardness?

Shore hardness is measured with a durometer, a spring loaded indenter that presses into the material; the hardness value, from 0 (very soft) to 100 (very hard), depends on how far the indenter penetrates against the calibrated spring. Because the spring force rises with penetration, the reading is a measure of the material resistance at a defined displacement rather than a fixed force. Many polymers creep under load, so the value can fall in the seconds after contact, which is why standards let you report either an instantaneous reading or a reading taken after a defined delay. The method is quick, inexpensive and portable, and it is the everyday choice for incoming and in process control of rubber and plastic parts.

Shore scales (A, D, O, OO)

Different durometer scales use different indenter shapes and spring forces so that each one resolves a particular hardness range. The most common are Shore A and Shore D, with the softer O and OO scales for foams, gels and very soft rubber.

What is IRHD hardness?

IRHD (International Rubber Hardness Degrees) measures the penetration of a defined indenter under a specified force, on a 0 to 100 scale where higher means harder. Unlike the spring based durometer, IRHD uses a dead load principle: a fixed force is applied and the additional penetration over a preliminary contact is measured, which makes it less sensitive to operator technique. It comes in several variants: normal (N) for medium hardness, high (H) for hard rubber, low (L) for soft rubber, and a micro (M) version for small or thin samples. IRHD is widely used in the rubber industry and on finished components such as O rings, seals and gaskets, where its small indenters and dead load principle give repeatable results on real parts.

Shore vs IRHD: which should you use?

How to measure correctly

For reliable results, use a flat sample of adequate thickness (generally at least 6 mm, stacking allowed only where the standard permits), keep the durometer perpendicular to the surface and away from edges, control temperature (commonly 23 degrees Celsius), take several readings at different points, and always report the timing of the reading, whether instantaneous or after the specified delay. A test stand that applies a controlled foot pressure removes much of the operator variation that hand held testing introduces.

Best practices

• Pick the right scale: use Shore A for soft and Shore D for hard materials, and the O or OO scales for foams and gels, so the reading sits in the well resolved part of the range.
• Mind thickness and edges: keep the sample at least 6 mm thick and test away from edges, because a thin sample or a nearby edge biases the value.
• Control temperature and timing: condition and test at the specified temperature, and report instantaneous or delayed readings consistently, since polymers creep.
• Use a stand and average: a durometer test stand gives repeatable foot pressure and timing, and several averaged points beat a single hand held reading.

Which standards apply?

Shore durometer hardness is defined by ISO 868 and ASTM D2240. IRHD is defined by the ISO 48 series (including micro IRHD) and ASTM D1415. Always follow the standard cited in your specification, and report the exact scale and conditions used.

Frequently asked questions