Hardness Measurement

Checkline Europe's Technical Support 10 Jul 2026

Hardness is an unusual quality-control number, and it helps to know why before you buy anything to measure it: there is no single "hardness" the way there is a single length or weight. What you get depends on the test you use. Press a pointed indenter into rubber and you read one kind of hardness; bounce a small hammer off steel and you read another — and the two numbers cannot be compared. So the first decision is not which instrument to buy, but which method suits the material in front of you, because the method is what defines the number.

That is why hardness testing turns up almost everywhere material condition has to be confirmed quickly — checking incoming rubber, steel or composite, watching a production line, or testing a part already in service. It is fast, needs little preparation and usually leaves the part usable, which is why it is reached for far more often than slow laboratory tests. But no single instrument covers the whole range: a gauge built for soft rubber tells you nothing about hardened steel. Settle what you are testing, and where, and the right method follows.

1. Why the Method Defines the Number

Hardness cannot be written in basic units the way force or length can. It is defined by the test itself — the shape of the indenter, how hard and how long it presses, and what the instrument measures in response. That is why a Shore reading, a Leeb rebound value and a Barcol depth all call themselves "hardness" yet do not convert cleanly into one another. Two families of test sit behind them. Indentation methods (Shore, IRHD, Barcol) press a defined tip into the surface and measure the impression it leaves. Rebound methods (Leeb) fire a small impact body at the surface and compare how fast it comes back with how fast it went in. Pencil and scratch tests on coatings are a different question again, handled with coating work rather than bulk-material hardness. The practical upshot is simple: a hardness number only means something when it is recorded with the method and scale that produced it.

2. Which material are you testing?

Match the method to the material; each has its own page. For rubber and flexible plastics, Shore Hardness covers durometer testing across the Shore A, D and OO scales — the everyday production rubber test. Where a laboratory needs tighter control than a hand-held durometer gives, Rubber Hardness (IRHD) uses a dead-weight ball on the same materials to take the operator's pressure out of the result. For rigid plastics and fibre-reinforced composites, Barcol Hardness confirms cure state in aerospace, marine and wind-energy work. For metals, Metal Hardness covers portable Leeb rebound testing and the conversions to Rockwell, Vickers and Brinell that field work relies on. Underneath all of them, Hardness Calibration and Standards holds the verification, test-block and traceability side that keeps any reading trustworthy. Coating-related hardness — pencil and scratch tests — sits apart under Coating Hardness, part of Coating Inspection.

With the method matched to the material, the hardness measurement selection guide is where the choice of instrument gets made.

3. Standards for the Different Hardness Methods

Because each method is its own test, each has its own standard. Shore durometer testing follows ASTM D2240 and ISO 868. IRHD rubber testing follows ISO 48. Barcol testing of rigid composites follows ASTM D2583. Portable Leeb rebound testing follows ASTM A956 and ISO 16859. The verification and traceability that keep all of them honest — the reference test blocks, the calibration intervals, the conversion tables — are set out in full under Hardness Calibration and Standards. The one habit that matters most in practice is to record the standard and the scale next to the number. Without them, a hardness value cannot be checked, converted or trusted later.

4. Frequently Asked Questions

1. What is the difference between Shore and IRHD hardness?

Shore hardness uses a spring-loaded indenter pressed against the specimen surface by the operator, and the reading depends on the applied force and timing. IRHD uses a ball indenter driven into the specimen by a calibrated dead-weight, removing operator force variability. The two methods often produce similar numerical values for mid-range rubbers, but they are not interchangeable — each follows a different standard and measures a slightly different material response.

2. Can hardness values be converted between different scales?

Conversions between hardness scales are possible using empirical tables published in standards such as ASTM E140 for metals. These conversions are approximate because different scales measure different physical interactions. Conversions between Shore and IRHD for rubber are documented in ISO 48 and related standards. Converting across material families (for example, from a Shore D reading on a plastic to a Rockwell value) is generally not meaningful.

3. How does temperature affect hardness readings?

Temperature has a pronounced effect on the hardness of polymers: elastomers become harder at lower temperatures and softer at higher temperatures, which can shift a Shore A reading by several points over a 20 °C range. Metals are less temperature-sensitive in the ambient range, but significant temperature changes can affect the instrument's electronic accuracy. Standards typically specify a reference temperature (usually 23 ± 2 °C) for comparable results.

4. Is hardness testing truly non-destructive?

Most hardness tests leave a small impression on the specimen surface. For many applications — testing a production rubber moulding, verifying a steel forging — this impression is insignificant and the test is considered non-destructive. For sensitive surfaces such as optical coatings or precision-machined components, even a small indentation may be unacceptable. In such cases, testing is performed on a sacrificial witness sample or on a non-critical area of the part.

5. Glossary

Barcol hardness	A hardness scale for rigid plastics and composites, determined by the depth of penetration of a sharp, spring-loaded indenter under standardised conditions.
Durometer	An instrument that measures indentation hardness of elastomers and plastics by pressing a spring-loaded indenter into the surface and reading the depth of penetration on a graduated scale.
Indenter	The precisely shaped tip (cone, ball, flat rod) pressed into the material surface during a hardness test to create the indentation from which hardness is derived.
IRHD	International Rubber Hardness Degrees; a hardness scale for rubber based on ball indentation under dead-weight loading, designed to reduce operator variability.
Leeb hardness	A rebound hardness method where a spring-loaded impact body strikes the surface and the ratio of rebound to impact velocity is measured, expressed as HL with an impact device suffix.
Shore hardness	An indentation hardness scale for elastomers and plastics, measured by pressing a spring-loaded indenter of defined geometry into the material surface and reading the penetration depth.
Test block	A certified reference specimen of known hardness, used to verify and calibrate hardness testing instruments.
Traceability	An unbroken chain of documented comparisons linking a measurement result to a recognised reference standard, typically maintained by a national metrology institute.
Viscoelasticity	The property of materials that exhibit both viscous (time-dependent) and elastic (instantaneous) behaviour when deformed, causing hardness readings to change with loading duration.