Shore A vs. Shore D: How to Read Coating Hardness Values

A coating data sheet lists a hardness value of 50 Shore D. Another brochure shows a larger number on Shore A. Which coating is harder? The missing step is to read the letter with the number. Shore A and Shore D are different measurement scales, so the larger numeral alone does not settle the comparison.

Understanding that distinction helps you ask better purchasing questions and keep a specification readable. It does not turn a single hardness result into a complete coating recommendation. Start by preserving the full reported value, its test method and the source document.

What a Shore hardness number measures

Shore hardness is an indentation measurement. A durometer uses a defined indenter and force to measure how the material responds under specified test conditions. ASTM D2240 describes this as an empirical test and explains that indenter geometry and applied force influence the result.

In plain language, the measurement concerns resistance to that particular indentation test. It is not a percentage of “toughness,” a coating thickness or a count of years the system will last. Keeping the property name beside the result prevents a number from acquiring a meaning it was never intended to have.

Keep Shore A and Shore D in separate columns

Instrument manufacturer DeFelsko identifies separate Shore A and Shore D instruments for different hardness ranges. Its Shore A applications include softer rubbers and elastomers; Shore D covers harder materials such as hard rubber, epoxies and hard plastics. Both instruments report a numerical range from 0 to 100, but they do not use an interchangeable scale.

ASTM specifically cautions that there is no simple relationship between results from different durometer types. Do not subtract a Shore A value from a Shore D value, rank two products by the bare numerals or treat a generic conversion chart as an approved product specification.

For example, if one supplier writes “80A” and another writes “50D,” a comparison sheet should retain those two complete entries. Leave a separate “directly comparable?” field unresolved until the relevant technical contact confirms an appropriate basis. A blank field is better than a confident-looking conversion that nobody has approved.

Read a real coating entry without stripping away context

The linked ArmorThane HighLine 410 technical data sheet lists hardness as 50 ± 5 Shore D, ASTM D-2240. Record the central value, the stated variation, the D scale and the method together. Writing only “hardness 50” loses important information.

The same sheet lists tensile strength, tear strength, elongation, abrasion resistance and adhesion separately. It also notes that equipment and application parameters affect physical properties and describes the values as approximate. Those qualifications belong with a technical comparison, rather than disappearing when a number moves into a quote or purchasing email.

You can use the HighLine 410 shop listing to identify the product being discussed, then confirm the current technical sheet with ArmorThane for your project. The example here explains how to read a value; it does not establish suitability for a particular substrate, exposure or installation.

Do not make hardness stand in for every performance question

Ask the question you actually need answered. If the concern is wear, ask for the relevant abrasion evidence. If it is bonding to a particular surface, ask about the specified preparation, primer and adhesion requirements. If it is chemical exposure, ask for compatibility information for the actual substance and conditions.

This is a useful way to organize a conversation because it keeps unlike questions apart. A hardness entry can be recorded correctly while the coating selection remains open. Similarly, two products with similar hardness entries may still have different intended applications and different supporting documentation.

Avoid “highest hardness wins” as a purchasing rule. Write the service requirement first, then have the manufacturer or responsible specifier explain which evidence addresses it. You will get a more useful answer than asking whether a coating is simply the hardest option in the catalog.

Agree on measurement conditions before comparing field readings

DeFelsko's Shore measurement guidance calls attention to test support, instrument contact and measurement timing. These details matter when someone tries to compare a handheld reading with a published number. A reading without its method and conditions is incomplete evidence.

For an inspection plan, ask the responsible technical team to specify the instrument scale, specimen requirements, conditioning, reading time and acceptance criteria. Record who supplied that procedure and which document revision applies. Do not invent a pass/fail threshold from a typical-property table.

This article is a reading guide, not a field-test procedure or authorization to release a coating into service. Follow the project specification, current product documents and trained personnel's approved inspection process.

A five-line note that keeps the comparison honest

• Product: exact name and formulation.
• Reported hardness: full number, variation and Shore scale.
• Method: standard and any stated test conditions.
• Source: document title, revision and date obtained.
• Open question: the service requirement still needing confirmation.

For help assembling the rest of the paperwork, see our guide to coating technical documents. A good comparison keeps each measurement attached to its meaning, and leaves product selection to the full requirements of the job.