Shore A vs Shore D: Why 'Shore 70' Is Not a Spec

Shore A vs Shore D is not a precision question, it is a different-instrument question, and a purchase order that says "Shore 70" tells the factory almost nothing. Shore 70A is a shoe sole you can bend in your hands. Shore 70D is a hard hat. Both are "70", both are Shore, and the standard that defines them states outright that a reading from one durometer type cannot be converted into a reading from another.

That sentence is not a technicality. It is the reason hardness is one of the quietest sources of rejected rubber, gasket, wheel and moulded-plastic shipments in export manufacturing: the buyer, the factory and the lab each fill in the missing letter from their own habit, and nobody finds out until the parts are on a container.

Shore A vs Shore D: What Each Scale Measures

Shore A is the durometer scale for flexible materials: a spring-loaded instrument presses a 35° truncated cone with a 0.79 mm flat tip into the surface under 822 gf (8.05 N) of force, and the depth of that indentation is read on a 0–100 dial.

Shore D is the scale for rigid materials: the same instrument family, but with a sharp 30° cone and 4,536 gf (44.45 N) of spring force — five and a half times as hard a push — so it can still register a difference in something Shore A can no longer separate.

Everything below is a version of the same mistake: treating that table as decoration rather than as part of the number.

Myth 1: "Shore 70" Is a Hardness Spec

It is a hardness rumour. Without the scale letter, 70 could be a soft sealing profile or a structural plastic, and the factory will read it as whatever it usually makes. This is the most common and most expensive one on the list, because it survives all the way to production — a drawing note that says "rubber, Shore 70" looks complete and gets signed off by three people.

Write the scale letter every single time, on the drawing, on the spec sheet, on the PO line, in the email. "Shore A 70" and "70 Shore A" are both fine. "Shore 70" is not.

Myth 2: There Is a Conversion Chart, So You Can Convert

Conversion charts for Shore A to Shore D circulate widely, and they are all approximations produced by measuring specific material families. The standard itself closes the door: ASTM D2240 states that the geometry of the indenter and the applied force influence the measurements such that no relationship exists between values from one durometer type and another.

The practical meaning: you may use a chart internally to sanity-check a material choice. You may not use one to convert a customer's requirement into a test result. If the PO says Shore D 45 and your lab reports Shore A 95, you have not met the spec — you have measured a different thing and produced a plausible-looking number.

Myth 3: The Overlap Region Is a Grey Area You Can Ignore

The scales overlap, and the overlap is exactly where the arguments happen. ZwickRoell's guidance is the usable rule of thumb most labs work to:

• A Shore A reading above about 90 is at the top of the instrument's resolution — retest on Shore D.
• A Shore A reading below about 20 is off the bottom — retest on a softer scale (Shore OO).
• A Shore D reading below about 30 is off the bottom of that scale — retest on Shore A.

A hard polyurethane wheel sitting at Shore A 95 / Shore D 45 will be described both ways by two honest suppliers, and both will be right. Whoever specifies the part has to choose one scale and stay on it for the life of the part number, including on the incoming inspection sheet.

Myth 4: Sample Thickness Is a Lab Detail

It is a result-changing variable. All three standard families require a minimum specimen thickness of 6 mm for a standard Shore reading. Below that, the indenter feels the bench under the sample and the reading climbs.

This is why the number on a material datasheet and the number your customer measures on the finished part often disagree without anyone lying:

• A 3 mm gasket cannot be measured to the standard as a single layer. Stacking plies is permitted, but a single specimen is preferred and stacked readings are not identical.
• A curved surface — a hose wall, an O-ring, a wheel tread — is not a flat 6 mm specimen either.
• The reading must be taken at least 12 mm from any edge, which small parts rarely allow.

If the part cannot host a compliant test, that belongs in the agreement: state that hardness is verified on a moulded test plaque from the same batch, not on the part.

Myth 5: A Reading Is a Reading

Rubber creeps under the indenter, so the number depends on when you look at the dial, and the three standards do not ask you to look at the same moment:

On a soft, high-creep compound the 1 s and 15 s readings can differ by several points — enough to move a part from inside tolerance to outside it, with both labs following a standard correctly. A hardness spec that names a value but not a dwell time is only two-thirds of a spec.

Myth 6: ASTM D2240 and ISO 868 Are the Same Test for Everything

They agree on the two scales, and they diverge on scope, which is where mixed-market suppliers get caught:

• ISO 868 covers plastics and ebonite with Shore A and Shore D.
• ISO 48-4 covers vulcanised or thermoplastic rubber.
• ASTM D2240 covers rubber, elastomers and plastics together, with twelve durometer types (A, B, C, D, DO, E, M, O, OO, OOO, OOO-S, R), not two.
• In China, GB/T 2411 is the plastics-and-hard-rubber route and GB/T 531.1 is the rubber route.

So "tested to ISO" is not a complete claim for a rubber part. A European buyer asking for a rubber seal expects ISO 48-4 and a 3 s reading; quoting ISO 868 signals you tested it as a plastic.

What a Complete Hardness Callout Looks Like

Six fields, one line, no ambiguity:

Hardness: Shore A 70 ±5, ASTM D2240, instantaneous reading, moulded plaque 6 mm, 23 °C ±2 °C, 5 readings averaged.

Field by field:

The same discipline applies to how the number reaches the buyer. A hardness value that lives only in a PDF attachment gets lost; the ones that stop questions sit on the product image itself, labelled against the part they describe — the seal, not the housing. That means the callout has to be pinned to real, measured geometry on the photo rather than typed into a caption, which is also the difference between an annotated spec image and an AI-generated one: an image generator will render a confident "70A" on a part it never touched, and the buyer has no way to tell. The same rule that governs Ra vs Rz surface roughness callouts governs this one — the parameter, the value and the method travel together or they mean nothing. For a layout that keeps all of them legible on one drawing, see the industrial spec diagram format.

Hardness spec checklist

• Scale letter written on every document (drawing, PO, spec sheet, test report)
• Standard named, and it matches the material class (rubber vs plastic)
• Reading time stated (instantaneous / 3 s / 15 s)
• Tolerance band stated, not implied
• Test specimen defined: part or plaque, thickness ≥ 6 mm, edge distance ≥ 12 mm
• Number of readings and conditioning temperature agreed
• No conversion between scales anywhere in the acceptance chain
• The hardness value appears on the product image next to the component it applies to

Run the same check on the tolerance line while you are there — hardness bands and dimensional bands fail for the same reason, which is covered in product dimension tolerance.

FAQ

What is the difference between Shore A and Shore D?

Shore A and Shore D are two different durometer scales in the same standard. Shore A uses a 35° truncated cone under 822 gf and measures flexible materials; Shore D uses a sharp 30° cone under 4,536 gf and measures rigid ones. They are separate instruments with separate indenters, not two zoom levels of one scale.

Can you convert Shore A to Shore D?

Not officially. ASTM D2240 states that no relationship exists between readings taken with different durometer types. Approximate conversion charts exist and are useful for choosing a material, but they are not valid for demonstrating conformance to a specified hardness.

What is Shore 70A equivalent to in Shore D?

There is no standard equivalent. Shore A 70 is a moderately soft elastomer — typical of many seals and shoe soles — and sits well below the useful range of a Shore D durometer, which starts around 30 D. Any single-number answer you find is an approximation for one material family, not a conversion.

Is ASTM D2240 the same as ISO 868?

They overlap but do not match. ISO 868 covers plastics and ebonite on the Shore A and D scales; rubber has its own ISO route in ISO 48-4 with a 3 s reading. ASTM D2240 covers rubber, elastomers and plastics in one standard and defines twelve durometer types. For a rubber part sold into Europe, ISO 48-4 is the expected reference.

How thick does a sample need to be for a Shore hardness test?

At least 6 mm. Thinner specimens read harder because the indenter senses the supporting surface. Plies may be stacked to reach the thickness, but a single specimen is preferred, and readings must be taken at least 12 mm from an edge.

Sources & References

• CCSI — Basic Durometer Testing Information (ASTM D2240 indenter geometry, spring forces, reading rule, no cross-type relationship)
• ZwickRoell — Shore hardness test: ISO 868, ISO 48-4, ASTM D2240 scope, ranges and reading times
• Intertek Testlopedia — Shore Hardness ASTM D2240
• Industrial Physics — ASTM D2240 testing overview
• National Digital Standards Library — GB/T 2411-2008, plastics and ebonite, indentation hardness (Shore hardness)