The drawing that reached Nadia’s desk at a mid-sized appliance manufacturer carried a single hardness callout: “70.” No scale letter. No test standard.
Her purchasing team read it as a rigid, hard plastic. The design engineer meant a soft, grippy elastomer. Two entirely different materials, one ambiguous number, and a purchase order about to go wrong.
That gap is the whole story of the shore hardness scale. It looks like one simple 0 to 100 number, but it’s a family of scales, each with its own indenter and spring force. Read the value without the scale letter, and you can be off by an entire material category.
If you specify, buy, or inspect plastics, you’ve probably already run into this. A datasheet quotes Shore D 84. A supplier’s chart quotes Shore A 95. An online conversion table claims they’re the same thing.
None of those numbers can be compared directly, and the conversion table is the least trustworthy of the three.
This guide explains how the shore hardness scale works, how Shore A and Shore D differ at the level of the instrument itself, when each applies, and why published conversions are estimates at best. You will also get typical values for the plastics and elastomers you buy, the factors that shift a reading, and a way to keep a specified band such as 95A ± 3 stable from lot to lot.
For the wider picture of how Shore fits alongside Rockwell and other methods, our complete guide to plastics hardness covers every scale in one place. Ready to match a grade to your hardness spec? Contact our technical team and we will come back with a COA-documented option within 24 hours.
What Is the Shore Hardness Scale?
The shore hardness scale is an empirical measure of how far a spring-loaded indenter penetrates a material’s surface. The reading runs from 0, where the indenter sinks fully by about 2.5 mm, to 100, where it barely moves. A higher number means a harder surface.
Shore hardness measures resistance to indentation, not strength or wear
A durometer does one thing. It pushes a shaped metal point into the surface under a defined spring load and reports how far it traveled. That’s indentation resistance, and only indentation resistance.
It isn’t tensile strength. It isn’t stiffness. It isn’t abrasion or scratch resistance.
MatWeb’s Shore reference states this plainly: durometer hardness shouldn’t be used alone to predict strength, wear, or abrasion. Engineers get into trouble when they treat a hard number as a guarantee of performance.
How the 0 to 100 durometer reading works
The scale is anchored to full penetration. At 0, the indenter tip sits at its maximum travel of roughly 2.46 to 2.54 mm. At 100, it’s barely broken the surface. Everything between is a relative position on that travel, which makes the reading dimensionless.
Because the number is relative, it only means something when you also know the scale, the standard, and the condition of the specimen. “Shore D 84, ASTM D2240, dry as molded” is data. “Hardness 84” isn’t.
Why a Shore value is meaningless without its scale letter and standard
Shore isn’t one scale. ASTM D2240 defines twelve of them: A, B, C, D, DO, E, M, O, OO, OOO, OOO-S, and R. Shore A and Shore D are the two that matter for plastics and elastomers; the others exist for foams, gels, and specialty materials.
Related standards include ISO 868 and ISO 7619-1 for plastics, ISO 48-4 for rubber, DIN 53505, and JIS K 6253. For the method itself, our guide to how plastic hardness is measured covers the procedure and specimen preparation.
Shore A vs Shore D: How the Two Scales Differ
Shore A and Shore D don’t measure different amounts of the same thing. They’re two instruments aimed at two different ranges of material, and the difference starts with the shape of the indenter.
| Feature | Shore A | Shore D |
|---|---|---|
| Indenter shape | 35° truncated cone (blunt, flat tip) | 30° pointed cone (sharp) |
| Spring force at reading 100 | ~822 g / 8.05 N | ~4536 g / 44.5 N |
| Typical materials | Rubber, silicone, TPE, TPU, flexible PVC, EVA | Rigid PVC, ABS, PC, POM, PA66, PMMA |
| Typical applications | Seals, gaskets, grips, flexible hose, soft overmolds | Structural parts, housings, gears, hard hats |
| Useful reporting range | ~20 to 95 | ~20 to 90 |
What is Shore D hardness?
Shore D hardness is the reading on the scale that uses a 30-degree pointed indenter and a roughly 4536 g spring, aimed at semi-rigid and rigid plastics. It isn’t a harder version of Shore A. It’s a different measurement, taken with a different instrument, which is why the two don’t line up.
Indenter geometry is why the numbers do not convert
The Shore A indenter is a truncated cone with a 35-degree flank and a flat tip about 0.79 mm across. The Shore D indenter is a 30-degree cone with a sharp point. A blunt tip spreads the load over a wider area and meets more resistance. A sharp point concentrates the same force into a tiny patch and sinks further.
Change the tip and you change what the material is asked to resist. That’s the mechanical reason a converted value can’t be trusted. You’re not rescaling a number, you’re describing a different physical event.
Spring force is why the same number means different things
At a reading of 50, Shore A applies roughly 4.3 N to the material. At the same reading of 50, Shore D applies roughly 22.2 N. Same number, more than five times the force.
“50” on Shore A and “50” on Shore D describe materials of completely different stiffness and completely different states of the test. There’s nothing proportional to convert between them.
The other Shore scales
Beyond A and D, the durometer scale family steps in both directions. Shore OO and OOO handle very soft foams, gels, and sponge. Shore O covers soft elastomers below the A range. Shore B and C appear in older specs and some specialty rubbers, while E and M extend toward harder materials where D starts to roll off.
You’ll rarely specify these, but imported datasheets occasionally report them. The rule stays the same: read the scale letter first.
Consider what happens when the letter is dropped. Marcus, a product designer at a consumer-goods company, sent a silicone grip specification to two molders and got back a quote that was 18 percent lower. Both cited the same “50” durometer.
The cheaper molder had sourced a TPE compound on Shore A 50, not the silicone the design assumed. Same number, but different compression set, a lower temperature ceiling, and a grip that went tacky in summer heat. The hardness matched. The material didn’t.
When to Use Shore A and When to Use Shore D
The choice of scale isn’t a preference. It’s determined by the material under test, and ASTM D2240 gives a rule for the boundary.
The ASTM D2240 switching rule
If a Shore A reading comes back above 90, switch to Shore D. If a Shore D reading comes back below 20, switch to Shore A. The reason is resolution. Near the ends of either scale, the indenter is either barely moving or sinking almost fully, so small material differences produce nearly identical numbers.
Useful readings on both scales sit between about 20 and 90. Below 10 and above 90, the number carries little information, and careful labs don’t report it.
Which materials belong on which scale
- Shore A: silicone, EPDM, nitrile, natural rubber, TPE, flexible PVC, EVA, soft TPU grades.
- Shore D: rigid PVC, ABS, PC, POM, PA66, PMMA, HDPE, hard TPU grades.
- Rockwell, not Shore: very hard thermoplastics where even Shore D saturates, such as PEEK and highly filled grades.
The last row matters commercially. For rigid engineering plastics, most producers report Rockwell values, not Shore. If your drawing says “Rockwell M94” and a generic chart hands you “Shore D 84,” those are different scales describing the same material from different angles. Our guide to Rockwell hardness for rigid plastics covers when Rockwell is the right instrument.
Can You Convert Shore A to Shore D?
No, not reliably. A shore A to shore D conversion is an estimate, and estimation is sometimes useful, but a converted value must never appear on a specification or a receiving inspection.
Why the scales overlap but are not interchangeable
A50 and D50 both exist, and both describe a spring-loaded indentation test, so the scales feel comparable. They aren’t. The indenter geometry and spring force differ, so the same reading corresponds to different forces and different depths of penetration. A very stiff elastomer might read 95A and also register on Shore D; neither converts cleanly into the other.
What published conversion formulas actually are
You’ll find estimates such as D ≈ A − 50 (so 90A would be about 40D) or A ≈ 0.173 × D + 86.58 (so 40D would be about 93.5A). ASTM D2240 includes a guideline chart, but the standard is explicit that such comparisons are rough.
The formulas assume similar elastic behavior between materials, and most don’t hold that assumption. Published tables even disagree: one equates 90A with about 40D, while another describes 40D as considerably harder. When two tables can’t agree, a converted number isn’t a specification.
The saturation problem
Above roughly 95A, the Shore A scale loses resolution. Hard rubber, stiff plastic, wood, and even some very hard non-polymers can all read near 100A. The scale is compressed against its ceiling, so materials that behave nothing alike report the same number. This is exactly where people try to convert to Shore D, and exactly where conversion is least trustworthy.
The rule to follow
Use conversions as a rough sanity check and nothing more. If you need a Shore D value, measure it on Shore D. If your part was validated at 95A ± 3, then 95A ± 3 is your spec, and any supplier quoting a Shore D equivalent has already shown they will substitute a number for a measurement.
Want to get a grade whose Shore hardness matches your validated spec? Tell us the scale, the value, and the tolerance, and we will quote a branded prime grade that meets it. Send us your spec and we respond within 24 hours.
Shore Hardness Chart: Typical Values for Plastics and Elastomers
This is the table most searchers want. Read the scale column first. The elastomeric grades are on Shore A, the rigid grades on Shore D, and the two columns aren’t comparable.
| Material | Scale | Typical range | Notes |
|---|---|---|---|
| Silicone rubber | A | 10–60 | Platinum-cure grades span very soft to firm |
| EPDM / EPM | A | 50–90 | Common seal and weatherstrip material |
| Nitrile (NBR) | A | 50–90 | Oil-resistant seals and gaskets |
| Flexible PVC | A | 55–95 | Plasticizer content sets the value |
| TPE (generic) | A | 30–95 | Wide family; check the specific compound |
| TPU, soft grades | A | 60–95 | 85A and 95A are the volume workhorses |
| TPU, hard grades | D | 50–75 | 60D and above approach rigid plastic |
| HDPE | D | 60–70 | Toughness, not hardness, is the selling point |
| PP | D | 70–75 | Copolymer grades soften slightly |
| PA66 (dry) | D | 75–80 | Conditioned nylon reads lower |
| ABS | D | 70–80 | High-impact grades sit at the soft end |
| PC | D | 75–88 | Also quoted as Rockwell M70–M80 |
| POM copolymer | D | 80–85 | Tough, low friction, stable in moisture |
| POM homopolymer | D | 84–86 | The hardest high-volume engineering plastic |
| PMMA | D | 85–90 | Hard, glossy, brittle under impact |
| PTFE | D | 50–65 | Filled grades read higher |
Read the caveat before you use a single row. These are representative ranges for common unfilled grades, not guaranteed values. Hardness moves with grade, filler, plasticizer, moisture, and preconditioning, so confirm against the specific grade datasheet.
Two patterns here are worth calling out. The elastomeric materials (TPU, TPE, flexible PVC, EVA) live on Shore A because that is the scale their hardness spec uses, which is why TPU shore hardness is quoted from 60A up to 75D depending on grade. The rigid engineering plastics you buy as pellets (POM, PA66, PC, ABS, HDPE) live on Shore D and, for the harder grades, on Rockwell.
For the full cross-material view, our engineering plastic hardness comparison lines up the rigid grades side by side. For single-material depth, see our guides to POM hardness, polycarbonate hardness, and PTFE hardness.
What Changes a Measured Shore Hardness Scale Reading
Two parts from the same mold can read different numbers, and often do. Five factors explain most of the variation.
Plasticizer and filler content. Flexible PVC and many TPU compounds are tuned by formulation; more plasticizer lowers the reading, more mineral filler or glass raises it.
Crystallinity and hard-segment content. Hardness tracks how fully a semi-crystalline plastic crystallizes, which is why POM homopolymer reads higher than copolymer. In TPU, the hard-to-soft segment ratio sets the value, which is why 85A, 95A, and 60D are distinct products rather than one material at three settings.
Temperature and dwell time. Thermoplastics soften as they warm, so a room-temperature reading says little about behavior at 80 °C. Because the material is viscoelastic, a reading at 1 second differs from one at 15 seconds; a value without a contact time is incomplete.
Specimen thickness and support. ASTM D2240 calls for roughly 6 mm minimum thickness, or stacked thin samples on a rigid base. A thin, unsupported section reads low because the material deflects rather than resists, and a rough surface reads high.
Moisture and conditioning. Nylon absorbs water that plasticizes the polymer and lowers measured hardness. POM, the polyolefins, and TPU move far less, but the condition should always be stated.
Reading Shore Hardness on a Datasheet
A complete hardness callout carries three parts: the scale letter, the number, and the tolerance.
Read all three. “95A” is incomplete. “Shore A 95 ± 3, ASTM D2240” tells a QC inspector exactly what to accept. Real TPU grades are commonly quoted as 85A ± 2, 95A ± 3, or 60D ± 3, and that tolerance band, not the nominal value, is what an incoming check must hold.
Match the scale to the material. If a datasheet quotes a flexible grade in Shore D, or a rigid grade only in Shore A, treat it as a documentation problem and ask for the value on the correct scale.
Check the condition. Dry versus conditioned, plus the stated dwell time, can move a number by several points, especially for nylon. And confirm the value came from a calibrated shore hardness tester held on a rigid surface, not a handheld reading taken on a finished part.
What hardness does not tell you
Hardness doesn’t predict tensile strength, elongation at break, compression set, or abrasion resistance. Two TPU grades can both read 95A and still differ substantially in tensile strength and compression set, because hardness describes one behavior, not a performance profile. A matched hardness number isn’t a matched material.
Sourcing Consistency: Keeping a Specified Shore Band Stable
For elastomeric grades, Shore hardness isn’t a secondary property. It’s the release spec. A TPU or TPE part is often specified at a single Shore A value with a tolerance, and that band decides whether the part feels right, seals correctly, and meets the drawing.
Why hardness drifts between lots
Lot-to-lot hardness drift usually has one of four causes: off-spec or contaminated resin where the hard-segment ratio, plasticizer, or filler is out of range; regrind blended back into prime resin, which changes crystallinity on cooling; plasticizer or moisture variation between deliveries; or processing shifts, especially melt temperature and cooling rate.
Priya, a QC inspector at an automotive Tier 2 supplier, caught this exact failure. An incoming lot of 95A TPU read 89A on the durometer, well outside the specified band. Rather than reject on the spot, she pulled the COA and found the melt flow index had drifted as well.
The lot turned out to be prime resin blended with regrind. The hardness check flagged it in minutes, before a single seal reached the molding line.
For a part validated at 95A ± 3, a lot reading outside the band is a genuine early warning. Hardness testing is fast, cheap, and non-destructive, which makes it one of the best incoming-inspection checks available.
What to verify on the COA
The Certificate of Analysis is where a supplier proves the lot meets spec. Check the Shore value with its scale and tolerance, the lot number so drift across reorders is visible, the melt flow index (MFI) as the flow fingerprint that correlates with how the material will process and crystallize, and the density and filler content that confirm the grade. A hardness reading outside spec, paired with a COA check, catches a resin problem before it reaches the line.
How Yifuhui keeps hardness consistent
We supply branded prime resin, which means grades whose hardness ranges are documented by the manufacturer and whose lots are traceable. TPU ships as BASF Elastollan, Covestro Desmopan, or Huntsman IROGRAN. POM ships as BASF Ultraform, Celanese Hostaform, or Polyplastics DURACON. PC ships as Covestro Makrolon, and PA66 as BASF Ultramid.
Every shipment carries a manufacturer-issued COA with the hardness value, MFI, and lot number, plus MSDS and full export documentation. Our Suzhou warehouse sits minutes from the Port of Shanghai, standard terms are FOB Shanghai, and the minimum order is 25 kg, so you can validate a lot on your own line before committing to volume. The families most often specified by hardness are POM, PA66, PC, HDPE, and PTFE, alongside TPU and TPE elastomer grades.
Frequently Asked Questions
Is Shore D harder than Shore A?
Neither scale is simply “harder.” They’re different instruments for different ranges. Shore A uses a blunt indenter and a light spring for soft elastomers; Shore D uses a sharp cone and a spring more than five times stiffer for semi-rigid and rigid plastics. The same material measured on both scales won’t produce matching numbers.
Can you convert Shore A to Shore D?
Only as a rough estimate, never as a specification. Published formulas such as D ≈ A − 50 assume similar elastic behavior that most materials don’t share, and the scale saturates above about 95A. If you need a Shore D value, measure it on Shore D.
What does 95A mean in Shore hardness?
It is a reading of 95 on the Shore A scale, taken with a 35-degree truncated-cone indenter per ASTM D2240. It describes a fairly firm elastomer, common for TPU and TPE grades. Because it sits near the top of the A range, a tolerance is usually quoted with it, such as 95A ± 3.
What is the Shore hardness of TPU?
Commercial TPU grades span roughly 50A to 85D. Soft and medium grades such as 85A and 95A sit in the elastomeric range, while 60D and above behave more like rigid plastics. Hardness is set by the hard-segment-to-soft-segment ratio of the formulation.
What is the Shore D hardness of POM, ABS, and polycarbonate?
POM homopolymer reads about 84 to 86 Shore D and POM copolymer about 80 to 85. ABS sits around 70 to 80 Shore D, and polycarbonate around 75 to 88 Shore D, also quoted as Rockwell M70 to M80. All values depend on grade.
What is Shore 00 used for?
Shore 00 and Shore OOO are much softer scales for foams, gels, and sponge materials that fall below the range of Shore A. They are common for cushioning, padding, and very soft elastomers.
Does Shore hardness tell me how abrasion-resistant a plastic is?
No. Abrasion and wear depend on the counterface, load, speed, lubrication, and friction coefficient, not on indentation hardness alone. Two materials with the same Shore value can wear at very different rates, which is why POM is chosen for wear parts on the strength of low friction, not the highest hardness.
Conclusion
The shore hardness scale is one of those specifications that looks simple and punishes assumptions. It’s a family of scales, not a single number, and a value without its scale letter, standard, and tolerance isn’t a specification at all.
Three rules carry most of the value. Use Shore A for soft and elastomeric materials and Shore D for semi-rigid and rigid plastics. Never treat a converted value as a spec, because the scales use different indenters at very different spring forces and Shore A saturates near its ceiling. And remember that hardness measures indentation resistance only, not strength, abrasion, or impact performance.
Then protect the number you validated. For TPU, TPE, and flexible PVC, the Shore hardness band is the release spec, and holding it lot after lot starts with branded prime resin and a COA that documents the value, the tolerance, and the lot.
At Yifuhui, we supply branded prime POM, PA66, PC, HDPE, PTFE, and TPU/TPE grades with a Certificate of Analysis on every batch, so the shore hardness band you validate on a 25 kg sample is the band your production lots hold. Ready to source a grade that holds it? Request a quote with your material, application, and target hardness range, minimum order 25 kg, FOB Shanghai, response within 24 hours.
