Resources · Specification
Shore A, Shore C, and the number that means nothing on its own.
Hardness is the most quoted figure on a sole spec sheet and the most often quoted wrongly. Two scales, no reliable conversion between them, and a number without its scale is not a specification.
Published
Two scales, and no reliable conversion between them.
Shore hardness measures how far a standardised indentor sinks into a material under a standardised force. Sole work uses two of the scales.Shore A is for vulcanised rubber, foamed rubber and TPR. Shore C is for EVA sole stock, whether it is used as a midsole or as the wear surface of a casual sole.
They are not two ways of saying the same thing. The scales use different indentor geometry and different spring force, so they respond differently to the same material, which is why there is no conversion factor worth printing. Published A-to-C equivalences exist, but each is an approximation fitted to one material over a narrow band, and applying one outside that band produces a number that looks authoritative and is wrong. On a sole specification, the fix is not a conversion. It is writing the scale next to the number.
This is also the fastest way to read a supplier. A spec sheet quoting an EVA midsole in Shore A, or a rubber outsole in Shore C, was assembled by someone who has not stood next to a durometer. We measure hardness on site against SATRA TM205, on both scales.
What each material can reach.
Published industry ranges for the material as a class, not a statement of what any one factory holds. Read the next table for that.
| Material | Hardness range | Density |
|---|---|---|
| Vulcanised rubber | 30–90 Shore A (typically 50–65) | Highest of the four |
| Foamed rubber | 40–65 Shore A | Lower than solid rubber |
| TPR | 40–80 Shore A (40–55 casual, 50–65 urban) | Light |
| EVA sole stock | 20–35 Shore C | 80–120 kg/m³ |
What we actually run.
Narrower than the category, split by application, and the figures we quote against. A supplier's real window is always narrower than its material's.
| Application | Hardness | Density |
|---|---|---|
| Rubber outsole, casual / lifestyle | 60–65 Shore A | 1.15 g/cm³ |
| Rubber outsole, safety / work | 65–70 Shore A | 1.15 g/cm³ |
| Foamed rubber outsole | 60–65 Shore A | 0.40 g/cm³ |
| TPR outsole | 60–65 Shore A | 1.00 g/cm³ |
| EVA sole, casual / slipper | 55–60 Shore C | 0.25 g/cm³ |
| EVA midsole, sport / running | 40–45 Shore C | 0.20 g/cm³ |
Two constructions are missing from that table on purpose. Ourcombination sole andIP injection sole are quoted per programme: our own parameter review returned nothing usable for the first and an unscaled figure for the second, and this is not the page on which to publish a durometer number we cannot stand behind. Send the brief and we will come back with a figure we can hold, scale included.
What moving the number does.
Hardness is not a quality rating. It is a position on a trade-off, and both ends cost something.
Softer compound
More grip on smooth and wet surfaces, more cushioning, a more forgiving flex through the tread.
Wears faster, deforms sooner under load, and lug edges round off earlier in the shoe’s life.
Harder compound
Longer wear life, sharper tread definition retained, better lateral support and torsional stability.
Less grip on smooth wet floors, a harsher ride, and less tolerance for a thin or highly sculpted section.
Which is why the useful conversation is rarely about the number. Tell us the surface the shoe meets, the wear life you need and whether grip or support leads, and hardness comes out of that as a consequence. It is the same reason our own windows split by application rather than by material: a casual rubber outsole at 60–65 Shore A and a safety outsole at 65–70 are the same family answering two different briefs.
Four ways a hardness spec goes wrong.
All four reach us regularly. None is exotic, and each costs a round trip before anything can be quoted.
A hardness with no scale
"60 hardness" reaches us most weeks. On a rubber outsole that is a normal casual specification; read as Shore C it would describe something closer to a firm EVA sole. We will always come back and ask, but a specification that needs a clarifying email is not yet a specification.
One figure for a two-layer sole
A combination sole has an outsole hardness and a midsole hardness, quoted on different scales, and they move independently. A single number on a drawing for a rubber-over-EVA construction cannot be actioned without asking which layer it belongs to.
Copying a competitor’s durometer figure
Hardness interacts with section thickness, tread geometry and compound. The same 65 Shore A behaves differently under a 4 mm slab and a deeply lugged outdoor sole. Send the shoe and the performance target rather than the number you measured off someone else’s sole.
Treating a category range as a supplier capability
Vulcanised rubber spans roughly 30–90 Shore A as a material. No factory holds all of it on one line. Ask for the window a supplier holds in production, which is always narrower, and ours is in the table above.
How to write hardness into an RFQ.
Give the scale, a range rather than a point, and the layer it applies to:outsole 63 ± 3 Shore A, midsole 45 ± 3 Shore C. A tolerance matters because a compound is mixed, not machined, and asking for a single value is asking for a rejection rate. If you are unsure of the figure, describe the use instead. That is a complete brief too, and it is the one our engineers can act on fastest.
Hardness is one line of six on a full enquiry. Drawing or sample, size range, colour, volume and the standards you certify against are on thedevelopment page. If you have not settled on a material yet, start withEVA, rubber and TPR compared, and seehow we test for the rig behind every figure on this page.
Unsure which scale your spec is in?
Send the drawing or the sample. We will measure it, tell you the scale it was written on, and quote a window we can hold in production.