What is Shore hardness?

Shore hardness is a standardised measurement (per ASTM D2240 and ISO 48-4) indicating how much a material resists indentation. It is measured with a durometer: an instrument with a spring-loaded needle pressed into the surface. The less the needle penetrates, the higher the Shore value and the harder the material.

Each type runs from 0 to 100. A low value means soft and flexible, a high value means stiff and hard. Important: a Shore value only means something when the scale is stated. "Shore 40" says nothing, while "Shore A 40" or "Shore D 40" tells the full story.

The most common Shore scales

Each scale uses its own durometer with a different needle shape and spring force, matched to a specific hardness range. The four you encounter in practice:

Shore OO (soft)

For very soft, gel-like materials such as soft silicone gels, foams and soft cushioning. Potting and encapsulation gels also fall in this range.

Shore A (soft to semi-hard)

By far the most used scale for rubbers and silicones. From very flexible casting silicones to firm, semi-hard rubbers. Most mould and casting silicones in this shop are expressed in Shore A.

Shore C (medium-hard)

A less well-known scale sitting between Shore A and Shore D. Used mainly for medium-hard materials such as firm silicone and rubber foams, EVA and some technical rubbers. Note: very soft foam is more often rated in Shore OO; Shore C is intended for the firmer foams.

Shore D (hard)

For hard rubbers and plastics such as hard polyurethane casting resin, hard rubber wheels and thermoplastics. Begins where Shore A tops out.

How do the scales relate?

This is where most explanations go wrong. The scales are not an extension of one another on a single straight line. They are separate measurements, each with its own instrument. They overlap only in narrow regions, and even there the relationship is indicative, not exact.

There are several zones where a rough comparison is possible:

• At the soft end, the top of Shore OO overlaps the bottom of Shore A (roughly OO 80 to 100 falls in the A 0 to 15 region).
• Shore C largely coincides with the top of Shore A and the bottom of Shore D, filling the medium-hard region between them.
• At the hard end, the top of Shore A overlaps the bottom of Shore D (roughly A 80 to 100 falls in the D 0 to 50 region).

This overlap is approximate. The further you are from the ends, the less reliable a comparison between two scales becomes.

Hardness chart with overlap and examples

The chart shows the four scales as separate bars on a shared hardness axis, from very soft on the left to very hard on the right. Each bar carries recognisable materials at their approximate value, and the hatched zones mark where two scales roughly overlap.

The values and positions shown are indicative and may vary by manufacturer and material. The overlap zones roughly indicate where two scales describe the same feel of hardness, but are not an exact conversion.

Which Shore value do I need?

Frequently asked questions

Is a higher Shore value better?

No, higher only means harder. Which is better depends on your application. A mould with fine detail and undercuts needs a low Shore A value so the rubber stays flexible enough to demould.

Can I convert Shore A to Shore D?

Only approximately, in the overlap zone. Roughly Shore A 90 to 100 corresponds to Shore D 40 to 50. Outside that range a direct conversion is not reliable.

What is Shore C used for?

Shore C is intended for medium-hard materials between Shore A and Shore D, such as firm silicone and rubber foams and EVA. It is a less common scale than A and D.

How is Shore hardness measured?

With a durometer, per ASTM D2240 or ISO 48-4. The material must be thick enough (usually at least 6 mm) and the measurement is taken on a flat, clean surface.