What durometer measures
Hardness is how much an O-ring resists indentation, measured on the Shore A scale. 70A is the general-purpose default; 90A is hard; 60A and below are soft. The number is not about quality — it is a design trade-off between sealing, extrusion resistance and friction, and the right value depends mostly on pressure and gap.
The core trade-off
- Softer (e.g. 60–70A) conforms to surface imperfections, seals at low pressure with less squeeze, and runs with lower friction. But it extrudes into clearance gaps more easily under pressure.
- Harder (e.g. 80–90A) resists extrusion, tolerates larger gaps and higher pressure, and holds shape better. But it needs more force to seat, generates more friction on dynamic seals, and seals less readily on rough surfaces.
The single strongest driver is pressure: as pressure rises, the ring is forced into the extrusion gap, and a harder durometer (or a back-up ring) is what keeps it in the groove.
Pressure and gap guidance
| Condition | Typical choice |
|---|---|
| Low pressure, static, rough surface | 60–70A |
| General static and dynamic service | 70A |
| Higher pressure or larger clearance | 80–90A |
| High pressure without a back-up ring | 90A, or add a back-up ring |
Parker’s extrusion limit curves are drawn separately for 70A, 80A and 90A precisely because hardness shifts the allowable gap so much. For a given pressure, a 90A ring tolerates a larger diametral gap before it extrudes than a 70A ring does.
Dynamic seals
On reciprocating and rotary seals, hardness also sets friction and wear. Too soft and the ring drags and wears quickly; too hard and it seals poorly and can score the counterface. 70–80A is the usual balance for dynamic duty, biased harder as pressure or gap increases.
Interaction with squeeze and material
Hardness does not replace correct sizing. A hard ring still needs the right squeeze to seal, and a soft ring still needs a controlled extrusion gap. Hardness and material are also linked — the same compound family is offered in several durometers, and softer grades of a heat-resistant polymer may take a set faster. Decide the polymer for the media and temperature first, then pick the durometer for the pressure and gap.
Check it with the calculators
The extrusion calculator uses the 70A/80A/90A curves to show the allowable diametral gap at your pressure and hardness, including the material correction for silicone and fluorosilicone. The radial static and radial dynamic tools then confirm squeeze and fill for the chosen ring.
Frequently asked questions
What is the most common O-ring hardness? 70 Shore A is the general-purpose default and covers most moderate-pressure static and dynamic applications.
When should I use a 90A O-ring? At higher pressure or with larger clearance gaps, where a softer ring would extrude. 90A resists extrusion but needs more seating force and seals less easily on rough surfaces.
Does harder mean it seals better? No. Harder resists extrusion but seals less readily, because it conforms less to surface texture. Match hardness to pressure and gap, not to a belief that harder is always better.