Short answer: squeeze = (cord diameter − groove depth) ÷ cord diameter. Use 15–25% for static seals and 8–15% for dynamic seals (excess squeeze raises friction and heat, cutting life), and keep the fill ratio under 75% to leave room for thermal expansion and swelling. Once those three numbers are set, material and hardness come next.
Three quantities, one formula
| Application | Squeeze | Why |
|---|---|---|
| Static sealing (flanges, end caps) | 15–25% | Enough contact stress to compensate surface finish and minor eccentricity |
| Low-pressure static (air, water resistance) | 15–20% | Balances sealing against fitting effort |
| High-pressure static (hydraulic faces) | 20–25% | Higher initial stress, paired with a back-up ring against extrusion |
| Dynamic sealing (reciprocating / rotary) | 8–15% | More squeeze means more friction and heat, and sharply shorter life |
Why the fill ratio must stay under 75%
Fill ratio = seal cross-section ÷ groove cross-section. Under pressure the O-ring is pushed toward the clearance gap; with no spare volume in the groove for thermal expansion and media swell, the ring extrudes into the gap, suffering shear damage or being cut clean through. The 75% ceiling exists to reserve volume for swell — and where media swelling is high (certain oils, for example), pull it lower still.
Deriving groove dimensions, in order
- Step 1: fix the sealing face from the pressure direction (internal pressure usually seals on the inner diameter)
- Step 2: choose cord diameter W (commonly 1.5–5.0 mm; thinner cord is more sensitive to dimensional precision and groove finish)
- Step 3: pick the squeeze for the application and back out groove depth H = W × (1 − squeeze)
- Step 4: compute the fill ratio; if above 75%, deepen and widen the groove
- Step 5: set hardness from media and temperature (60–70 Shore A static, 70–80 Shore A dynamic are typical)
- Step 6: add a back-up ring against extrusion in high-pressure joints
Three most common mistakes
Assuming "tighter is safer"
Beyond about 30% squeeze the material lives under high stress, compression set accelerates, and the classic outcome is a seal that works at first and weeps later. The recommended static range is deliberate, not conservative for its own sake.
Ignoring how media swelling eats the fill ratio
Swelling varies from a few percent to well into the teens depending on the compound and media. Where swelling is high, the initial fill ratio must start lower or the groove is nearly full at assembly. Requesting swell data for your specific media is the cheapest step available.
Reusing a static groove for a dynamic seal
Dynamic seals need less friction and heat, so both squeeze and hardness drop, while groove finish requirements rise. Taking a static groove into reciprocating service typically runs fine at first and starts weeping after a few thousand cycles.
Send your cord diameter, groove dimensions and duty conditions as a one-page table and we will check the squeeze and propose a custom O-ring or profile seal, from 50 pieces. Contact us for the calculation and a quote.
Frequently Asked Questions
What compression ratio should an O-ring use?+
Target 15–25% for static seals and 8–15% for dynamic ones. Too little squeeze fails to seal; too much raises friction and heat and accelerates permanent set. Tilt toward the low end for low-pressure static joints, and the high end plus a back-up ring for high-pressure ones.
Why keep the fill ratio under 75%?+
To reserve volume for thermal expansion and media swelling. Above 75% the ring has nowhere to go when pressure pushes it toward the clearance gap, so it extrudes into the gap and suffers shear damage or outright cutting. Where media swelling is high, go lower still.
How is groove depth calculated?+
Groove depth H = cord diameter W × (1 − target squeeze). For a 2.0 mm cord at 20% squeeze, that is about 1.6 mm. Then recheck the fill ratio: if it exceeds 75%, deepen and widen the groove rather than forcing the squeeze lower.