STRETCH The percent stretch, usually simply called the stretch, is defined by the diameter of the hardware minus the O-ring inner diameter (ID), all divided by the O-ring ID. The diameter of the hardware value represents the diameter the O-ring is being stretched about. For male designs, this diameter is the groove. For female designs, it is the rod. For face seals, this can be the ID of the groove. This formula can also be used for installation stretch where the hardware diameter is the outer diameter of the installation tool or the maximum diameter that the O-ring must pass over for installation.  NOTE: Face seals are usually designed with the Groove ID equal to the ORing ID (internal pressure) or Groove OD equal to the the ORing OD (external pressure so stretch/shrink only applies if the Groove ID or OD is modified from these conditions. Ideal: 0-5% SQUEEZE To form a robust seal, adequate compression or squeeze is a critical consideration. Compressing the seal energizes it, and the O-ring pushes back on the mating surfaces. This contact force between the O-ring and hardware creates a seal. Squeeze is calculated by the cross-section diameter minus the gland depth, all divided by the cross-section diameter. Note that in this calculation, the gland depth is the distance from the bottom of the groove to the mating surface. This accounts for any clearance gap. Ideal: 22-32% VOLUME FILL The final essential criterion for O-ring design is volume fill, also called gland fill. The compressed O-ring must have extra space within the groove without overfilling the groove. Volume fill is the O-ring volume divided by the gland volume. NOTE: Since the volume of the gland is calculated by mulitplying the area of the gland by the circumerence of the gland and the volume of the oring is calculated by multiplying the aread of the oring by the circumference of the centerline of the oring and both circumferences are roughly equal when the oring is compressed in the gland, the Volume Fill is roughly the quotient of the areas. Ideal: 60-90% Stretch(ORingID,GrooveID)=(GrooveID-ORingID)/ORingID Squeeze(ORingCS, GrooveDepth)=(ORingCS-GrooveDepth)/ORingCS ORingAreaRound(ORingCS)=(pi/4)*ORingCS^2 ORingAreaSquare(ORingCS)=ORingCS^2 GlandArea(GrooveDepth, GrooveWidth)=GrooveDepth*GrooveWidth FOR SQUARE O-RINGS, FACE SEAL, INTERNAL PRESSURE Nom Min Max min Tol max Tol Stretch (0-5%) Squeeze (22-32%) Volume Fill (60-90%) ORing Area Gland Area From inPHorm ORring ID 2.5 mm 2.37 mm 2.63 mm -0.13 mm 0.13 mm min 0.7533333333333333 0.13043478260869565 0.6045714285714285 0.8464 mm^2 1.125 mm^2 ORing CS 1 mm 0.92 mm 1.08 mm -0.08 mm 0.08 mm nom -0.4 0.25 0.8888888888888888 1 mm^2 1.125 mm^2 Groove ID 1.5 mm 1.5 mm 1.75 mm -0 mm 0.25 mm max 0.35428571428571437 0.3055555555555556 1.0368000000000002 1.1664 mm^2 1.4000000000000001 mm^2 Groove Width 1.5 mm 1.5 mm 1.75 mm -0 mm 0.25 mm inPHorm min Groove Depth 0.75 mm 0.75 mm 0.8 mm -0 mm 0.05 mm inPhormMax Modified ORring ID 2.5 mm 2.37 mm 2.63 mm -0.13 mm 0.13 mm min 0.0785761154855642 0.13043478260869565 0.6045714285714285 0.8464 mm^2 1.125 mm^2 ORing CS 1 mm 0.92 mm 1.08 mm -0.08 mm 0.08 mm nom -0.02463999999999995 0.25 0.8888888888888888 1 mm^2 1.125 mm^2 Groove ID 2.4384 mm 2.4384 mm 2.6884 mm -0 mm 0.25 mm max -0.11843475673262907 0.3055555555555556 1.0368000000000002 1.1664 mm^2 1.4000000000000001 mm^2 Groove Width 1.5 mm 1.5 mm 1.75 mm -0 mm 0.25 mm inPHorm min Groove Depth 0.75 mm 0.75 mm 0.8 mm -0 mm 0.05 mm inPhormMax References The Parker O-Ring Handbook ORD 5700 Some summaries here https://www.gallagherseals.com/blog/how-to-design-o-ring-gland-stretch?srsltid=AfmBOopdY2-1nzLdpUIv2eeZ4DyXf6IrnN1HKR_MgUjIhIAMUFkDazwW https://www.gallagherseals.com/blog/how-to-design-an-o-ring-gland-compression https://www.gallagherseals.com/blog/how-to-design-an-o-ring-gland-volume-fill https://www.gallagherseals.com/blog/how-to-design-effective-face-seals