O Ring Size Calculator
Select the correct O-ring size from groove dimensions, ID, and cross-section. Enter values for instant results with step-by-step formulas.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Squeeze % = (CS - Groove Depth) / CS x 100 | Fill % = O-ring Area / Groove Area x 100
Where CS = O-ring cross-section diameter, Groove Depth = depth of the seal groove. Squeeze creates initial sealing contact. Fill percentage must leave room for thermal expansion (target 60-85%).
Worked Examples
Example 1: Static Face Seal Design
Problem:Design an O-ring seal for a static flange with groove ID 25mm, groove width 3.5mm, and groove depth 2.5mm at 10 MPa operating pressure.
Solution:Estimated cross-section = 3.5 x 0.7 = 2.45mm\nO-ring ID = 25 - (2.45 x 0.02) = 24.95mm\nSqueeze = (2.45 - 2.5) / 2.45 x 100 -- note: need CS > depth\nUsing standard CS 2.62mm (AS568-1xx series):\nSqueeze = (2.62 - 2.5) / 2.62 x 100 = 4.6% -- too low for static\nAdjust groove depth to 2.0mm:\nSqueeze = (2.62 - 2.0) / 2.62 x 100 = 23.7% -- within 15-30% range\nBackup ring: needed at 10 MPa for static seal
Result:Use CS 2.62mm O-ring | Groove depth 2.0mm | Squeeze: 23.7% | Backup ring recommended
Example 2: Hydraulic Cylinder Piston Seal
Problem:Select an O-ring for a 50mm bore hydraulic cylinder piston, dynamic reciprocating application at 7 MPa.
Solution:Bore diameter = 50mm\nRecommended cross-section for 50mm bore: 3.53mm (AS568-2xx)\nGroove depth for 10% squeeze: 3.53 x 0.90 = 3.18mm\nSqueeze = (3.53 - 3.18) / 3.53 x 100 = 9.9% (within 8-16%)\nGroove width = 3.53 x 1.3 = 4.59mm\nFill = (Pi x 1.765^2) / (4.59 x 3.18) x 100 = 67% (within 60-85%)\nBackup ring: needed at 7 MPa for dynamic seal
Result:O-ring CS: 3.53mm | Groove: 4.59 x 3.18mm | Squeeze: 9.9% | Fill: 67%
Frequently Asked Questions
How do you select the correct O-ring size for an application?
Selecting the correct O-ring size requires knowing the groove dimensions (inner diameter, width, and depth), the type of seal (static or dynamic), the operating pressure, temperature range, and the fluid being sealed. The O-ring inner diameter should be slightly smaller than the groove inner diameter to create a stretch of 1-5 percent, which keeps the O-ring seated properly. The cross-section diameter must be larger than the groove depth to achieve the required squeeze percentage. Standard O-ring sizes follow the AS568 series in North America or metric ISO 3601 series internationally. Always verify that the resulting squeeze and groove fill percentage fall within recommended ranges for the application type.
What is O-ring squeeze and why is it critical for sealing?
O-ring squeeze is the percentage of cross-section compression when the O-ring is installed in its groove, calculated as (cross-section minus groove depth) divided by cross-section times 100. Squeeze creates the initial contact stress that forms the seal before system pressure is applied. For static seals, recommended squeeze is 15-30 percent, while dynamic seals require 8-16 percent to balance sealing effectiveness against friction and wear. Too little squeeze allows leakage at low pressures, while too much squeeze causes excessive friction, premature wear in dynamic applications, and can permanently deform the O-ring through compression set. The squeeze must be sufficient to maintain sealing over the expected service life as the elastomer gradually takes a compression set.
What is the difference between static and dynamic O-ring seals?
Static seals have no relative motion between the mating surfaces, such as flange gaskets, pipe fittings, and cover plates. Dynamic seals must accommodate relative motion between surfaces, including reciprocating seals in hydraulic cylinders, rotary seals on shafts, and oscillating seals in valves. Dynamic seals require lower squeeze percentages (8-16 percent versus 15-30 percent for static) to minimize friction and wear. They also require smoother surface finishes on the moving surfaces (Ra 0.1-0.4 micrometers versus Ra 0.8-1.6 micrometers for static). Dynamic applications generate heat from friction, limiting speed and pressure capabilities. Material selection also differs, with dynamic seals requiring compounds with better wear resistance and lower friction coefficients.
When are backup rings needed for O-ring seals?
Backup rings (also called anti-extrusion rings) are needed when operating pressure exceeds the O-ring material ability to resist being forced into the extrusion gap between mating surfaces. For static seals with standard elastomers (70-90 Shore A), backup rings are recommended above 10 MPa. For dynamic seals, the threshold is lower at approximately 3.5 MPa because the dynamic gap is typically larger. At very high pressures above 35 MPa, backup rings on both sides may be required. The backup ring material is typically a harder material like PTFE, nylon, or polyacetal that bridges the extrusion gap. Without backup rings at high pressure, the O-ring material extrudes into the gap, causing nibbling damage that rapidly degrades the seal.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy