Calculate O-ring inside diameter, outside diameter, or cross-section from any two measurements, or check all three for consistency, in millimeters or inches.

Find the outside diameter from the opening and ring thickness.

Related Calculators

O-ring Size Formula

The following relationship calculates one dimension of an undeformed circular O-ring with a round cross-section. Use consistent units:

OD = ID + (2 × CS)

Variables:

  • OD is the outside diameter of the O-ring (mm)
  • ID is the inside diameter of the O-ring (mm)
  • CS is the cross-sectional diameter of the O-ring (mm)

To calculate the O-ring size, multiply the cross-sectional diameter by 2 and add the result to the inside diameter. This gives the outside diameter. The other modes rearrange the same relationship for ID or CS, or compare measured OD with the expected value using your allowed difference. Geometry does not determine a standard part number or seal design.

What is an O-ring Size?

An O-ring size refers to the dimensions of an O-ring, a mechanical gasket in the shape of a torus or doughnut. The size is typically defined by its internal diameter (ID) and its cross-sectional diameter (CS). These dimensions are crucial in ensuring a proper fit and effective seal in a variety of applications, such as in mechanical devices and plumbing systems, to prevent the escape or entry of gases or fluids.

How to Calculate O-ring Size?

The following steps outline how to calculate the O-ring Size using the formula: OD = ID + (2 * CS).

  1. First, determine the inside diameter of the O-ring (ID) in millimeters (mm).
  2. Next, determine the cross-sectional diameter of the O-ring (CS) in millimeters (mm).
  3. Next, use the formula OD = ID + (2 * CS) to calculate the outside diameter of the O-ring (OD) in millimeters (mm).
  4. Finally, calculate the O-ring Size.
  5. After inserting the variables and calculating the result, check your answer with the appropriate measuring tools.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Inside diameter of the O-ring (ID) = 10 mm

Cross-sectional diameter of the O-ring (CS) = 5 mm