The size of the O-ring can be accurately calculated using the following formula:

L = π * (D + d) / 2 + 2 * C

Among them:

L represents the length of the O-ring, which is the initial length when not stretched or compressed.

Pi is pi, usually taken as 3.14 or 3.1416, and the appropriate precision can be selected according to actual needs.

D is the outer diameter of the O-ring.

D is the inner diameter of the O-ring.

C is the cross-sectional thickness of the O-ring.

When calculating the length of O-rings, it is necessary to maintain consistency in units. Usually, millimeters or inches are used as units of length. If the units of outer diameter and inner diameter are different, unit conversion is required to ensure the accuracy of the calculation results.

In practical applications, accurate values can be obtained by measuring the outer and inner diameters of equipment or workpieces. Then, according to the calculation formula, substitute the outer diameter, inner diameter, and cross-sectional thickness to calculate the length of the O-ring.

In addition, the specifications and standards of O-rings are also factors that need to be considered. The specifications and models of O-rings mainly include UHSO ring specifications, UHPO ring specifications, UNO ring specifications, DHO ring specifications, piston rod O-ring specifications, etc. Each specification has its specific size range and application scenarios. When selecting O-rings, it is necessary to choose the appropriate specifications and sizes based on specific application scenarios and requirements.

The above information is for reference only. If necessary, it is recommended to consult a professional engineer or refer to relevant standards.