Designing the Inner Diameter

• The size of the o-ring's inner diameter should be designed to account for it stretching to fit the groove diameter---the diameter of the gland the inner diameter of the o-ring butts up against. The gland is the chamber that the two parts being connected together join. This area has a circular groove that the o-ring fits in to act as the seal between the two parts.
  
  The inner diameter should stretch between 1 percent to 5 percent, with 2 percent as the preferred amount. The inner diameter's size should be calculated by multiplying the groove diameter by the difference between 1 minus the stretch amount (1 to 5 percent). For example: ID = Groove diameter (1 -- stretch value). If going with 2 percent, then the equation goes to ID = Gd x 0.98.
  
  By accounting for the stretch, you ensure that the o-ring will not fall out or twist out during assembly.
  
  

Designing the Cross Section/Outer Diameter

• To figure out the cross section diameter of the o-ring, you have to take into account the compression the ring will take once in the gland, being pinched by the groove diameter and the bore diameter. The bore diameter is the inner diameter of the bore that butts against the outside diameter of the o-ring.
  
  For the o-ring to compress in the gland (and stay put), its cross section diameter must be greater than the depth of the groove. The most basic way to calculate the cross section diameter is bore diameter minus groove depth, divided by two.
  
  

Making the O-Ring

• Once you have the inner diameter and outer diameter of the o-ring figured out, you have your basic design. You either select from a manufacturer's pre-made catalog of standard sizes or have your piece custom-made.