Definition
Gland fill (or groove fill) is the percentage of the groove’s cross-sectional volume occupied by the O-ring once installed. Fill % = O-ring volume ÷ groove volume × 100.
Leave room to expand
Rubber is essentially incompressible — it changes shape, not volume. When it heats up or swells in service fluid, it needs somewhere to go. If the groove is already full, the ring over-stresses the gland, can extrude, and loses sealing reliability.
Typical limit
The applicable window depends on the verified Parker module. Face seals use 60%–90% for liquid and 75%–98% for gas; vacuum is mapped to 75%–98% as an engineering interpretation of Parker’s shared Vacuum/Gas narrow-gland reference, not as a separately observed Ideal value. The five radial bands use 60%–90%. The calculator does not interpolate a criterion for a non-standard cross-section. It displays the complete tolerance stack and applies expected media swell only to the maximum O-ring volume case.