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
A common design rule keeps maximum gland fill around 85%, leaving roughly 15% for thermal expansion and swell. Account for the worst-case tolerances and the expected volume swell of the compound in your media.