An O-ring is a torus of elastomer that seals by being squeezed between two surfaces. The squeeze deforms the rubber, which pushes back against both surfaces with a contact stress greater than the fluid pressure trying to escape. Get the squeeze wrong and you either leak (too little) or destroy the ring (too much).

The gland is the geometry that matters. An O-ring doesn't seal — the gland seals, and the O-ring is just the deformable filler. A gland has three critical dimensions:

• Squeeze (compression): typically 15–30% of cross-section diameter for static seals, 10–20% for dynamic. Too little = leak path. Too much = compression set, where the rubber forgets its shape.
• Gland fill: 75–90% of the groove volume. Elastomers are nearly incompressible — they need somewhere to bulge when squeezed and when they swell from heat or fluid absorption. A 100%-filled groove will extrude the ring through the clearance gap.
• Stretch: 1–5% on the inside diameter for face seals. More than 5% and the cross-section thins (think pulling on a rubber band) and seal force drops.

Real-world example: hydraulic cylinder rod seal. A 1-inch rod with a Parker 2-214 O-ring (0.139" cross-section). Recommended squeeze for a reciprocating dynamic seal is ~12%, so groove depth = 0.139 × 0.88 ≈ 0.122". Pressure rating drops fast above 1,500 psi without a backup ring — the ring extrudes into the rod-to-bore clearance and gets nibbled off on each stroke. Add a PTFE backup ring on the low-pressure side and the same gland holds 3,000+ psi.

Material picks the fight, not the size. Buna-N (nitrile) is the default — cheap, oil-compatible, –40°F to 250°F. Viton (FKM) handles 400°F and aggressive chemicals but cracks below –15°F. EPDM loves brake fluid and steam but dies in petroleum. Silicone has wide temperature range but lousy tear strength. Always check chemical compatibility against the actual fluid, not just the family — a "synthetic oil" might contain an additive that swells nitrile 30%.

Common failure modes to recognize:

• Spiral failure: diagonal cuts on dynamic seals — the ring twisted instead of sliding. Fix: lower friction lube or smoother surface finish (Ra 8–16 µin on the dynamic surface).
• Extrusion (nibbling): ring gets chewed at the clearance gap. Fix: backup rings, or harder durometer (90A vs. 70A).
• Compression set: ring is flat on one side. The rubber aged out — replace and check operating temperature.

Rule of thumb: for a static face seal, groove depth = 0.75 × cord diameter, groove width = 1.5 × cord diameter. That gets you ~25% squeeze and ~85% fill, which works for almost anything below 1,500 psi.