O-Ring Guide — Materials, Sizes, and How to Choose the Right Seal
21 Mar 2026
O-rings are simple but critical components — a single rubber ring that prevents leaks in hydraulics, plumbing, pneumatics, and thousands of other applications. Choose the wrong material or size, and you get leaks, premature failure, or chemical degradation.
How O-Rings Work
An O-ring is a loop of elastomer (rubber) with a circular cross-section. It sits in a groove (called a gland) and gets compressed between two surfaces. The compression creates a seal that prevents fluid or gas from passing through.
Two types of sealing:
- Static seal: No relative movement between surfaces (pipe flanges, covers, housings)
- Dynamic seal: One surface moves relative to another (pistons, rotary shafts)
O-Ring Materials Compared
| Material | Abbreviation | Temp Range | Best For | Avoid |
|---|---|---|---|---|
| Nitrile | NBR (Buna-N) | -30°C to 120°C | Petroleum oils, fuels, hydraulic fluid, water | Ozone, sunlight, ketones |
| Silicone | VMQ | -60°C to 230°C | Food/medical, extreme temps, dry heat | Fuels, oils, strong acids |
| Viton® | FKM | -20°C to 200°C | Chemicals, acids, high temp fuels | Ketones, ammonia, steam |
| EPDM | EPDM | -50°C to 150°C | Hot water, steam, brake fluid, ozone | Petroleum oils, fuels |
| PTFE | PTFE | -200°C to 260°C | Almost all chemicals, extreme temperatures | Expensive, not elastic |
| Neoprene | CR | -40°C to 120°C | Refrigerant (Freon), moderate oils, outdoor | Strong oxidizing acids |
Which Material Should You Use?
- General purpose / hydraulics / automotive: → NBR (Nitrile) — cheapest and most versatile
- Food processing / medical / drinking water: → Silicone — FDA-approved versions available
- Chemical processing / fuel systems: → Viton (FKM) — handles aggressive chemicals
- Hot water / steam / brake systems: → EPDM — excellent steam resistance
- Extreme temperatures / chemical inert: → PTFE — works almost everywhere but costs more
How to Measure an O-Ring
Three measurements define an O-ring:
- ID (Inner Diameter) — the diameter of the hole
- OD (Outer Diameter) — the overall outside diameter
- CS (Cross Section) — the thickness of the rubber cord
The relationship: OD = ID + (2 × CS)
When ordering, always specify ID and CS. The OD is calculated automatically.
Common O-Ring Sizes (Metric)
| ID (mm) | CS (mm) | OD (mm) | Typical Application |
|---|---|---|---|
| 3 | 1 | 5 | Small pneumatic fittings |
| 5 | 1.5 | 8 | Tap/faucet washers |
| 8 | 2 | 12 | Hydraulic fittings |
| 10 | 2 | 14 | Cylinder pistons |
| 15 | 2.5 | 20 | Pump housings |
| 20 | 3 | 26 | Pipe flanges |
| 25 | 3 | 31 | Filter housings |
| 30 | 3.5 | 37 | Large cylinder seals |
| 40 | 4 | 48 | Industrial pipe joints |
| 50 | 5 | 60 | Large flange seals |
Tips for Longer O-Ring Life
- Lubricate during assembly. Use silicone grease or the system fluid to prevent pinching and tearing.
- Avoid sharp edges. Chamfer all groove edges to prevent cutting the O-ring during installation.
- Don't stretch more than 5%. An over-stretched O-ring will have a thinner cross-section and fail sooner.
- Store in bags away from sunlight. UV and ozone degrade rubber over time.
Shop O-Rings at RPI Shop
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