What is AS568?
AS568 is the standard that defines o-ring sizes in the United States. Every standard o-ring you’ll encounter in American engineering has a “dash number” (like -214 or -028) that maps to specific inner diameter (ID) and cross-section (CS) dimensions.
The dash number tells you everything:
- First digit = cross-section series (0XX = 0.070″ CS, 1XX = 0.103″ CS, 2XX = 0.139″ CS, 3XX = 0.210″ CS, 4XX = 0.275″ CS)
- Remaining digits = size within that series (larger number = larger ID)
Standard O-Ring Size Tables
| Series | Dash Numbers | Cross-Section | ID Range | Link |
|---|---|---|---|---|
| 000 Series | 001–114 | 0.070″ | 0.029″ – 5.234″ | View table → |
| 100 Series | 115–178 | 0.103″ | 0.487″ – 11.196″ | View table → |
| 200 Series | 201–264 | 0.139″ | 0.171″ – 16.955″ | View table → |
| 300 Series | 265–349 | 0.210″ | 4.975″ – 22.940″ | View table → |
| 400 Series | 350–439 | 0.275″ | 0.176″ – 26.940″ | View table → |
How to Pick an O-Ring Size
- Determine your groove or bore diameter
- Decide face seal or radial seal — this determines whether you match on ID or OD
- Pick the right cross-section — bigger CS = more forgiving seal, but needs a bigger groove
- Look up the dash number from the tables above
- Design your groove — see the Static O-Ring Groove Design Guide
Common Materials
| Material | Abbreviation | Temp Range | Best For |
|---|---|---|---|
| Buna-N (Nitrile) | NBR | -40°F to 250°F | General purpose, oils, fuels |
| Viton | FKM | -15°F to 400°F | High temp, chemicals, fuels |
| Silicone | VMQ | -80°F to 400°F | Food grade, medical, dry heat |
| EPDM | EPDM | -60°F to 300°F | Water, steam, brake fluid |
| PTFE | PTFE | -350°F to 500°F | Chemical resistance, non-elastic backup rings |