O-ring size chart (AS568).
AS568 dash-number O-ring sizes — the US standard. Dash number, inside diameter, cross-section, and series. ISO 3601 metric equivalents noted where applicable.
The chart
| Dash # | ID (inch) | ID (mm) | Cross-section | Series / Note |
|---|---|---|---|---|
| 001 | 0.029" | 0.74 mm | 0.040" (1.02 mm) | Very small (0xx) |
| 004 | 0.070" | 1.78 mm | 0.070" (1.78 mm) | Small bore |
| 008 | 0.176" | 4.47 mm | 0.070" (1.78 mm) | Small bore |
| 010 | 0.239" | 6.07 mm | 0.070" (1.78 mm) | Small bore |
| 012 | 0.364" | 9.25 mm | 0.070" (1.78 mm) | Small bore |
| 014 | 0.489" | 12.42 mm | 0.070" (1.78 mm) | Small bore |
| 016 | 0.614" | 15.60 mm | 0.070" (1.78 mm) | Small bore |
| 020 | 0.864" | 21.95 mm | 0.070" (1.78 mm) | Small bore |
| 024 | 1.114" | 28.30 mm | 0.070" (1.78 mm) | Common small (0xx) |
| 030 | 1.614" | 41.00 mm | 0.070" (1.78 mm) | 0xx series |
| 040 | 2.864" | 72.74 mm | 0.070" (1.78 mm) | Larger 0xx |
| 104 | 0.139" | 3.53 mm | 0.103" (2.62 mm) | 1xx series |
| 110 | 0.362" | 9.19 mm | 0.103" (2.62 mm) | 1xx series |
| 120 | 0.987" | 25.07 mm | 0.103" (2.62 mm) | 1xx series |
| 200 | 0.114" | 2.90 mm | 0.139" (3.53 mm) | 2xx series begins |
| 210 | 0.734" | 18.64 mm | 0.139" (3.53 mm) | 2xx common |
| 214 | 0.984" | 25.00 mm | 0.139" (3.53 mm) | Common metric-equivalent |
| 218 | 1.234" | 31.34 mm | 0.139" (3.53 mm) | 2xx common |
| 222 | 1.484" | 37.69 mm | 0.139" (3.53 mm) | 2xx common |
| 226 | 1.984" | 50.39 mm | 0.139" (3.53 mm) | 2xx common |
| 230 | 2.484" | 63.09 mm | 0.139" (3.53 mm) | 2xx larger |
| 240 | 3.734" | 94.84 mm | 0.139" (3.53 mm) | 2xx large |
| 312 | 0.612" | 15.54 mm | 0.210" (5.33 mm) | 3xx series |
| 318 | 0.987" | 25.07 mm | 0.210" (5.33 mm) | 3xx series |
| 325 | 1.487" | 37.77 mm | 0.210" (5.33 mm) | 3xx series |
| 335 | 2.987" | 75.87 mm | 0.210" (5.33 mm) | 3xx series |
| 345 | 4.487" | 113.97 mm | 0.210" (5.33 mm) | 3xx series |
| 425 | 4.475" | 113.67 mm | 0.275" (6.99 mm) | 4xx series |
| 440 | 6.975" | 177.17 mm | 0.275" (6.99 mm) | 4xx large |
About AS568. The Aerospace Standard 568 (SAE) defines 369 specific O-ring sizes by dash number. Each series has a constant cross-section, with inside diameters scaling by series: 0xx = 0.070" (1.78 mm) CS, 1xx = 0.103" (2.62 mm) CS, 2xx = 0.139" (3.53 mm) CS, 3xx = 0.210" (5.33 mm) CS, 4xx = 0.275" (6.99 mm) CS. The above shows a representative sample; a full catalog is from Parker, McMaster-Carr, or AS568 reference.
Common applications
| Application type | Recommended material | Notes |
|---|---|---|
| Standard fluid (oil/water 0-100°C) | NBR (Buna-N) | Cheapest, default for most applications |
| Hot oil (100-150°C) | FKM (Viton) | High temperature; petroleum oils |
| Steam, hot water | EPDM | Polar fluids; NOT petroleum |
| Aerospace fuel | FKM or FFKM | Resistant to JP-fuels |
| Cryogenic | Silicone | Down to ~-100°C |
| Food contact | FDA-grade silicone or EPDM | Confirm FDA 21 CFR 177.2600 compliance |
| High vacuum | FKM or low-outgassing FKM | Avoid plasticizer-rich elastomers |
| Strong acids | FFKM or Aflas | PTFE may also be used (with care) |
| High pressure (>3000 psi) | Standard + backup ring | Or harder durometer (90A vs 70A) |
Common pitfalls
- 'AS568-214' specifies size only, not material. Different materials (Buna-N, Viton, EPDM, silicone, FFKM) work for very different chemical and temperature conditions. Always specify both size and material.
- O-ring stretches in service. An installed O-ring should be stretched to about 1-5% of its free ID for static face seals. Less stretch = leakage; more stretch = premature failure.
- Squeeze ratio matters. A static face seal is typically squeezed 15-25% of cross-section thickness. Less = leakage; more = extrusion or compression set. Manufacturer recommendations vary by application.
- Hardness (durometer) affects performance. Standard is 70 Shore A. Softer (50A) seals at lower pressures but extrudes under high pressure. Harder (90A) handles high pressure but needs higher squeeze and surface finish.
- Surface finish matters more than people think. For dynamic seals (sliding/rotating), the mating surface should be Ra 8-32 µin (0.2-0.8 µm) — neither too rough (wears the ring) nor too smooth (microscratching). For static seals, Ra 32-63 µin is typical.
Common questions
How do I choose between standard sizes for a custom application?
Pick a groove first (depth and width based on standard size charts), then choose the O-ring size that fits with proper squeeze (typically 15-30% for static, 10-20% for dynamic). The cross-section affects squeeze; the inside diameter affects stretch. AS568 size charts list the standard combinations — start there to avoid custom tooling.
What's 'squeeze' and why does it matter?
Squeeze is how much the O-ring cross-section is compressed in the groove. Too little (< 8%) and it won't seal under pressure spikes; too much (> 30% for static, > 20% for dynamic) and it extrudes through gaps or fails from compression set. Static seals tolerate more squeeze than dynamic ones — moving seals heat up from friction at high squeeze.
What material should I use for fuel exposure?
Nitrile (Buna-N, NBR) is the standard for gasoline and most petroleum fuels. For diesel and biodiesel, hydrogenated nitrile (HNBR) is better — biodiesel attacks standard nitrile over time. For aviation fuels, fluorocarbon (Viton, FKM) is the choice. Always check the chemical compatibility chart against your specific fluid.
Why is my O-ring leaking even though it's the right size?
Common causes in order: surface finish too rough (need 16-32 µin Ra for static seals), wrong material for the fluid, twisted during installation, groove dimensions out of spec, extrusion through clearance gaps under pressure, or too low squeeze. Inspect the O-ring for cuts, flat spots, or chemical degradation before assuming sizing is the problem.
What's the difference between AS568 and metric O-rings?
AS568 is the US standard with 369 dash numbers, mostly fractional-inch dimensions. Metric O-rings use mm dimensions and aren't standardized to AS568. Some sizes are close to interchangeable — e.g. AS568 -010 (0.239" ID × 0.07" cross-section) is very close to metric 6×1 mm — but always verify before swapping.
Sources
- O-ring sizes (US): SAE AS568 — Aerospace Standard for O-ring Sizes.
- O-ring sizes (international): ISO 3601-1 — Pneumatic and hydraulic fluid systems — Sealing elements — Part 1: Inside diameters, cross-sections, tolerances and designation codes.
- O-ring design: Parker O-Ring Handbook (free PDF, industry standard reference).
- Materials: ASTM D2000 — Standard Classification for Rubber Products in Automotive Applications.
Disclaimer. O-ring selection is a function of fluid, temperature, pressure, sliding velocity, surface finish, and groove geometry. For pressure-critical or safety-critical applications, consult the Parker O-Ring Handbook or equivalent industry reference.