An O-ring is a circular, torus-shaped elastomer seal that closes the gap between two mating surfaces — a bore and a shaft, a flange and a cover, a fitting and a boss. It seals by being compressed ("squeezed") between the gland walls, so the rubber's elastic memory keeps pushing outward against both surfaces. Static seals (flanges, covers, plugs) simply hold that squeeze in place; dynamic seals (rotating shafts, reciprocating pistons) also have to survive repeated motion without losing that seal — which is where material choice and cross-section start to matter as much as the size itself.
Because an O-ring's performance depends on squeeze percentage, fluid compatibility and temperature simultaneously, getting the size right is only half the job — the compound has to match the application before the size chart is even relevant.
| Your situation | Start with |
|---|---|
| Mineral oil, hydraulic fluid, general machinery | NBR |
| Hot oil, fuel, oil & gas, aromatics up to 200°C | FKM |
| Water, steam, outdoor/weather exposure | EPDM |
| Aggressive or unpredictable chemical exposure | FFKM |
| Food, beverage, pharma, medical contact | Silicone |
| Sour gas, amines, steam beyond FKM's range | AFLAS |
| Amine/high-pH chemical exposure, FKM-family performance | Viton Extreme (ETP/GBL/TBR) |
Full detail on temperature range, resists/avoids, and typical applications for every compound is on our Materials page.
Full standard cross-section range shown: 1.0–10.0mm metric series plus AS568 -0xx to -4xx. Sizes listed cover our most-requested variants per series — exact match to your drawing confirmed on quotation. Manufactured to AS568 · BS 4518 · GB/T 3452.1 · JIS B2401 (P/G/S/V) — send your standard and dash number for a confirmed cross-reference.
Send O-Ring Requirement →Measured a ring that doesn't match a standard size? Enter the exact dimensions and we'll confirm against our tooling or quote a custom mold.
Every O-ring is defined by two numbers: cross-section (CS) and inside diameter (ID) — outside diameter is derived as OD = ID + 2×CS. Measuring CS first with a caliper narrows you to one standard series before you ever look for a size.
| CS Series | Standard | Typical Use |
|---|---|---|
| 1.0 / 1.5 / 2.0 / 2.5 / 3.0 mm | Metric round series | Small precision, instrumentation and light-duty seals |
| 1.78 mm — AS568 -001 to -050 | AS568 (Imperial) | Small hydraulic & pneumatic fittings, instrumentation |
| 2.62 mm — AS568 -102 to -178 | AS568 (Imperial) | General hydraulics, pump housings |
| 3.53 mm — AS568 -201 to -284 | AS568 (Imperial) | Most common industrial series — valves, flanges, pumps |
| 4.0 / 5.0 / 6.0 mm | Metric round series | Medium-to-heavy static & dynamic seals |
| 5.33 mm — AS568 -309 to -395 | AS568 (Imperial) | Large flange & static seals, heavy hydraulics |
| 6.99 mm — AS568 -401 to -475 | AS568 (Imperial) | Large-bore static seals, tank & vessel closures |
| 7.0 / 8.0 / 10.0 mm | Metric round series | Very large-bore static seals |
| Standard | Region / Origin | Notes |
|---|---|---|
| AS568 (A/B/C/D) | USA — SAE International | Dash-number system, first published 1971; most common worldwide reference for imperial sizing |
| ISO 3601 / Metric | International | Metric ID/CS sizing and tolerance classes; default across Europe and most non-US OEM drawings |
| JIS B2401 (P/G/S/V) | Japan | Japanese Industrial Standard — separate series for piston, gland, static and vacuum applications |
| BS 4518 | United Kingdom | British metric O-ring standard, common on legacy UK-designed equipment |
| SMS 1586 | Scandinavia | Swedish/Nordic standard occasionally specified on process equipment |
Not sure which standard your drawing references? Send us the dash number, ID, or CS you have — we'll confirm the standard and cross-reference an equivalent FITCO size.
General-purpose seal for mineral oils, fuels and hydraulic fluids.
Material detail →-20°C to +200°CHigh-temperature, chemical-resistant sealing for oil & gas and process industries.
Material detail →-15°C to +327°CNear-universal chemical resistance for the most demanding process seals.
Material detail →-40°C to +150°CWater, steam and weather-resistant sealing — the wrong choice for mineral oil.
Material detail →-30°C to +150°CHigher heat and abrasion resistance than standard NBR for demanding drives.
Material detail →-60°C to +230°CFood-, medical- and electrically-safe sealing across a wide temperature band.
Material detail →-60°C to +200°CSilicone's temperature range with added fuel and oil resistance standard silicone doesn't have.
Material detail →-10°C to +230°CExtreme chemical + heat resistance for sour gas and amine service beyond standard FKM.
Material detail →-20°C to +200°CGeneral-purpose FKM dipolymer, 66% fluorine — the baseline Viton grade for standard petroleum and fuel contact.
Material detail →-20°C to +200°CTerpolymer with ~68% fluorine — enhanced resistance to inorganic acids, caustics and harsh industrial fluids.
Material detail →-15°C to +200°CHigh-fluorine terpolymer (~70%) engineered for oxygenated automotive fuels and alternative fuel blends.
Material detail →-40°C to +200°CLow-temperature specialist grades — cold flexibility down to -40°C without giving up FKM's fluid resistance.
Material detail →-20°C to +200°CHigh-hardness FKM engineered to prevent Anti-Explosive/Rapid Gas Decompression failures in high-pressure systems.
Material detail →-20°C to +230°CThe core Viton Extreme terpolymer — amine and high-pH chemical resistance beyond standard FKM.
Material detail →-20°C to +230°CPeroxide-cured Viton Extreme with added steam and acid resistance.
Material detail →-20°C to +230°CThe broadest base-resistance Viton Extreme formulation, for multi-chemical process exposure.
Material detail →-40°C to +120°CGeneral-purpose weather- and ozone-resistant rubber with moderate oil resistance.
Material detail →ID (inside diameter) is the hole through the middle, OD (outside diameter) is the total width across the ring, and CS (cross-section) is the thickness of the rubber itself. OD = ID + (2 × CS).
NBR is the standard choice for mineral oil and hydraulic fluid at moderate temperatures. Move to FKM if temperatures exceed roughly 100°C or the fluid includes aromatic content.
AS568 sizes in inches using a dash-number system; ISO 3601 sizes in millimetres with its own tolerance classes. They aren't directly interchangeable — confirm which standard your drawing references before ordering.
Yes — use the custom ID/CS section above, or send us a drawing or sample. Custom sizes are quoted with tooling lead time depending on volume.
