O-Ring Size Chart — Complete Guide to AS568, ISO 3601 and International O-Ring Standards

O-rings are among the most important and widely used sealing elements in mechanical engineering. Found in everything from hydraulic cylinders and pneumatic systems to medical devices, automotive engines, aerospace components, and household appliances — the humble O-ring is responsible for preventing fluid and gas leaks in countless applications worldwide. Selecting the correct O-ring size is critical to system performance and safety. This complete guide covers everything you need to know about O-ring sizing, international standards, material selection, and how to find the right O-ring for any application.

What is an O-Ring?

An O-ring is a torus-shaped (doughnut-shaped) sealing element made from an elastomeric material. It is designed to sit in a machined groove and be compressed between two mating surfaces — creating a seal that prevents fluid or gas from passing through the joint. When correctly sized and installed an O-ring creates a leak-proof seal that can withstand significant pressure, temperature extremes, and chemical exposure depending on the material chosen.

O-rings were first patented by Danish-American inventor Niels Christensen in 1937 and became widely adopted during World War II in aircraft hydraulic systems. Today billions of O-rings are manufactured and used annually across virtually every industry.

How O-Rings Are Measured

Every O-ring is defined by two critical dimensions:

1. Inner Diameter (ID) — the diameter of the hole through the centre of the O-ring, measured across the inside of the ring. This determines how the O-ring fits around a shaft or into a bore.

2. Cross Section (CS) — also called the cord diameter or wire diameter, this is the thickness of the rubber cord that forms the ring. This determines how much the O-ring compresses in the groove.

From these two dimensions a third value is derived:

3. Outer Diameter (OD) — the total outside diameter of the O-ring.

OD = ID + (2 × CS)

For example an O-ring with ID = 50 mm and CS = 3 mm has an OD = 50 + (2 × 3) = 56 mm.

When ordering or specifying O-rings you need to provide at minimum the ID and CS — or the standard code number which contains this information implicitly.

International O-Ring Standards

O-ring sizes are standardised under several international and national standards. The most important are:

AS568 — American Standard (Most Widely Used Globally)

The AS568 standard (formerly MS28775) is published by the Society of Automotive Engineers (SAE) and is the most widely used O-ring standard in the world. It is used extensively in the United States, United Kingdom, Australia, and internationally across aerospace, defence, oil and gas, automotive, and general engineering.

AS568 O-rings are identified by a dash number preceded by a hyphen — for example -214 or AS568-214. The dash numbers range from -001 to -475 and are divided into series:

Series	Dash Numbers	Cross Section	Typical Applications
000 series	-001 to -050	1.78 mm (0.070”)	Small precision seals, instruments
100 series	-101 to -178	1.78 mm (0.070”)	Small hydraulic and pneumatic
200 series	-201 to -284	2.62 mm (0.103”)	Medium hydraulic and pneumatic
300 series	-301 to -395	3.53 mm (0.139”)	Larger hydraulic cylinders
400 series	-401 to -475	5.33 mm (0.210”)	Large bore hydraulic systems
900 series	-901 to -932	6.99 mm (0.275”)	Very large seals

ISO 3601 — International Metric Standard

ISO 3601 is the international metric O-ring standard published by the International Organization for Standardization. It is used primarily in Europe, Asia, and countries that have fully adopted the metric system. ISO 3601 is divided into four parts:

• ISO 3601-1: Dimensional requirements — specifies standard O-ring sizes in metric dimensions
• ISO 3601-2: Housing dimensions for general applications
• ISO 3601-3: Quality acceptance criteria
• ISO 3601-4: Anti-extrusion rings (back-up rings)

ISO 3601 sizes are specified directly by inner diameter and cross section in millimetres — for example 50 × 3 indicates ID = 50 mm, CS = 3 mm.

BS1806 — British Standard

BS1806 is the British Standard for O-rings, historically widely used in UK engineering. It is similar to AS568 in many respects and uses the same dash number system. With the UK’s adoption of ISO standards BS1806 is now largely superseded by ISO 3601 for new designs, though many legacy specifications still reference BS1806.

JIS B2401 — Japanese Standard

JIS B2401 is the Japanese Industrial Standard for O-rings. It uses a letter-number designation system (P for piston series, G for general purpose) and specifies dimensions in millimetres. JIS O-rings are commonly found in Japanese-manufactured equipment including vehicles, industrial machinery, and hydraulic equipment.

JIS Series	Applications
P series	Piston and cylinder applications
G series	General purpose fittings and connectors
V series	Vacuum applications
S series	Straight thread fittings

Complete AS568 Size Chart — Selected Sizes

The following table shows selected AS568 O-ring sizes across all series. For the complete chart use the CalcGlobe O-Ring Size Calculator.

100 Series (CS = 1.78 mm / 0.070”)

Dash No.	ID (mm)	ID (inch)	CS (mm)	OD (mm)
-102	3.68	0.145”	1.78	7.24
-104	6.07	0.239”	1.78	9.63
-106	8.46	0.333”	1.78	12.02
-108	10.82	0.426”	1.78	14.38
-110	13.21	0.520”	1.78	16.77
-112	15.60	0.614”	1.78	19.16
-114	17.96	0.707”	1.78	21.52
-116	20.35	0.801”	1.78	23.91
-118	22.74	0.895”	1.78	26.30
-120	25.12	0.989”	1.78	28.68
-125	31.47	1.239”	1.78	35.03
-130	37.82	1.489”	1.78	41.38
-140	50.52	1.989”	1.78	54.08
-150	63.22	2.489”	1.78	66.78

200 Series (CS = 2.62 mm / 0.103”)

Dash No.	ID (mm)	ID (inch)	CS (mm)	OD (mm)
-201	5.94	0.234”	2.62	11.18
-204	11.43	0.450”	2.62	16.67
-206	14.86	0.585”	2.62	20.10
-208	17.86	0.703”	2.62	23.10
-210	20.35	0.801”	2.62	25.59
-212	22.23	0.875”	2.62	27.47
-214	25.12	0.989”	2.62	30.36
-216	27.99	1.102”	2.62	33.23
-218	30.48	1.200”	2.62	35.72
-220	33.38	1.314”	2.62	38.62
-225	41.28	1.625”	2.62	46.52
-230	49.61	1.953”	2.62	54.85
-240	63.09	2.484”	2.62	68.33
-250	76.20	3.000”	2.62	81.44

300 Series (CS = 3.53 mm / 0.139”)

Dash No.	ID (mm)	ID (inch)	CS (mm)	OD (mm)
-309	22.23	0.875”	3.53	29.29
-311	27.99	1.102”	3.53	35.05
-313	33.38	1.314”	3.53	40.44
-315	37.69	1.484”	3.53	44.75
-318	44.45	1.750”	3.53	51.51
-320	50.80	2.000”	3.53	57.86
-325	63.09	2.484”	3.53	70.15
-330	76.20	3.000”	3.53	83.26
-340	101.60	4.000”	3.53	108.66
-350	127.00	5.000”	3.53	134.06
-360	152.40	6.000”	3.53	159.46

400 Series (CS = 5.33 mm / 0.210”)

Dash No.	ID (mm)	ID (inch)	CS (mm)	OD (mm)
-425	63.09	2.484”	5.33	73.75
-430	76.20	3.000”	5.33	86.86
-440	101.60	4.000”	5.33	112.26
-450	127.00	5.000”	5.33	137.66
-460	152.40	6.000”	5.33	163.06
-470	177.80	7.000”	5.33	188.46

How to Measure an Existing O-Ring

If you need to replace an O-ring but do not have the part number you can measure the original O-ring to find the correct replacement. Use a digital calliper for the most accurate measurement.

Step 1 — Measure the Cross Section (CS): Place the O-ring on a flat surface. Measure the diameter of the rubber cord — this is the CS. Measure at a point where the O-ring has not been stretched or compressed. A worn O-ring may have a slightly reduced CS — measure carefully.

Step 2 — Measure the Inner Diameter (ID): Lay the O-ring flat. Measure across the inside of the ring from inner edge to inner edge. For small O-rings this is easier with the O-ring laid over a circular reference object.

Step 3 — Find the matching standard size: Use your measured ID and CS to find the closest standard size in the AS568 or ISO 3601 table. Standard O-rings are available in specific sizes only — select the closest standard size to your measurements.

Important note: Used O-rings may have been stretched in service and may measure slightly larger than their nominal size. The groove dimensions are a more reliable reference than the worn O-ring when identifying the correct replacement size.

O-Ring Groove Design

Correct groove design is as important as correct O-ring selection. The groove must be machined to the correct dimensions to achieve the right compression and seal performance.

Key groove parameters:

Compression (squeeze): The percentage by which the O-ring cross section is compressed in the installed position. Standard compression is:

• Static seals: 15–30% compression
• Dynamic seals (reciprocating): 10–20% compression
• Rotary seals: 5–15% compression

Too little compression and the O-ring will not seal. Too much compression causes premature wear and may extrude the O-ring material.

Groove width: Typically 1.2–1.5 times the O-ring cross section to allow for volumetric expansion of the elastomer.

Groove depth: Calculated to achieve the required compression: Groove depth = CS × (1 − target compression %)

For a CS = 3.53 mm O-ring with 20% target compression: Groove depth = 3.53 × (1 − 0.20) = 3.53 × 0.80 = 2.82 mm

Surface finish: Dynamic seal grooves require a smoother surface finish than static seals. Typical requirements: static ≤ 3.2 μm Ra, dynamic ≤ 0.8 μm Ra.

O-Ring Material Selection Guide

Choosing the right O-ring material is as important as choosing the right size. The wrong material will degrade, swell, harden, or crack in service — causing seal failure. Here is a comprehensive guide to the most common O-ring materials:

NBR — Nitrile Butadiene Rubber

Temperature range: −40°C to +120°C (−40°F to +248°F)

Excellent resistance to:

• Mineral oils and hydraulic fluids (most common application)
• Petroleum-based fuels (petrol, diesel)
• Greases and lubricants
• Water and water-based fluids
• Compressed air

Poor resistance to:

• Ozone and UV weathering
• Aromatic hydrocarbons (benzene, toluene)
• Polar solvents (ketones, esters)
• Brake fluid (glycol-based)

Applications: The most widely used O-ring material globally. Standard choice for hydraulic systems, automotive engines, pneumatic systems, and general industrial sealing. If you are unsure which material to use and the application involves mineral oil or water, NBR is usually the default choice.

Colour: Typically black (though not exclusively) Shore A hardness: 70 standard (also available in 60, 80, 90)

FKM / Viton — Fluoroelastomer

Temperature range: −20°C to +200°C (−4°F to +392°F) — up to +230°C for short periods

Excellent resistance to:

• Aggressive chemicals including concentrated acids and bases
• Aromatic and chlorinated solvents
• Petroleum and synthetic fuels
• High temperature steam (to limited temperatures)
• Ozone, UV, and weathering
• Silicone oils and greases

Poor resistance to:

• Low-temperature flexibility (stiffens below −15°C)
• Ketones (acetone, MEK)
• Low molecular weight esters
• Hot water above 200°C

Applications: Chemical processing, oil and gas, aerospace, automotive (fuel systems, turbochargers), and any application requiring high temperature resistance or compatibility with aggressive chemicals. The premium O-ring material — typically 5–10 times more expensive than NBR.

Colour: Typically brown or black Common brand name: Viton (DuPont/Chemours trademark — now a generic descriptor for FKM)

EPDM — Ethylene Propylene Diene Monomer

Temperature range: −55°C to +150°C (−67°F to +302°F)

Excellent resistance to:

• Hot water and steam
• Glycol-based brake fluids (DOT 3, DOT 4, DOT 5.1)
• Ozone, UV, and weathering — excellent outdoor durability
• Phosphate ester hydraulic fluids (Skydrol)
• Ketones and alcohols
• Dilute acids and alkalis

Poor resistance to:

• Petroleum-based oils and fuels — will swell significantly
• Mineral hydraulic fluids
• Silicone oils

Applications: Automotive cooling systems, brake systems, HVAC, water treatment, outdoor applications, and anywhere hot water or steam sealing is required. Never use EPDM in contact with petroleum oils.

Colour: Typically black or grey

Silicone (VMQ / PVMQ)

Temperature range: −60°C to +230°C (−76°F to +446°F)

Excellent resistance to:

• Extreme temperature range — best low-temperature flexibility of all common O-ring materials
• Ozone and UV
• Dry heat
• Food-grade applications (platinum-cured silicone)

Poor resistance to:

• Petroleum oils and fuels (significant swelling)
• Steam above 120°C
• Acids and alkalis
• Dynamic applications (poor abrasion resistance — not suitable for reciprocating or rotary seals)

Applications: Food processing equipment, medical devices, baking and cooking equipment, lighting, outdoor electrical connectors, and any application requiring extreme temperature range with static sealing only.

Colour: Typically translucent, red, blue, or white Note: Silicone has poor mechanical strength and tear resistance — it is generally not suitable for dynamic (moving) seals.

Neoprene (CR — Chloroprene Rubber)

Temperature range: −40°C to +120°C (−40°F to +248°F)

Excellent resistance to:

• Refrigerants (Freon and similar)
• Ozone and weathering
• Moderate petroleum oils
• Ammonia

Applications: Refrigeration and air conditioning systems, marine applications, weather seals. Has been largely superseded by EPDM for many applications but remains the standard choice for refrigerant sealing.

PTFE — Polytetrafluoroethylene

Temperature range: −200°C to +260°C (−328°F to +500°F)

Exceptional chemical resistance to virtually all chemicals — PTFE is essentially chemically inert and is resistant to almost every known fluid and chemical.

Limitations: PTFE is not an elastomer — it does not spring back when compressed. PTFE O-rings are used as backup rings (anti-extrusion rings) alongside elastomeric O-rings in high-pressure applications, or as primary seals in extremely aggressive chemical environments where no elastomer is suitable.

Applications: Chemical processing with extremely aggressive fluids, pharmaceutical and food processing, cryogenic applications.

O-Ring Material Quick Selection Guide

Application	Recommended Material
Hydraulic oil systems	NBR (first choice), FKM (high temp)
Pneumatic air systems	NBR or EPDM
Automotive fuel systems	FKM / Viton
Automotive cooling (water/glycol)	EPDM
Automotive brake systems	EPDM
Chemical processing	FKM (most chemicals), EPDM (alkalis/ketones), PTFE (extreme)
Hot water and steam	EPDM (to 150°C), PTFE (above 150°C)
Food and beverage	Silicone (static), EPDM, Platinum-cure silicone
Medical devices	Silicone, Platinum-cure silicone
Refrigeration/HVAC	Neoprene, EPDM
Outdoor/weathering	EPDM (first choice), FKM
Oil and gas (general)	NBR (general), FKM (H2S/sour service)
Aerospace	FKM, EPDM (Skydrol), Silicone
Cryogenic (very low temp)	Silicone (to −60°C), PTFE (below −60°C)

Common O-Ring Failure Modes

Understanding why O-rings fail helps prevent failures in service:

Compression set: The O-ring permanently deforms and does not spring back when the compression is removed. Caused by incorrect material for the temperature/chemical environment, excessive compression, or ageing. Result: loss of sealing force and leakage.

Extrusion and nibbling: The O-ring is forced into the clearance gap between mating components under pressure. Caused by excessive clearance gap, insufficient O-ring hardness, or excessive pressure without backup rings. Result: the O-ring is progressively destroyed, producing rubber debris and leakage.

Abrasion: Surface wear of the O-ring seal face in dynamic applications. Caused by inadequate lubrication, rough surface finish, or incorrect compression in dynamic seals. Result: gradual thinning of the O-ring leading to leakage.

Chemical degradation: The O-ring swells, hardens, cracks, or dissolves due to incompatibility with the fluid being sealed. Caused by incorrect material selection. Result: seal failure — potentially rapid in aggressive chemical environments.

Thermal degradation: Hardening and cracking of the O-ring due to excessive temperature. Caused by operating above the material’s temperature limit. Result: loss of elasticity and seal failure.

Installation damage: Cuts, nicks, or spiral twisting of the O-ring during installation. Caused by sharp edges on mating components or incorrect installation technique. Result: immediate or early seal failure.

Frequently Asked Questions

Q: What is the difference between AS568 and ISO 3601 O-rings? AS568 is the American standard using imperial-based dimensions expressed in inches, with sizes identified by dash numbers. ISO 3601 is the international metric standard with sizes expressed in millimetres. While some sizes are dimensionally close between the two standards they are not directly interchangeable — always use O-rings specified to the same standard as the groove design. If converting an AS568 groove to ISO 3601 or vice versa the groove dimensions should be recalculated.

Q: Can I use a slightly different size O-ring if the exact size is not available? In non-critical static applications a small size variation (within 5% of ID and CS) may be acceptable — but this should be assessed by a qualified engineer for each application. In dynamic seals, high-pressure seals, or safety-critical applications never substitute a non-standard size. The groove is designed for specific O-ring dimensions and the wrong size will result in incorrect compression, premature failure, or immediate leakage.

Q: How do I lubricate an O-ring for installation? O-rings should always be lightly lubricated before installation to prevent installation damage, reduce friction, and extend seal life. Use a lubricant compatible with both the O-ring material and the system fluid. For NBR O-rings in hydraulic applications use petroleum-based grease or the system hydraulic oil. For EPDM O-rings in water or brake fluid applications use silicone grease or the system fluid. Never use petroleum-based lubricants on EPDM O-rings — it will cause swelling.

Q: What is an O-ring backup ring? A backup ring (also called an anti-extrusion ring) is a non-elastomeric ring — typically made from PTFE or a hard plastic — installed in the O-ring groove on the low-pressure side of the O-ring. It prevents the O-ring from extruding into the clearance gap at high pressures. Backup rings are used in high-pressure hydraulic and pneumatic applications typically above 70–100 bar depending on clearance gap size and O-ring hardness.

Q: What is the shelf life of O-rings? Shelf life depends on material and storage conditions. Properly stored O-rings (away from heat, light, ozone, and oxygen — ideally in sealed bags in a cool, dry, dark environment) have the following approximate shelf lives per SAE AS5316:

• NBR: 15 years
• EPDM: 15 years
• FKM / Viton: 20 years
• Neoprene: 15 years
• Silicone: 20 years

O-rings stored in poor conditions (exposed to UV, ozone, heat, or chemicals) may degrade much sooner. Always inspect O-rings for cracking, hardening, tackiness, or surface damage before installation regardless of age.

Find Your O-Ring Size Instantly

Use the CalcGlobe O-Ring Size Chart to look up any AS568 or ISO 3601 O-ring size instantly. Enter a dash number or inner diameter to find ID, CS, and OD in both millimetres and inches. Includes the complete AS568 chart from -001 to -475 and the full ISO 3601 metric series.

No signup required. Free forever.

Disclaimer: This article is for educational and reference purposes only. O-ring selection and groove design for safety-critical applications must be performed by a qualified engineer and verified against the current published standard. Always refer to the current edition of AS568, ISO 3601, or the relevant national standard for authoritative specifications. CalcGlobe accepts no liability for seal failures or engineering decisions made based on information from this article.

Sources: SAE AS568B, ISO 3601-1:2012, Parker O-Ring Handbook ORD 5700, Trelleborg Sealing Solutions Engineering Guide.