O-Ring Size Chart: AS568, ISO 3601 & Measurement

Identifying an O-ring by size is one of the most common tasks in maintenance and procurement. Every O-ring is defined by exactly two measured dimensions — inside diameter (ID) and cross-section diameter (CS) — and one derived dimension (outside diameter: OD = ID + 2 × CS). The standard system you need depends on where the equipment was designed: AS568 for North American equipment, ISO 3601 for European and international metric equipment, and JIS B 2401 for Japanese equipment.

Quick answer: Measure CS first (the easier and more critical dimension), then ID. Match CS to the nearest standard value from the table below to identify the standard series. A 0.1 mm error in CS identification changes the compression rate by 2–5 percentage points — the difference between a functional seal and a chronic leak.

The Three Dimensions That Define an O-Ring

Inside Diameter (ID): The internal diameter measured from inner edge to inner edge when the ring lies flat and unstretched. The O-ring free-state ID should be 2–5% smaller than the groove ID so the installed ring sits under slight tension and maintains circumferential contact with the groove wall.

Cross-Section Diameter (CS): Also called cord diameter or wire diameter. The diameter of the circular cross-section that forms the ring body. CS is the critical sealing dimension: it determines compression rate (squeeze) in the groove:

Compression rate (%) = (CS − Groove Depth) / CS × 100

Target: 12–15% for dynamic seals; 15–25% for static seals.

Outside Diameter (OD): OD = ID + (2 × CS). A derived dimension used to verify that the installed O-ring does not contact the bore wall before the groove bottom is reached.

How to Measure an O-Ring: Step-by-Step Procedure

Tools Required

• Digital caliper, 0.01 mm resolution minimum (0.001 mm preferred for CS measurement)
• Flat reference surface
• Good lighting

Step 1: Measure Cross-Section Diameter (CS) First

CS is the more critical measurement and easier to measure precisely than ID.

1. Lay the O-ring flat on the surface
2. Place caliper external jaws at the O-ring cord at one point — grip perpendicular to the ring axis, as if measuring a round rod
3. Close gently until light contact is made — do not compress the elastomer
4. Read to 0.01 mm
5. Repeat at 3–4 positions around the circumference (top, bottom, left, right)
6. Average the readings — acceptable variation on new O-rings: ≤ 0.05 mm; variation > 0.10 mm indicates out-of-round cross-section or wear

Worn or compressed O-rings: A permanently deformed O-ring will show smaller CS than original. Add 10–20% to measured CS to estimate original design value, then match to nearest standard CS.

Step 2: Measure Inside Diameter (ID)

Three methods depending on O-ring size:

Method A — Direct caliper internal jaws (ID > 15 mm) Insert caliper internal jaws into the O-ring hole. Expand to measure inside span across the diameter. Measure at two perpendicular orientations and average. Record to 0.1 mm.

Method B — Pin or mandrel gauge (ID < 15 mm) Find a cylindrical pin gauge, calibrated mandrel, or drill bit shank that fits inside the O-ring without stretching it. The O-ring should rest on the pin with no visible tension. Pin diameter = O-ring free-state ID. Pin gauge sets with 0.5 mm increments are sufficient for standard size matching.

Method C — Circumference method (ID > 100 mm) Wrap a flexible tape measure around the inside circumference. Measure circumference (C) and calculate:

ID = C ÷ π = C ÷ 3.14159

Accurate to ±0.5 mm — sufficient for standard size matching at large diameters.

Step 3: Cross-Reference to Standard Size

Match CS first (it narrows the standard series), then match ID within that series.

Standard cross-section values across all major standards:

AS568 Standard: North American Reference

AS568 (SAE Aerospace Standard 568) is the dominant O-ring standard in North America and is widely used globally. It defines 369 O-ring sizes using a dash number system.

AS568 Dash Number Structure

Common AS568 Sizes with Full Metric Equivalents

AS568 Tolerance Classes

For critical applications (aerospace, semiconductor, oil & gas), always specify AS568 Class 2 — the tighter CS tolerance (±0.05 mm vs ±0.08 mm for small sizes) is significant when groove depth tolerance is tight and worst-case compression rate analysis is marginal.

ISO 3601 Standard: Metric International Reference

ISO 3601 is the international metric O-ring standard used throughout Europe, Asia, and in metric engineering globally. The current revision is ISO 3601-1:2012.

ISO 3601 Parts and Their Scope

ISO 3601 Tolerance Grades

Grade N is the default for industrial supply — most catalog O-rings are Grade N. Grade S requires additional inspection capability and costs 15–25% more.

Common ISO 3601 Sizes

When ISO 3601 and AS568 CS values don't match: ISO 3601 uses CS values (1.50, 2.00, 3.00, 4.00, 5.00 mm) that have no direct AS568 equivalent. Do not substitute between these CS values — a 3.00 mm CS ISO ring produces a compression rate approximately 4–5 percentage points different from a 3.53 mm CS AS568 ring at the same groove depth. Recalculate compression rate before any cross-standard CS substitution.

Other International Standards

JIS B 2401: Japanese Standard

JIS B 2401 is the Japanese metric O-ring standard, commonly encountered in Japanese-manufactured equipment (pumps, valves, hydraulic systems, automotive).

JIS B 2401 P-series uses CS values of 1.9, 2.4, 3.1, 3.5, 4.0, 5.7, 8.4 mm — none of which match the AS568 CS values exactly. For JIS P-10 (10.0 mm ID × 2.4 mm CS), the nearest ISO 3601 is 10.0 × 2.0 mm — but verify compression rate before substituting, as the 0.4 mm CS difference changes compression rate by 2–3 percentage points.

BS 1806: British Standard (Historical)

BS 1806 was the British O-ring standard, now superseded by ISO 3601. It defined inch-based sizes in four cross-section series similar to AS568 but with different ID increments. Equipment documented to BS 1806 before 1990 references BS dash numbers that are dimensionally similar to AS568 equivalents — BS and AS568 share CS values, and most IDs fall within ±1 mm. For sourcing replacement seals to BS 1806 specifications, cross-reference to AS568 by measured dimensions.

DIN 3771: German Standard (Historical)

DIN 3771 was the German metric O-ring standard, now replaced by ISO 3601. DIN 3771 O-rings are dimensionally equivalent to ISO 3601 in common size ranges. Equipment with DIN 3771 specifications is directly replaceable with ISO 3601 O-rings of identical dimensions.

Parker O-Ring Numbering Convention

Parker Hannifin's O-Ring Handbook (ORD 5700) is the most widely distributed O-ring reference in North America. Parker's internal numbering uses a prefix before the AS568 dash number:

Parker 2-214 = AS568-214 = ID 0.984 inch / CS 0.139 inch. This convention is common knowledge in North American maintenance — "Parker 2-214 NBR" is an AS568-214 in nitrile rubber.

Cross-Standard Size Identification Procedure

When equipment documentation specifies a standard you don't recognize:

1. Identify the standard (AS568, ISO 3601, JIS B2401, BS 1806, DIN 3771) — usually in the drawing title block or purchase specification
2. Look up CS and ID for that standard's size number
3. Identify the nearest equivalent in your required standard: CS must match within ±0.05 mm; ID within ±0.5 mm for diameters < 50 mm; ±1.0 mm for larger
4. Calculate compression rate for the replacement size in the existing groove — if it changes by more than ±2 percentage points, either modify the groove or the substitution is not acceptable

Measurement Tips for Worn or Degraded O-Rings

Best practice: Never rely solely on a failed O-ring for dimensions. Use the equipment drawing or groove dimensions — measure groove depth (add 15–20% for static, 12–15% for dynamic target squeeze) to estimate required CS; measure groove ID to estimate required O-ring ID. The groove dimensions are far more reliable than the failed part.

FAQ

Q1: How do I find an O-ring size if I only have the worn or failed part?

Measure CS first — it is less altered by most failure modes than ID. Compare to the standard CS table to identify the CS series. Then measure ID and find the nearest standard dash number in that CS series. For compressed or permanently deformed O-rings, add 10–20% to measured CS to estimate original. Better: measure the groove — groove depth and groove bore diameter give CS and ID directly from standard tables.

Q2: Can I substitute an ISO 3601 O-ring for an AS568 O-ring?

Only if CS matches within ±0.05 mm and ID matches within ±0.5 mm. Where AS568 and ISO 3601 share the same CS value (e.g., both use 3.53 mm CS), direct substitution at matching ID is straightforward. Where CS values differ (e.g., AS568 3.53 mm vs ISO 3.00 mm), the compression rates will differ by approximately 4–5 percentage points — a meaningful design difference requiring groove redesign, not a direct substitution.

Q3: What does an AS568 dash number tell me about the O-ring?

The dash number encodes both the cross-section series and the ID. The hundreds digit (or leading digit for -1xx, -2xx, etc.) identifies the CS series. The two remaining digits map to a specific ID in the AS568 table. AS568-214: series 2 = 3.53 mm CS; "14" within the -2xx series maps to ID = 0.984 inch (24.99 mm). AS568-320: series 3 = 5.33 mm CS; "20" maps to ID = 1.234 inch (31.34 mm).

Q4: What is the most accurate way to measure a small O-ring (under 10 mm ID)?

Use the mandrel (pin gauge) method: find a calibrated gauge pin that fits inside the O-ring without stretching it. Pin diameter = free-state O-ring ID. For CS, use a digital caliper at 0.01 mm resolution on the cord. For O-rings smaller than 5 mm ID, a calibrated optical comparator or vision measurement system is more reliable than contact caliper measurement.

Q5: Is there a standard for O-ring groove dimensions?

Yes — ISO 3601-2 specifies housing (groove) dimensions for general applications. Parker's O-Ring Handbook Table 3-4 specifies groove dimensions for all AS568 dash numbers. SAE AS4716 specifies face seal groove dimensions for hydraulic fittings. In practice, most engineers use Parker's groove tables or standard groove dimension tables provided by their O-ring supplier — these encode the compression rate and fill rate calculations for each O-ring size.

Q6: Why does Parker use a "2-" prefix before the AS568 dash number?

Parker's numbering system uses the first digit to indicate seal type: 2 = elastomeric O-ring; 4 = PTFE backup ring; 6 = specialty seal. The three remaining digits match the AS568 dash number exactly. So Parker 2-214 = AS568-214 in dimensions; Parker 4-214 = a PTFE backup ring designed for use with AS568-214. Other manufacturers use different prefix conventions — always convert to AS568 dash number for cross-manufacturer sourcing.

Q7: What does ISO Grade N vs Grade S mean for my application?

Grade N (Normal tolerance) is standard for general industrial applications — most catalog O-rings are Grade N. Grade S (Special tolerance) has approximately 50% tighter ID and CS tolerances, producing more consistent compression rates in precision grooves. Grade S is specified for precision hydraulics and pneumatics where compression rate variation directly affects sealing performance or friction. Grade S costs 15–25% more than Grade N; specify only when worst-case compression rate analysis shows Grade N tolerance is insufficient.

Q8: How do I identify an O-ring if there is no part number and the equipment has no documentation?

Measure CS with a caliper (stack 3–5 rings for accuracy) and match to the nearest standard CS value. Note the CS series this belongs to. Measure ID using the internal jaws method or mandrel method. Look up the combination in AS568 or ISO 3601 tables — start with whichever standard matches the equipment's country of origin. Also note any color code (many suppliers use black for NBR, red/brown for FKM, blue for EPDM food grade) and test the material's flexibility and oil compatibility to narrow down the elastomer type. From CS + ID + material, you can identify the part number in any standard.

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Need O-rings in AS568 or ISO 3601 sizes? Request a quote with your dash number or ID × CS dimensions, material, and quantity — we supply AS568 and ISO 3601 standard sizes in NBR, FKM, EPDM, HNBR, VMQ, and FFKM from MOQ 1 piece. Custom sizes available from cord splicing or compression molding.