O-Ring Size Chart: AS568, ISO 3601 and How to Measure an O-Ring
Identifying an O-ring by size is one of the most common tasks in maintenance and procurement. Whether you are replacing a failed seal or sourcing for a new design, accurate measurement is essential — a 1mm error in cross-section diameter can mean the difference between a seal that works and one that leaks immediately.
This guide covers how to measure an O-ring correctly, the main international size standards, and how to cross-reference between them.
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The Three Dimensions That Define an O-Ring
Every O-ring is defined by exactly three measurements:
1. Inside Diameter (ID) The internal diameter of the O-ring when lying flat and unstretched. Measured from inner edge to inner edge across the centre. This is the dimension that determines fit in the groove — the O-ring ID should be slightly smaller than the groove ID so the O-ring sits under slight tension and stays in position.
2. Cross-Section Diameter (CS) Also called the cord diameter or wire diameter. The diameter of the rubber cord that forms the ring — measured as the thickness of the O-ring material. This is the critical dimension for sealing — it determines compression rate in the groove.
3. Outside Diameter (OD) OD = ID + (2 × CS). The outside diameter is a derived dimension, not typically specified directly, but useful for checking fit in a groove bore.
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How to Measure an O-Ring: Step-by-Step
Tools Required
- Vernier caliper or digital caliper (0.01mm resolution or better)
- Flat surface
- Good lighting
Step 1: Measure the Cross-Section Diameter (CS)
Lay the O-ring flat. Open the caliper jaws and grip the O-ring cord at one point, measuring perpendicular to the ring axis — as if you were measuring a short piece of round rod.
Take measurements at three points around the circumference and average them. A worn or compressed O-ring may show variation.
Record to 0.01mm precision.
Step 2: Measure the Inside Diameter (ID)
Method A (caliper, direct measurement): Place the caliper tips inside the O-ring and expand to measure the inside span. This works well for O-rings above approximately 20mm ID.
Method B (diameter over mandrel): For small O-rings, place the O-ring over a cylindrical mandrel of known diameter. The O-ring should sit without stretch. Measure the mandrel diameter — this is the ID.
Method C (circumference, for large O-rings): Wrap a flexible tape measure around the inside circumference of the O-ring. Divide by π (3.14159) to get the inside diameter.
Record to 0.1mm precision for ID; matching to standard sizes uses nominal values.
Step 3: Cross-Reference to Standard Size
With your ID and CS measurements, look up the nearest standard size in the AS568 or ISO 3601 tables. Allow for measurement variation of ±0.1mm on CS and ±0.5mm on ID for worn seals.
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AS568 Standard — The US/Imperial Reference
AS568 (Aerospace Standard 568) is the dominant O-ring standard in North America, and widely used globally in industrial and aerospace applications. It defines O-ring sizes by a dash number system — each dash number corresponds to fixed inside diameter and cross-section dimensions.
AS568 Dash Number System
The dash number is a three-digit code:
- -0XX series (001–050): Small cross-section, 0.070" CS (1.78mm)
- -1XX series (102–178): 0.103" CS (2.62mm)
- -2XX series (201–284): 0.139" CS (3.53mm)
- -3XX series (309–395): 0.210" CS (5.33mm)
- -4XX series (425–475): 0.275" CS (6.99mm)
Common AS568 Sizes Reference
| Dash No. | ID (inch) | CS (inch) | ID (mm) | CS (mm) |
|---|---|---|---|---|
| AS568-010 | 0.239 | 0.070 | 6.07 | 1.78 |
| AS568-014 | 0.364 | 0.070 | 9.25 | 1.78 |
| AS568-110 | 0.239 | 0.103 | 6.07 | 2.62 |
| AS568-214 | 0.984 | 0.139 | 24.99 | 3.53 |
| AS568-222 | 1.359 | 0.139 | 34.52 | 3.53 |
| AS568-312 | 1.484 | 0.210 | 37.69 | 5.33 |
| AS568-332 | 2.984 | 0.210 | 75.79 | 5.33 |
| AS568-425 | 4.234 | 0.275 | 107.54 | 6.99 |
The full AS568 standard covers 369 dash numbers. Browse our complete AS568 size chart with searchable table and metric conversions.
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ISO 3601 Standard — The Metric Reference
ISO 3601 is the international metric O-ring standard, used throughout Europe, Asia and in metric engineering globally. It uses a direct metric notation: ID × CS in millimetres.
ISO 3601 Series
- ISO 3601-1: Dimensional and tolerance requirements
- ISO 3601-2: Housing dimensions for general applications
- ISO 3601-3: Quality acceptance criteria
- ISO 3601-4: Anti-extrusion rings
- ISO 3601-5: Dimensions for linear seals
ISO 3601 Tolerance Classes
- Class A: Tighter tolerances for precision applications (hydraulics, pneumatics)
- Class B: Standard tolerances for general industrial use
Common ISO 3601 Sizes
| ISO Size | ID (mm) | CS (mm) | Approximate AS568 Equivalent |
|---|---|---|---|
| 6.00 × 1.50 | 6.00 | 1.50 | Near AS568-006 |
| 10.00 × 2.00 | 10.00 | 2.00 | Near AS568-111 |
| 25.00 × 3.00 | 25.00 | 3.00 | Near AS568-214 |
| 50.00 × 3.00 | 50.00 | 3.00 | Near AS568-228 |
| 100.00 × 5.00 | 100.00 | 5.00 | Near AS568-347 |
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Other International Standards
BS 1806 (British Standard)
Used extensively in the UK and Commonwealth countries. Similar structure to AS568 but with different size increments. If you encounter a BS-specified O-ring, cross-reference to AS568 or ISO 3601 to find the nearest equivalent.
DIN 3771 (German Standard)
German metric standard, now largely superseded by ISO 3601 but still referenced in older German machine documentation. DIN 3771 sizes align closely with ISO 3601.
JIS B 2401 (Japanese Standard)
Japanese metric standard, commonly encountered in Japanese equipment. P series (for shafts), G series (for general) and V series available. Cross-reference with ISO 3601 for sourcing equivalents.
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Parker Size Chart Reference
Parker Hannifin publishes one of the most widely used O-ring reference charts, the Parker O-Ring Handbook (ORD 5700). Engineers familiar with Parker part numbering can convert to standard dash numbers as follows:
- Parker 2-XXX series → AS568 -XXX (direct equivalent)
- Parker metric sizes → correspond to ISO 3601 dimensions
Our AS568 size chart covers the same dimensional range as the Parker reference table. Use our searchable AS568 database to find any dash number with inch and metric dimensions.
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Measurement Tips for Worn O-Rings
Worn or failed O-rings may be difficult to measure accurately:
- Compressed/flattened O-ring: CS will measure smaller than original. Estimate original CS from the larger diameter measurement and application context.
- Swollen O-ring: Both ID and CS will be larger than original due to fluid absorption. The original size may be 10–20% smaller.
- Cut or damaged O-ring: Measure undamaged sections only.
- Best practice: If the equipment documentation is available, use the drawing or BOM specification rather than measuring a failed seal.
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Choosing Between Inch and Metric O-Ring Standards
If you are designing a new system:
- US/North American market: specify AS568
- European/international market: specify ISO 3601
- Japanese equipment: specify JIS B 2401
If you are sourcing a replacement:
- Identify the standard from equipment documentation if available
- If undocumented, measure the O-ring and cross-reference to the nearest standard size
- For critical applications, verify groove dimensions and calculate compression rate before ordering
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Frequently Asked Questions
How do I find an O-ring size if I only have the old (worn) part? Measure the cross-section first — CS changes less with wear than ID. Compare your CS measurement to standard cross-section values (1.78mm, 2.62mm, 3.53mm for AS568; or standard metric values for ISO 3601). Once you identify the CS series, measure ID and find the closest matching dash number in that series.
What is the most accurate way to measure a small O-ring (under 10mm ID)? Use the mandrel method: find a cylindrical pin gauge or drill bit that fits snugly inside the O-ring without stretching it. The pin diameter equals the O-ring ID. For cross-section, use a digital caliper directly on the cord.
Can I substitute a metric O-ring for an inch O-ring? Only if the dimensions are within the tolerance range for the application. The groove is designed around specific O-ring dimensions — substituting a different size changes the compression rate and may cause either leakage (under-compressed) or extrusion (over-compressed). Always verify the compression rate calculation when substituting between standards.
What does AS568-214 mean? AS568-214 is a specific O-ring size: inside diameter 0.984 inches (24.99mm), cross-section 0.139 inches (3.53mm). The -2XX series uses 0.139" cross-section. The number 14 within the series indicates the specific ID within that cross-section group.
Why does Parker's O-ring size chart use different numbers than AS568? Parker uses a two-prefix notation (e.g., 2-214) where the first digit indicates the cross-section series and the remaining digits match the AS568 dash number. Parker 2-214 = AS568-214. The dimensional values are identical.