O-Ring Size Chart & Standards
AS568, ISO 3601, JIS B 2401, DIN 3771 and metric O-ring sizing tables with free PDF downloads.
This O-ring sizing hub gives engineers one place to look up inside diameter, cross-section, dash numbers and groove dimensions across every major international standard. Whether you need AS568 imperial sizes, ISO 3601 metric tables, or JIS B 2401 references for Japanese equipment, each standard page includes searchable size data, tolerance classes and downloadable PDF charts for your documentation package. Use this O-ring specification chart alongside the size converter and compression calculator to move from measurement to verified seal specification quickly.
Choosing the correct O-ring size is critical for reliable sealing. Our size chart library covers the five most commonly used international standards, with full cross-reference tables, search-by-dimension tools, and downloadable PDFs for engineering documentation.
O-ring sizing is not merely a matter of finding a part number. The interaction between inside diameter, cross-section, groove dimensions, and compression rate determines whether a seal will perform reliably or fail prematurely. Our size chart pages go beyond simple dimension tables to include tolerance classes, recommended groove geometries, and compression rate guidance for each standard. This contextual information helps you specify not just a size, but a complete sealing system.
Different industries and regions favor different standards. North American aerospace, automotive, and hydraulic equipment traditionally uses AS568 dash numbers. European machinery and process equipment typically references ISO 3601 or DIN 3771 metric sizes. Japanese automotive and industrial equipment follows JIS B 2401. Understanding which standard your equipment was designed around is the first step in selecting the correct replacement or specifying a new seal.
Our online tools complement the static size charts. The size converter instantly translates between AS568 dash numbers and metric millimeter dimensions, eliminating manual calculation errors. The compression calculator validates that your chosen size will achieve the optimal squeeze percentage in your groove. The material selector helps confirm the compound matches the fluid and temperature.
O-Ring Sizing Tools
After you identify a size from the specification chart, use our compression calculator to verify groove squeeze, or the size converter to cross-reference AS568 dash numbers with ISO 3601 metric codes.
O-Ring Sizing Guide
O-ring sizing starts with two dimensions: inside diameter (ID) and cross-section (CS). Match the cross-section first to identify the standard series, then match the ID within that series. Always verify the resulting squeeze percentage in the groove before finalizing the specification.
1. Match cross-section first
Measure the cord diameter (CS) with digital calipers at three points around the ring. Match the average to the nearest standard CS value — 1.78 mm, 2.62 mm, 3.53 mm, 5.33 mm, 6.99 mm or 8.74 mm for AS568; 1.50 mm, 2.00 mm, 3.00 mm, 4.00 mm, 5.00 mm or 6.00 mm for common ISO 3601 sizes. The CS determines which series in the AS568 standard page table or ISO 3601 table to search.
2. Then match inside diameter
Measure the ID with internal caliper jaws or the circumference method for large rings. Convert to the unit used by your standard (inch for AS568, millimeter for ISO 3601) and find the closest dash number or metric code within the CS series identified in step one. Use the size converter to cross-check between inch and metric dimensions.
3. Verify compression in the groove
A correct size still fails if squeeze is wrong. Enter the CS and groove depth into the compression calculator to confirm 15–25% squeeze for static seals or 10–15% for dynamic reciprocating seals. If the calculated squeeze falls outside the recommended range, choose the next standard CS or redesign the groove rather than forcing a marginal fit.
AS568
Aerospace Standard 568 (Imperial)
The most widely used imperial O-ring standard in North America. Covers 349 dash numbers from -001 to -475 with inch-based inside diameters and cross-sections.
ISO 3601
ISO 3601-1 (Metric O-Rings)
The global metric standard for O-ring sizes. Organized by inside diameter and cross-section in millimeters. Fully compatible with DIN 3771.
JIS B 2401
Japanese Industrial Standard B 2401
The primary O-ring standard for Japanese machinery and automotive applications. Covers P-series, G-series, S-series and V-series sizes.
DIN 3771
DIN 3771 Metric O-Rings
The German metric standard, functionally aligned with ISO 3601. Widely used in European hydraulic and pneumatic equipment.
BS 4518
British Standard 4518
The established British metric O-ring reference for engineering, maintenance and European procurement teams.
Metric General
General Metric O-Ring Sizes
Non-standard metric sizes commonly used in Asian manufacturing. Available by ID x CS in millimeters with no dash number system.
SMS 1588
Swedish Standard SMS 1588
A Scandinavian sizing reference used in hygienic process equipment, marine systems and Nordic supply chains.
NF T47-501
French Standard NF T47-501
A French metric standard commonly referenced in hydraulic, industrial and automotive sealing applications.
JASO F404
Japanese Automotive Standard JASO F404
An automotive-focused O-ring reference for fuel, oil, coolant and air-conditioning sealing in JDM and global vehicle platforms.
How to Measure O-Rings
Accurate measurement is essential for identifying the correct replacement size or verifying a custom specification. Follow these guidelines to measure O-rings correctly using common shop tools.
1. Measure Inside Diameter (ID)
Place the O-ring on a flat surface. Use digital calipers to measure the inside diameter at two points 90 degrees apart. For large O-rings that cannot be measured directly, use the cord method: measure the cord length with the O-ring flattened, then calculate ID = (cord length / π) - CS. Record both measurements and use the average.
2. Measure Cross-Section (CS)
Measure the cross-section (wire diameter) at four points equally spaced around the circumference using the caliper's outside jaws. Do not compress the O-ring during measurement—apply only enough pressure to make contact. Record all four measurements. The average is your CS. If individual measurements vary by more than 0.05 mm, the O-ring may be out of specification or damaged.
3. Check for Ovality and Distortion
Place the O-ring on a surface plate and visually inspect for out-of-roundness or flat spots. Measure the ID in at least three orientations. If the maximum ID minus minimum ID exceeds 2% of the nominal ID, the seal has deformed and the measured dimensions may not reflect the original specification.
4. Compare to Standard Tables
Use our online Size Converter or download the relevant standard chart (AS568, ISO 3601, etc.). Find the size whose nominal ID and CS are closest to your measurements, allowing for manufacturing tolerance. If no standard size matches within tolerance, you may need a custom O-ring.
5. Document and Verify
Record the measured ID, CS, standard size identified, and any observed damage or deformation. If possible, verify the material and hardness using simple field tests (color, feel, smell) or send a sample to us for formal identification.
Important Notes
- Always measure O-rings at room temperature (20-25°C). Elastomers expand and contract with temperature, affecting dimensions.
- Worn or compressed seals will measure smaller than their original specification due to compression set. Allow for this when selecting a replacement.
- Cutting an O-ring to measure cross-section destroys the part and is unnecessary. Use calipers or a profile projector for non-destructive measurement.
- When in doubt, send us a photo with dimensions marked, or mail the physical sample for identification.
Engineering Tools
Size Converter
Convert between AS568 dash numbers and metric millimeter dimensions instantly.
Open ToolCompression Calculator
Calculate optimal squeeze percentage and gland fill ratio for any size.
Open ToolMaterial Selector
Match your size selection with the right material for temperature and chemical compatibility.
Open ToolFrequently Asked Questions
What is O-ring sizing?
O-ring sizing is the process of selecting an O-ring whose inside diameter (ID), cross-section diameter (CS) and manufacturing tolerance fit the groove and meet the target compression rate. Standard sizing references a dash number (AS568) or an ID × CS code (ISO 3601 / DIN 3771 / JIS B 2401). Tolerance class controls how much the actual dimensions may vary from nominal. Sizing is complete only when the chosen size also produces the correct squeeze in the installed groove — typically 15–25% for static seals and 10–15% for dynamic seals. Use our size chart hub to look up standard dimensions and the compression calculator to verify groove fit.
What is an O-ring specification chart?
An O-ring specification chart lists nominal inside diameter (ID), cross-section (CS), dash number or metric code, and manufacturing tolerances for a given standard such as AS568 or ISO 3601. Engineers use it to verify groove dimensions, select replacement seals, and document BOM specifications. Our charts include both online lookup and downloadable PDFs.
How do I use an O-ring sizing chart to find the right dash number?
Measure the groove or failed seal ID and cross-section with calipers, then search our AS568 or ISO 3601 tables for the closest nominal size within tolerance. Use the Size Converter tool to translate between dash numbers and millimeter dimensions, or download the specification chart PDF for offline reference.
What is the most common O-ring size standard?
AS568 is the most common standard in North America and globally for inch-based equipment. ISO 3601 is the dominant metric standard in Europe and Asia. JIS B 2401 is prevalent in Japanese automotive and industrial equipment.
Can I interchange AS568 and ISO 3601 sizes?
Some sizes are dimensionally close but not identical. Always verify the actual ID and CS dimensions before substituting one standard for another. Use our Size Converter tool for exact comparisons. Interchanging standards without verification can lead to incorrect compression rates and seal failure.
Do you stock all standard sizes?
We carry the full AS568 range and the most commonly used ISO 3601 and JIS B 2401 sizes in NBR, FKM and EPDM from stock. Non-stock sizes are available on 7-15 day lead times. If you have a recurring requirement for a non-stock size, we can add it to our safety stock program.
Can you supply custom non-standard sizes?
Yes. We manufacture custom O-rings to any ID and cross-section dimension. MOQ starts at 1 piece for standard profiles. No tooling is required for cut/vulcanized cord or small-run molding. Custom molded parts with special profiles can be produced in 7-15 days with in-house tooling.
How do I measure an O-ring if the part number is worn off?
Use a digital caliper to measure the inside diameter (ID) and cross-section (CS) at multiple points. Enter these dimensions into our online Size Converter tool, which will identify the closest standard size across AS568, ISO 3601, and JIS B 2401. If no standard size matches, the seal may be custom. Send us the measurements and we will identify or reproduce it.
What tolerance should I specify for an O-ring?
Standard molded O-rings are typically manufactured to ISO 3601-1 Class B or AS568 standard tolerances. For precision applications such as medical devices or aerospace, Class A or customer-specific tolerances can be achieved. Tighter tolerances increase cost and may require specialized tooling. We recommend Class B for general industrial applications.
Can I download the size charts as Excel files?
Yes. Contact us and we will provide the size chart data in Excel or CSV format for import into your CAD, ERP, or inventory management system. This service is free for registered customers and distributors.
What is the difference between a standard size and a general metric size?
Standard sizes belong to an organized numbering system such as AS568 or ISO 3601 with defined tolerances and groove recommendations. General metric sizes are specified simply by ID x CS in millimeters without a dash number. General metric sizes are common in Asian manufacturing but may lack the dimensional consistency and tooling availability of standardized sizes.
How do I select the right cross-section for my application?
Cross-section selection depends on groove depth, compression rate requirements, and extrusion risk. Thinner cross-sections (1-2 mm) are suitable for compact assemblies with limited groove depth. Thicker cross-sections (3-5 mm) provide better stability in high-pressure and large-diameter applications. Use our Compression Calculator to validate that your chosen cross-section achieves 15-25% compression in the groove.
How do I measure an O-ring for sizing?
Use a digital caliper to measure inside diameter (ID) at two points 90° apart and cross-section (CS) at four points around the circumference without compressing the seal. Enter the dimensions in our Size Converter to find the closest AS568 dash number or ISO 3601 metric code. See the How to Measure section on this specification chart page for step-by-step guidance.
Do size charts include groove dimensions?
Yes. Our downloadable size chart PDFs include recommended groove width, groove depth, corner radii, and surface finish for each standard size. These groove recommendations are based on industry best practices and ensure optimal compression rate and gland fill ratio.
Need a Custom Size?
Any ID and cross-section. MOQ starts at 1 piece. Quote within 24 hours.
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