Metric vs AS568 O-Rings: What Buyers Need to Know
When you are looking for a replacement O-ring, you may see sizes written in two different ways: a direct metric size, or a standard number such as AS568.
This can be confusing at first, but the basic idea is simple. A metric O-ring is usually listed by its actual size in millimeters. An AS568 O-ring is listed by a standard number that points to a specific inch-based size.
If you already know your ID and CS, you can search O-ring small packs or use the O-Ring Finder. If you are not sure how to measure the ring, start with our guide on how to measure an O-ring by ID, CS, and OD.
What is a metric O-ring?
A metric O-ring is usually written as:
ID × CS in millimeters
For example:
10.00 × 2.00 mm
This normally means:
10.00 mm inner diameter × 2.00 mm cross section
Metric sizes are easy to understand because the size is shown directly. If you can measure the old O-ring with a caliper, you can usually compare your measurement with the listed size.
What is an AS568 O-ring?
AS568 is a common O-ring size standard used for inch-based O-rings, especially in North America.
Instead of writing the full inch size every time, AS568 uses standard numbers. For example, you may see a size listed as:
AS568-214
That number refers to a specific standard O-ring size. It is not just a random product code.
If you already know the AS568 number, searching the standard number is usually better than guessing from a rough measurement.
Metric size vs AS568 number
The main difference is how the size is shown.
| Type | How it is shown | Best when you know |
|---|---|---|
| Metric O-ring | ID × CS in mm | The measured size in millimeters |
| AS568 O-ring | AS568 standard number | The standard number or inch-based size |
If your old part, drawing, manual, or supplier label gives an AS568 number, search that number directly. If you only have the old ring in your hand, measure the ID and CS first.
For more help matching ID and CS, read the O-Ring Size Chart Guide.
Can a metric size be close to an AS568 size?
Sometimes a measured metric size may look close to an AS568 size. That does not always mean they are exactly the same.
Old O-rings can stretch, swell, flatten, or shrink after use. A used ring may also measure slightly different from its original size.
If the application is simple and low-risk, a close standard size may be enough. If the O-ring is used with fuel, pressure, heat, steam, chemicals, or expensive equipment, do not guess. Use Bulk Quote and include your measurements, photos, or drawing.
Should you search by size or by standard number?
Use the information you trust most.
If you do not trust the size, standard number, or old sample, start with what to do if you don't know your O-ring size.
If you know the exact AS568 number:
Search the AS568 number directly.
If you measured the old O-ring:
Search by ID × CS in millimeters, or use the O-Ring Finder.
If you only know an inch size:
Search the inch size if possible, or include both inch and metric measurements in a quote request.
If the size comes from a worn old ring:
Measure more than once and check the groove if possible.
Do material and hardness change the choice?
Yes. Size and standard number only tell you the shape. They do not confirm the right material.
The same size may be available in NBR, EPDM, FKM, Silicone, FFKM, and different hardness levels. These materials are not the same.
NBR is often used for oil and grease. EPDM is often used for water, weather, and outdoor exposure. FKM is often considered for fuel, oil, and higher heat. Silicone is often used where flexibility or cleaner applications matter. FFKM is usually for more demanding chemical or temperature conditions.
If you are not sure, check the O-Ring Material Guide. For fuel, oil, chemicals, steam, or unknown fluids, check the Chemical Compatibility Guide before ordering.
When should you use Bulk Quote?
Use Bulk Quote if any of these apply:
- You need 100+ pcs.
- You cannot find the size in small packs.
- You have a drawing or sample but no clear size.
- You only know an inch size and are not sure about the standard number.
- The O-ring is used with fuel, chemicals, pressure, steam, or high temperature.
- You need a special material, hardness, or custom size.
When sending a quote request, include the size, standard number if known, material, hardness, application, quantity, and any photo or drawing you have.
Simple buying path
You know the metric size and material.
Search Shop Small Packs.
You know the AS568 number.
Search the AS568 number directly.
You know the size but not the material.
Read the Material Guide first.
You do not know the size.
Start with how to measure an O-ring.
You need 100+ pcs or a special requirement.
Use Bulk Quote.
FAQ
Is AS568 metric or inch?
AS568 is an inch-based O-ring size standard. Metric O-rings are usually listed directly by ID × CS in millimeters.
Can I replace an AS568 O-ring with a metric O-ring?
Sometimes a metric size may be close, but it should be checked carefully. For pressure, fuel, chemicals, heat, or critical equipment, do not guess.
What should I search if I know AS568-214?
Search the full AS568 number directly. Standard numbers are usually more reliable than rough measurements from an old ring.
What if I only know the inch size?
Search the inch size if possible. If you are not sure, send both inch and metric measurements through Bulk Quote.
Does AS568 tell me the material?
No. AS568 tells you the size standard. You still need to choose the right material and hardness for the application.
