What to Do If You Don't Know Your O-Ring Size

If you need a replacement O-ring but do not know the size, do not guess too quickly.

O-rings can look simple, but a small difference in size or material can change how well they seal. The good news is that you can usually get close by checking a few basic details first.

This guide explains what to measure, what information to collect, and when to search O-ring small packs or use Bulk Quote.

Start with the old O-ring if you have it

If you still have the old O-ring, start there.

Place it on a clean, flat surface. Do not stretch it. If possible, use a digital caliper instead of a ruler.

The two most important measurements are:

• ID — inner diameter, the open space inside the ring
• CS — cross section, the thickness of the rubber

You can also check OD, or outer diameter, with this formula:

OD = ID + (2 × CS)

If you are not sure how to measure these, start with our guide on how to measure an O-ring by ID, CS, and OD.

If the old O-ring is damaged

Old O-rings are not always reliable for measurement.

Heat, oil, fuel, water, chemicals, pressure, and age can make rubber swell, shrink, flatten, crack, or stretch. If the ring is damaged, measure it more than once and check different spots around the ring.

If the ring looks flat or uneven, the groove or sealing area may tell you more than the old O-ring itself.

When the size is unclear, write down your best measurements and use Bulk Quote instead of forcing a guess.

Check the groove or sealing area

If the O-ring sits in a groove, look at the groove carefully.

You may be able to check:

• The inside diameter of the groove
• The width of the groove
• The depth of the groove
• Whether the seal is static or moving
• Whether the part seals oil, water, air, fuel, steam, or chemicals

You do not need to know every engineering detail before asking for help. But the more information you provide, the easier it is to avoid the wrong size.

Look for a standard number

Some O-rings are listed by a standard number instead of a direct size.

For example, you may see an AS568 number such as:

AS568-214

If you know the AS568 number, search that number directly. Standard numbers are often more reliable than measuring a worn old O-ring.

If you are not sure whether your O-ring is metric or AS568, read our Metric vs AS568 O-Rings Guide.

If you know the size but not the material

Size is only part of the choice. Material matters too.

An O-ring used with oil may need a different material than one used with water, fuel, steam, outdoor weather, or chemicals.

As a simple starting point:

• NBR is often used for oil and grease.
• EPDM is often considered for water, weather, and outdoor exposure.
• FKM is often considered for fuel, oil, and higher heat.
• Silicone may be used where flexibility or cleaner applications matter.
• FFKM is usually for more demanding chemical or high-temperature conditions.

For a simple overview, read the O-Ring Material Guide. For fuel, oil, chemicals, steam, or unknown fluids, check the Chemical Compatibility Guide before ordering.

When you can search small packs

You can search Shop Small Packs when you know:

• The inner diameter
• The cross section
• The material
• The hardness, if required
• The quantity you want

If you know the size but want to understand how it matches a listed size, read the O-Ring Size Chart Guide.

If the size and material are clear, small packs are the fastest route.

When you should use Bulk Quote instead

Use Bulk Quote if any of these apply:

• You do not know the exact size.
• The old O-ring is damaged, swollen, or stretched.
• You only have a photo, sample, or drawing.
• You need 100+ pcs.
• You need a size that is not listed.
• You need a special material or hardness.
• The O-ring is used with fuel, chemicals, pressure, steam, or high temperature.

Bulk Quote is also better when the application is unclear. It gives you room to include notes, photos, drawings, and quantity requirements.

What to send when asking for help

If you do not know the exact O-ring size, send as much of this as you can:

• Inner diameter, if measured
• Cross section, if measured
• Outer diameter, if measured
• Metric size or inch size, if known
• AS568 number, if known
• Material, if known
• Hardness, if known
• Application or equipment type
• Fluid exposure, such as oil, water, fuel, air, steam, or chemicals
• Temperature range, if known
• Quantity needed
• Photo, drawing, or old sample details

You do not need to have every detail. But sending more information helps reduce back-and-forth and lowers the risk of choosing the wrong O-ring.

A simple decision path

You have the old O-ring and it is still round.
Measure ID and CS first.

You have the old O-ring but it is damaged.
Measure it, check the groove, and use Bulk Quote if the size is uncertain.

You know the AS568 number.
Search the AS568 number directly.

You know the size but not the material.
Read the Material Guide before ordering.

You only know the application.
Use Bulk Quote and include the application, fluid, temperature, and quantity.

FAQ

Can I buy an O-ring if I only know the outside diameter?

Outer diameter can help, but it is not enough by itself. You should also confirm the inner diameter and cross section.

What if my old O-ring is stretched?

Measure it in more than one place and check the groove if possible. If the size is still unclear, use Bulk Quote instead of guessing.

Can I search by AS568 number?

Yes. If you know the AS568 number, searching the full standard number is usually a good starting point.

What if I know the size but not the material?

Check the application first. Oil, water, fuel, steam, outdoor exposure, and chemicals may require different O-ring materials.

Should I order small packs or use Bulk Quote?

Order small packs when the size and material are clear. Use Bulk Quote for unclear sizes, 100+ pcs, special materials, drawings, samples, or higher-risk applications.