How to Determine Your O-Ring Size in 5 Steps
Follow these five steps to narrow down the size you need.
Determining an O-ring’s likely size starts with a few basic measurements. Once you know the inside diameter and cross-section, you can compare those dimensions with an O-ring size chart to identify possible standard sizes.
Follow these five steps to help narrow down the size you may need.
1. Place the O-Ring on a Flat Surface
Lay the O-ring on a clean, flat surface without stretching or pulling it.
If the O-ring has already been used, inspect it for visible damage or distortion. Swelling, stretching, wear, and compression set may cause an older O-ring to measure differently than it did when it was new.
2. Measure the Inside Diameter
Measure straight across the open center of the O-ring, from one inside edge to the opposite inside edge.
This measurement is called the inside diameter, or ID.
A ruler may work for larger O-rings, but calipers generally provide a more precise measurement, especially for smaller sizes. Measure carefully without stretching or compressing the O-ring, since deformation can affect the result.
3. Measure the Cross-Section
Next, measure the thickness of the O-ring itself.
This dimension is called the cross-section, or CS. Measure an area that has not been noticeably flattened, worn, or damaged.
Inside diameter and cross-section are the two primary dimensions used to compare an O-ring with recognized standard sizes.
4. Compare Your Measurements With an O-Ring Size Chart
Once you have measured the inside diameter and cross-section, compare those dimensions with a recognized O-ring size chart.
Many standard inch O-rings used in the United States are sized according to the AS568 standard and identified by dash numbers.
The Parker O-Ring Selector can be used as a reference for comparing dimensions, reviewing standard sizes, and considering basic application requirements.
For more detailed technical information, the Parker O-Ring Handbook includes guidance on O-ring sizes, materials, tolerances, installation, and gland design.
5. Confirm the Material and Application Requirements
Finding the correct size is only part of selecting an O-ring.
Two O-rings can have the same dimensions but perform very differently if they are made from different materials. Before ordering a replacement, consider:
- The fluid or chemical being sealed
- Operating temperature
- System pressure
- Static or dynamic movement
- Groove dimensions
- Required hardness
- Applicable industry or customer specifications
Common O-ring materials include nitrile, FKM, EPDM, silicone, and FFKM. The right choice depends on the conditions in which the seal will operate.
What If the O-Ring Does Not Match an AS568 Size?
Not every O-ring follows the AS568 sizing system.
An O-ring that does not match a standard AS568 dash number may be:
- A metric size
- Manufactured to another sizing standard
- A custom size
- Worn or permanently distorted
- Swollen from chemical exposure
- Shrunk or hardened from heat and aging
When measurements fall between two standard sizes, do not assume the closest size will provide an effective seal. Additional measurements or application information may be needed.
Can You Measure a Used O-Ring?
A used O-ring can provide a helpful starting point, but it may not provide the exact original dimensions.
Before using its measurements to select a replacement, check the seal for:
- Flattened areas
- Stretching
- Swelling
- Cracks
- Hardening
- Cuts or surface damage
- Permanent deformation
When available, the original part number, equipment documentation, groove dimensions, or manufacturer specifications may provide more reliable information.
Measurements taken from a used O-ring should be treated as a starting point rather than confirmation of the original size. Final selection should be verified against the equipment requirements, gland dimensions, and operating conditions.
How Performance Seals Can Help
If you are having trouble identifying an O-ring, gather as much information as possible, including:
Inside diameter
Outside diameter
Cross-section
Material, if known
Equipment or application
Operating temperature
System pressure
Fluid or chemical exposure
Original part number, if available
Groove dimensions, if available
Performance Seals has specialized inspection equipment, including a vision measurement system, that can help measure an O-ring sample and compare its dimensions with recognized standard sizes.
Measurements are only part of the identification process. Used O-rings may be stretched, swollen, flattened, worn, or otherwise distorted. Visual inspection and dimensional measurements may not confirm the original size, material, or compound.
Application details, equipment documentation, groove dimensions, and original part information can help narrow down the options. Performance Seals can assist with reviewing standard and custom O-ring options and selecting a material based on the information provided.
Final O-ring selection should be verified for the specific equipment, gland design, operating conditions, and application requirements.