O-Ring Dash Size Chart

What it is

Inch-size O-rings in North America follow SAE AS568. Each size has a three-digit dash number (such as -116) that sets the inside diameter (ID) and the cross section (CS). The first digit groups the sizes by cross section. Within a group, a higher number means a larger ID. Material and hardness are ordered separately from the size.

How to use it

1. Measure the cross section with calipers, pressing lightly so you do not squeeze the ring.
2. Measure the ID. A cone gauge or O-ring sizing tool works better than calipers on larger rings.
3. Use the CS to find the series in the chart.
4. Look up the dash number with that ID in the AS568 table or the maker's catalog.
5. Order with the material and durometer, for example nitrile (NBR) 70 or FKM 75.

Series chart

Sizes -001 to -003 are very small rings with thinner cross sections.

Key points

• 900 series rings fit SAE straight thread ports. The last two digits are the tube OD in sixteenths of an inch, so -908 fits a -8 port.
• A used O-ring may have swelled, shrunk or taken a flat set. Measure the groove when possible.
• Metric O-rings (ISO 3601) use different cross sections, such as 1.5, 2, 2.5 and 3 mm. Some are close to AS568 sizes but are not the same.
• Durometer (Shore A) and material must suit the pressure, fluid and temperature.

Common mistakes

• Measuring a squeezed cross section and dropping a series.
• Ordering by dash number with no material.
• Using a general -0xx ring in place of a 900 series port seal.

Governing standards: SAE AS568 (O-ring sizes); ISO 3601 (metric O-rings); O-ring maker catalogs and gland design data.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.