How to Size O Rings

by Justin Cupler

All standard O-rings come in are sized by the "AS568-" system, after the dash is a series of three number that determines the physical size of the O-ring. There are three parts of the O-ring that are measured to determine its size: the outside diameter, inside diameter and cross section or width. Finding the proper replacement O-ring is important to ensure a proper fit and seal of the component the O-ring is attaching to. Measuring the O-ring only takes a few minutes and requires only one tool.

Lay the O-ring on a clean, flat and dry surface.

Close the micrometer jaws and press the "reset" button to calibrate it prior to measuring.

Place the rear jaws of the micrometer in the center of the O-ring. Slowly open the micrometer until the inner jaws lightly touch the inner part of the O-ring. Do not stretch the O-ring, or the measurement will be inaccurate. Read the measurement, known as the inside diameter, given by the micrometer and take note of it.

Open the micrometer jaws and place them over the outside of the O-ring. Slowly close the jaws until the lightly touch the outer part of the O-ring. Do not compress the O-ring with the jaws, as this will cause an inaccurate measurement. Take note of the measurement, known as the outside diameter, given by the micrometer.

Open the micrometer jaws and place the bottom jaw through the O-ring, so that the O-ring is hanging on the lower jaw. Close the micrometer jaws until they are lightly holding the O-ring, do not press too hard and squash the O-ring, as this will provide an incorrect reading. Take note of the measurement, known as the width or cross section, given by the micrometer.

Read the O-ring sizing chart (see Resources) and compare the inside diameter, outside diameter and cross section or width to the chart to get the proper "AS568-" number of the O-ring.


  • check The O-ring's inner diameter and cross section have a range of tolerance, so do not be alarmed if it does not match up exactly with the measurements on the chart. The tolerances are listed on the chart.

Items you will need

About the Author

Justin Cupler is a professional writer who has been published on several websites including CarsDirect and Cupler has worked in the professional automotive repair field as a technician and a manager since 2000. He has a certificate in broadcast journalism from the Connecticut School of Broadcasting. Cupler is currently studying mechanical engineering at Saint Petersburg College.