How to measure an O-Ring

Just a heads up on the Olypsys™ for O-Rings App now that we left the Beta phase. The O-Ring Store LLC has entered into an agreement with Olypsys to provide the Lite Version of the App FREE to users that have or acquire a pre-calibrated Olypsys™ Measurement Grid. They will be good for 12 months from the QR scan. We will continue to send out free Grids on qualifying orders or you may purchase them on our web site.

Olypsys™ for O-Rings Lite (Free) Brought to you by The O-Ring Store LLC

Olypsys™ for O-Rings Lite Pro $500.00 Annually

What is Olypsys™ for O-Rings?

A revolutionary means of measuring All O-Rings within industry standards using your cell phone. Our App uses your phone's modern camera technology and our machine learning techniques to achieve results rivaling expensive computer vision inspection equipment. It replaces digital calipers, size cones, Pi Tapes and even expensive computer vision inspection equipment.

As an Olypsys™ Industrial Measurement Distributor, The O-Ring Store LLC is bringing Olypsys Lite for O-Rings App for free of use. You can download, print, and calibrate a temporary use grid. Once you acquire a pre-calibrated Olypsys™ Measurement Grid, by purchasing a set of Grids, placing a qualified order of just by being a great customer of ours, you have an unlimited number of uses as a single user for 12 months. Due to the near Aerospace Standard Tolerances a fresh grid will be required each year.

High quality professional measuring grids are recommended for optimum results. Currently available in two sizes:-

Accurately measure and identify O-Rings using your phone



O-Ring Size Diagram






How to measure an O-Ring.

O-rings are actually really easy to measure. All you need to do is measure the internal diameter (ID) or outside diameter (OD) and the width or cross section (C/S) of the o ring. The diagram shows exactly where these dimensions are to be found. Once you have these measurements you then need to decide if the size you need is AS568 inch sizes or if the best fit will be a metric oring. If you're planning to replace an O-ring, a good fit is critical.

O-rings are measured in millimeters.

  • Place the O-ring on a clean, flat, level surface.
  • Determine the inside diameter. Use a ruler or calipers to measure from one inner edge to another.
  • To measure the outside diameter, use a ruler or calipers to measure from one outer corner of the O-ring to the other outer corner.
  • Measure cross section. Place the O-ring into a vernier caliper. Lightly clamp the jaws of the caliper onto the O-ring, but do not compress it. Record the thickness of the ring (the cross section).

Measurement Formulas:

  • Outside Diameter (OD) = Inside Diameter + (2 x Cross Section).
  • Inside Diameter (ID) = OD - (2 x CS).
  • Cross Section (CS) = OD - ID, ÷ 2.

If the o-ring is cut, measure the cross section (thickness) and the length.

  • Converting Length to Inside Diameter: (Length ÷ 3.142) - CS = Inside Diameter .


There are two main o-ring sizing types: standard and metric. Standard was developed for use in the United States, while the international community uses metric sizes. The metric sizes are further broken down by region, notably Europe and Japan.

Standard Sizes (AS568B)

The standard sizes used by O-ring manufacturers in the United States are defined by Aerospace Standard AS568B, Aerospace Size Standard for O-rings. That document, published by the Society of Automotive Engineers (SAE), lists the sizes of O-rings in six series or groups in both inches and millimeters. The first five series are based on cross-sectional diameter. The sixth series includes 20 sizes for boss seals. The standard sizes are also used for most military specifications.

Cross-sectional diameters range from 0.040 to 0.275 inch. Inside diameters range from 0.029 to 25.940 inches. There are 369 standard sizes.

Metric Sizes

Standard metric sizes for O-rings are defined by International Standard ISO 3601-1:2002: Fluid power systems O-rings Part 1: Inside diameters, cross-sections, tolerances and size identification code. That standard groups metric sizes into two series, G and A. The G series is used for general purpose applications and includes a wide range of inside diameters. The A series is used for aerospace applications where tighter tolerances are recommended. The G series has 445 sizes and the the A series has 383 sizes.

Currently, ISO sizes do not use a dash numbering system like the Aerospace Standard. ISO cross-sectional diameters differ from the Aerospace Standard by less than 0.001 inch. Therefore, many AS568B sizes are interchangeable with an ISO size. Be sure to consult ISO 3601-1 for specific dimensions, as inside diameters may differ.

Information within is believed to be accurate and reliable. However, The O-Ring Store, LLC makes no warranty, expressed or implied, that parts supplied in this material will perform satisfactorily in specific applications. It’s the customer’s responsibility to evaluate the material prior to use.

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Phone 208-413-6377 - Fax 208-413-6719