How To Measure An O-Ring

The three main dimensions of any o-ring are: Internal Diameter (ID), Outside Diameter (OD) and the Cross Section (CS). In most cases, only two measurements are needed in order to calculate the third.

Here are three formulas that can be used to get the third measurement: 

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 and the length and use the following formula to convert the length to the Inside Diameter:

Inside Diameter (ID) = (Length ÷ 3.142) - CS

Tips for Measuring O-Rings

  • Place the O-ring on a clean, flat, level surface
  • Try to ensure the o-ring is not deformed while taking measurements
  • When using a vernier, use light pressure
  • Wherever possible, try to measure the groove instead of the o-ring itself, this can give the most accurate dimensions

One Last Thing

If you do not see the size available in our inventory, we will still be able to get it for you. Reach out to us via our live chat, Contact Form, or give us a call at 407-906-2736. You can even send us an email at

We always respond within 12 hours and our office hours are: Monday to Friday 9 am to 5 pm and Saturdays from 9 am to 12 pm.