We have an existing fiber infrastructure that's several years old. I can't for the life of me read the jacket to get the core size of the fiber, and was wondering if there was a way to measure it without depending on the markings.

The fiber is for sure multimode, but I'm more concerned if it's 62.5 or 50 micron as we're looking to do 10GBASE-SR over them.


  • I wonder if a cabling contractor would have a way of determining this - I expect they would. Maybe you can pay for an audit or even finagle a free site survey. Commented Jul 23, 2015 at 12:05
  • That's not a bad idea, I'll check the documentation we have, but unfortunately the company who installed the cable went out of business a few years ago. Commented Jul 23, 2015 at 16:41
  • What are the optics of SFPs are you currently using over this fibre?
    – Baldrick
    Commented Jul 23, 2015 at 21:06
  • We're currently using 1000Base-SX transceivers. Our goal is to budget for new equipment with 10GBase-SR uplinks next year, but I have to take into account if our existing fiber can handle the speeds. Commented Jul 23, 2015 at 22:12
  • 2
    Terminology nitpick - the cladding is is the outer part of the glass, and never has anything written on it (nor can it be removed.) The buffer is the acrylate layer over the glass, and also never has anything written on it. The place where the fiber type should be imprinted every 2-3 feet is the jacket.
    – Ecnerwal
    Commented Jul 28, 2015 at 16:00

3 Answers 3


If you have a fiber scope (you really should, to check if connections are clean) the core size (but not OM level) is obvious by inspection, especially if you look at a known connector and then the unknown connector (or better yet, two known connectors, one of each size.)

Image from theFOA.org

enter image description here

  • Remarkable how few decent images there are on the web with an accurate representation of the core sizes.
    – Ecnerwal
    Commented Jul 28, 2015 at 16:07
  • 2
    For those not familiar with fiber, in the above image the grey is the core size and the yellow is the cladding. If you are looking at this through a scope, many new users mistakenly think the ferrule (white) is the cladding and the cladding (grey) is the core (white). In some conditions or with the focus a bit off, you may not see the core clearly. You can get an idea of what I am trying to say if you search images on your search engine of choice.
    – YLearn
    Commented Jul 29, 2015 at 2:32

If you have a light meter, get some readings over the fiber as a baseline. Then, connect a 50 micron patch cord at one end and take readings again in both directions. If you see much more loss than expected (assuming the label that came with the patch cord shows the real loss over that cord), then the fiber is 62.5 micron. If not, try it with a 62.5 patch cord at one end, repeat the readings. Basically, if there's a mismatch between the patch cord and the fiber you should see 4 dB loss in one direction, with the matching patch cord giving the expected loss in both directions.

But if it's only a few years old it's probably 50.


We have a large quantity mix of fiber types in our network.

If the fiber is only a few years old as others have mention it is most likely 50u. With that said on the jacket should have at least have the manufacturer name and part number. I know what you are saying in that many jackets do not have a 50 or 62.5 stamped on it. Best is by name and part number and look that up.

The poor man's way to make an educated guess is one I have used many times over the years at many installations and is generally pretty accurate.

Most fiber installers are generally smart enough to match the patch cords to the fiber that has been pulled to the closet.

Here is one guideline to use if there truly isn't any manufacturer name and/or part number.

Color coding, If the patch cords and many times the jacketing are orange then there is a high possibly that it is 62.5 OM2 cabling. There are rare exceptions and that is usually done by a 3rd tier fiber manufacturer. Grey can also be 62.5, especially if it is the patch cords. You can technically run 10GB upto 80 meters on 62.5 but in a darken room you will see alot of light leaking around the connectors. Also will see a lot of crc errors under the show int status.

Aqua color for patch cord and jacketing of the fiber runs is almost always 50u OM3 & OM4. I doubt you have OM4 if it is several years old unless your employer paid the higher price at the time. About the same price now but not several years ago.

Doubt you have indoor single mode but if you do then generally it's yellow patch cords.

These suggestions are for indoor fiber. Outside plant can and in many cases are black jacketed.

Others many not agree but this has been my experience and I've done my share of indoor and outside plants when I worked for Ciena Communications selling fiber transmission gear.

I would highly suggest lifting some ceiling tiles and try to locate the manufacturer name and part number. That is going to be the most accurate way.

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