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Definition of 'Micron'

Q. There seems to be two schools of thought on the size of a "micron". We all know that a mil is 1/1000 of an inch, but what is a micron? I have heard that a micron is 1 millionth of an inch. I have also heard that a micron refers to a micrometer (1 millionth of a meter). So which is it, a millionth of an inch or a millionth of a meter?

Sally Almond
- Knoxville, Tennessee, USA

A. A micron (μm) is a micro-meter (a millionth of a meter). It's NOT a microinch. However, I have also heard, for decades, the strong recommendation that the term micrometer be used instead of micron -- because "micron" does sound like a contraction of "micro in.", which can perhaps lead some people to this erroneous conclusion.

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

A. A micron is one thousandth of a millimeter or one millionth of a meter. It is also one thousand nanometers and 10,000 Angstroms. There are 25.5 microns in one thousandth of an inch.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

A. It may be that part of the confusion indicated by the original question here stems from the name of the instrument used in the U.S. for measuring thousandths of an inch, called a 'micrometer caliper.'

Joseph Boike
- Bloomington, Indiana
July 17, 2011

A. I was an engineer at GE in 1974. I believe that one millionth of an inch was called a micron. This was before metric had taken over.

I think physics used metric but engineering still used the English system.

Ginny Bair
Retired - Ohio
October 9, 2021

thumbs up sign Thanks, Ginny. It may well be that that usage was a commonplace :-)

Surely we should try to use "micro-inch" and "micro-meter" to eliminate ambiguity, but two problems remain. Firstly, in some fields (like filtration -- see dialog below), the term "micron" is universally used and means micro-meter, and those fields are unlikely to ever switch to 'micro-meter'. Secondly the greek letter 'µ' is widely used as an abbreviation for 'micro-meter', for example on surface finish designations (we try to use 'µm' here -- redundancy is a smaller error than building something wrong).


pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey
October 2021

Q. So what is a millionth of an inch called?

Lee Holden
- Rossland, BC, Canada
August 26, 2008

A. A millionth of an inch is called a microinch (μin.), Lee.


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey
August 31, 2008

Q. Hi there!

I am Bon from philippines. I would like to know what is the meaning of 5 micron in a sediment filter (for water filters)

Please help me thanks!

bon lucas
purchaser - tuguegarao city, cagayan, Philippines
November 4, 2010

A. Hi, Bon. Basically, a 5-micron filter is designed to catch particles that are 5 microns in diameter or larger. Filters are sometimes rated such that the filter will catch this size particle 98.7 percent of the time (essentially all of the time) and they are sometimes rated to catch that size particle 50 percent of the time (figuring that the fluid will usually be filtered multiple times so you'll eventually catch all of the 5 micron particles).

Since the water only passes through a water filter once, I'd probably expect that the sediment filter is probably rated 5 microns absolute.


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey
November 4, 2010

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