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topic 22671

Definition of 'Micron'


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

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


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

August 26, 2008

So what is a millionth of an inch called?

Lee Holden
- Rossland, BC, Canada

August 31, 2008

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


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

November 4, 2010

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

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,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

July 17, 2011

I 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

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