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  1. #1
    Join Date
    Aug 2004
    Location
    Milan Michigan
    Posts
    1,723

    Default Converting MM to inches

    In my steel book it says 1 MM dec. equivalent is .0394 when I x that x 100 I come up with 3.94 but the book says 100 MM is actually 3.937

    The decimal equivalent for 100 MM is 3.937 so when I devide that x 100 I get .03937

    If I take .03937 x say 78 MM I come up with 3.07086 but the book says its actually 3.0709

    What am I missing.

  2. #2

    Default Converting MM to inches

    The fact that they are rounding

  3. #3
    Join Date
    Oct 2009
    Posts
    306

    Default

    Simply rounded the last few decimal places to keep it simple. Unless you are measuring precision machined parts there is no need to be exact down to the 4th decimal place.
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  4. #4
    Join Date
    Dec 2011
    Posts
    401

    Default

    They are rounding up. What are you doing that requires that level of accuracy? That difference is well within the tolerances for cold rolled steel in my information.---Meltedmetal

  5. #5
    Join Date
    May 2013
    Location
    Cantonment, FL
    Posts
    46

    Default

    The chart is rounding to the nearest 10-thousandth inch.1 MM = .03937 but it is rounded to .0394 for the chart"If I take .03937 x say 78 MM I come up with 3.07086 but the book says its actually 3.0709"Again, this is due to the chart being rounded to 4 decimal places - your 3.07086 rounds to 3.0709.The chart at http://mdmetric.com/tech/cvtcht.htm says for greater accuracy when converting MM to IN - divide MM by 25.4. So, in your example (78 MM to IN) -- 78 / 25.4 = 3.07086614173 (which would be rounded to 3.0709 in your chart)

  6. #6
    Join Date
    Aug 2007
    Location
    Northern California
    Posts
    193

    Default .039371

    Use .039371 for decimal/millimeter equivalent.

    100 X .039371 = 3.9371.

    78 X .039371 = 3.070938
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  7. #7
    Join Date
    Aug 2004
    Location
    Milan Michigan
    Posts
    1,723

    Default

    Thanks guys, Yea I finally realized that, I'm just not used to rounding up and was surprised that alro steel did.

    When they were teaching us the metric system 30 plus years ago they should have stuck with it.

  8. #8
    Join Date
    Apr 2009
    Posts
    361

    Default

    inches to mm= multiply by 25.4

    mm to inches= divide by 25.4

    For most shop work, that is close enough.

  9. #9
    Join Date
    Sep 2012
    Location
    AZ
    Posts
    52

    Default converting mm to in, etc.

    Quote Originally Posted by Portable Welder View Post
    Thanks guys, Yea I finally realized that, I'm just not used to rounding up and was surprised that alro steel did.

    When they were teaching us the metric system 30 plus years ago they should have stuck with it.
    To get an answer without any loss of precision simply open up Google. In the search area type in, for example, 100 mm in inches. It has a built in calculator and will display 3.93701. You can, of course, also go from inches to mm. For example, type in 3 inches in mm and you'll get 76.2.

    You can convert light years into yards, or fathoms into furlongs. Try it. It will do any conversion!

  10. #10
    Join Date
    Sep 2005
    Location
    16919 Pole Rd. Brethren, MI 49619
    Posts
    4,395

    Default

    Buy a metric tape measure,,, ha

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