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# Thread: Converting MM to inches

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1. Senior Member
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## 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. Member
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Mar 2012
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## Converting MM to inches

The fact that they are rounding

3. Senior Member
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Oct 2009
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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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## .039371

Use .039371 for decimal/millimeter equivalent.

100 X .039371 = 3.9371.

78 X .039371 = 3.070938

5. Senior Member
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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.

6. Senior Member
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inches to mm= multiply by 25.4

mm to inches= divide by 25.4

For most shop work, that is close enough.

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## converting mm to in, etc.

Originally Posted by Portable Welder
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!

8. Senior Member
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Dec 2011
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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

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Cantonment, FL
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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)

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I have an app on my phone that is a unit converter. Very handy. I can convert any measure of a unit and transfer it to another with a button. Free too.

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