Inch vs. Metric

When it comes to GD&T training, I am often asked which units of measurement are preferred. The answer: It doesn’t really matter! The GD&T system works the same using inches or millimeters; the only thing that changes is the number.

The technical standard ASME Y14.5M-1994 uses SI units (millimeters). Paragraph 1.1.2 phrases it this way: “The International System of Units (SI) is featured in this standard because SI units are expected to supersede United States (U.S.) customary units specified on engineering drawings. Customary units could equally well have been used without prejudice to the principles established.”

This may be humorous to those companies that have always used inches and continue to do so. (Weren’t we all told back in the late 1970s that everything would soon be metric?) The millimeter is widely used by countries besides the United States, and within the U.S. many industries have made the complete changeover to metric (including the automotive industry). But other industries, such as the aircraft industry, continue to use inches, as do smaller suppliers and machine shops.

Obviously, dimensions and tolerances given in one system can be easily converted to the other, but several things need to be addressed when doing this. First, keep in mind that rounding error may occur. Second, there are different customs to follow when displaying millimeters on a drawing than for inches. One custom is that a number less than one millimeter is to be preceded by a zero (such as 0.5 mm) but a number less than one inch should not (such as .500 in). Other minor differences are spelled out in the Y14.5 standard in paragraphs 1.6 and 2.3.

Finally, I should mention that our training is available in either system of units. Our GD&T seminars are based on the ASME standard, so our training manual was originally developed using millimeters. But we recently finished converting the manual to inches. So if your company engages us to do GD&T training, the same class can be taught using either system!


  1. I am trying to find the paragraph or the standard that says to use 3 place decimals in basic dimensions for imperial (inch) units. The company I work for uses 2 place decimal basic dimensions on 3 place decimal hole callouts.
    Thanks for any help

    • Since your question is about basic dimensions, there is actually no requirement about the number of decimal places. In the standard ASME Y14.5-2009, paragraph 2.3.2, subparagraph (d), it states:
      “There is no requirement for the basic dimension value to be expressed with the same number of decimal places as the tolerance.”
      Then they give an example that shows 1.00 as a basic dim even though the geometric tolerance is .005.

      Earlier in 2.3.2, it does say that the tolerance values and the dimensions shall have the same number of decimal places, but basic dimensions don’t have such a rule.

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