Micke Tong

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Imperial or Metric: Which Way to Measure in AutoCAD

Shaun Bryant

So here’s the deal: I am British and proud to be. However, we caused ourselves a headache many years ago by giving our American friends (and a number of other countries) the imperial method of measurement (feet and inches), and then, in our wisdom, going metric and using the decimal method of measurement. Patriotism aside, the ongoing issues caused by global projects using imperial or metric or both measurement methods have to be addressed to ensure both accuracy and integrity of your drawings and models.

The imperial method of measurement, known as British Imperial, was originally set by the British Weights and Measures Act of 1824. Most imperial drawings and/or models use yards, feet, and inches, where one yard is three feet, and a foot (plural = feet) is 12 inches.

The metric method of measurement, known as the International System of Units or SI (from the French, Le Systeme International D’Unites), was originally set up in 1960 to address the issue of a number of diverging metric systems and to coordinate and standardize those systems. Most metric drawings and/or models use kilometers, meters, and millimeters, where one kilometer = 1,000 meters, and one meter = 1,000 millimeters.

This is where it gets interesting. The conversion factors between imperial and metric have to be carefully considered, primarily due to the number of decimal places needed to calculate them effectively. You can see why when you look at the conversions below:

1 yard = 3 feet = 914.4 millimeters = 0.9144 meters
1 foot = 12 inches = 304.8 millimeters = 0.3048 meters
1 inch = 1/12 foot = 25.4 millimeters = 0.0254 meters

If you don’t use the appropriate conversion, or the appropriate number of decimal places (as you can see from the above), the integrity of the drawing and/or model can be compromised. This is especially important when the project standard specifies that both methods of measurement have to be used. I have been in many project meetings where the above conversions have been discussed at length to make sure the project maintains true measurements.

imperial or metric

Applying Imperial and Metric on the Same Project
Let’s consider AutoCAD here, at a simplistic level, because I could happily write a book on this subject! AutoCAD uses Dimension Styles when placing dimensions when annotating a drawing. In the Dimension Style, you have the facility to set both a primary dimension setting and an alternate dimension setting. So, for example, on a UK/European project that I might be working on, I would set the primary setting to decimal (metric) and the alternate setting to architectural (imperial). AutoCAD automatically calculates the conversion factors between each setting and displays them accordingly.

So, you might have a dimension annotation that reads like the following:
1,000 [39.37]—1,000 represents 1,000 millimeters (metric), and the value in the square brackets is the distance in inches (imperial) using a conversion factor of 0.03937 (Source: Google).

You’ll notice my source above is Google. In the “old” days, I used to keep a small notebook with my calculator, nicknamed my “Drafter’s Almanac.” It was full of all of these conversion factors, scale factors, and the like. Now, with an Internet connection, you can find them instantaneously, thus improving your productivity.

When applying these factors on a project, the key is consistency. Generate a CAD standard that applies across the broad spectrum of all of your disciplines, and stick to it. An example would be that you maintain a level of precision of four decimal places on all conversion factors in a project. This would have to be rigidly maintained, so that all drawings/models use the same standard; otherwise, small discrepancies on distances (and also angles) can creep in, leading to small transferred errors. In mechanical CAD work, this can cause big problems when it comes to machining and fabrication.

In my previous article, I said that CAD standards are necessary and very important to the industry, allowing for designs and drawings to be interchanged between teams on projects. Sometimes, large projects use multiple measurement philosophies, such as metric and imperial, so the above-mentioned CAD standard would be essential for the project to function.

For those of you working in a small-business design firm, it is essential that you consider these types of standards. Again, as per my previous article, “boring is good.” The more standardization you have, the more accuracy you have, and the drawings/models maintain integrity.

So, when implementing a combined metric and imperial CAD standard, consider this: There is nothing to stop you from using both. All it takes is a standard that can be maintained by your team and that can be checked easily. This can be done purely by using the Units command in AutoCAD. Make sure that your drawing templates (DWT) are set correctly and that regular checks are done before the drawings go out. This checking and approval process should be being done religiously anyway, so no problems there. Revit can be set the same way using the Revit templates (RVT files).

So, imperial or metric? Or perhaps both?

How do you check to make sure your drawings go out in a consistent manner? Please share your experiences below in the comments section.


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