The X dimension is the width, in thousandths of an inch, of the narrowest element (bar or space) in a barcode. This is usually referred to in mil. So a .005" bar width would be referred to as a 5-mil label. This is relatively easy to set in most printing applications. Sometimes, though, it is difficult to ascertain the X dimension in a pre-printed barcode for which you have no X dimension specification (or a barcode diagnostic scanner that will calculate it for you).
The X dimension can also be determined with an eye loupe that has a reticule giving you a comparison to the exact width of the bars and spaces in the barcode. But for most conversations regarding the need to know an X dimension of a label, a close approximation is sufficient. The attached document contains a wide variety of Code 39 labels showing the letter "A" printed in different X dimensions.
If you are trying to gauge the X dimension of an unknown barcode, print the attached sheet on a laser printer. Then compare the narrowest bars and spaces of the unknown to those in the various labels from the attachment and you will be able to make a good estimate of the X dimension. Although in different symbologies there may be varying thicknesses of the elements, it is only necessary to look at the narrowest element to determine the X dimension.
Be aware that printing errors can occur that will make a bar look narrower or wider than it really is, and hence one of the adjoining spaces will inversely be also wider or narrower. See also the document on Wide-to-Narrow Ratio ( Click Here).
Note 1: When printing the following attachment for the smaller X dimensions (5 and 7.5 mil) it is wise to use a light toner setting or toner saver setting in your printer properties to get more accurate labels. The labels you get will not be considered to be Class A quality. They are provided for reference purposes only.
Note 2: When printing on a Barcode printer the printer is built with a minimum X dimension that it is capable of printing. For intermec printer the most common of these is 2.5, 5, and 3.3 mil. The printers normally can only print the minimum X dimension when printing in the Picket Fence orientation. Normally when printing in Ladder orintation the minimum X dimension is doubled.