Specifications

Additionally, the bar code height for the extension should be 27 dots
(0.135 inches) shorter than that of the Primary code. A Primary UPC
code height of 183 dots (0.900 inches) would require a extension
height of 155 dots (0.765).
The figure below shows an example of how to create a normal UPC-A
code for the value 7000002198 with an extension equal to 04414.
ZPL II COMMAND REFERENCE 165
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UPC-A Bar Code with an Extension