Code 39 generator

The mod 43 check, worked through

Every character has a value: digits 0 to 9 are themselves, and A to Z are 10 to 35. Then come hyphen 36, period 37, space 38, dollar 39, slash 40, plus 41 and percent 42. Add the values and divide by 43, and the remainder names the check character.

For CODE39, that is C 12 + O 24 + D 13 + E 14 + 3 + 9, which comes to 75. Divided by 43, the remainder is 32, and value 32 is the letter W. The symbol reads CODE39W, and the scanner sends the W along unless it is set to strip it. You can run this for any text with the check digit calculator.

How Code 39 draws a character

Each character is five bars and four spaces, and exactly three of those nine are wide. Characters sit apart, with a narrow gap between them that carries no data. So at a 3 to 1 ratio, each character takes 15 modules plus 1 for the gap.

That gap costs width but also makes Code 39 forgiving, because a scanner reads each character on its own. It also means the barcode gets long fast:

The same text in Code 39 at two ratios and in Code 128

Text	Code 39 at 3:1	Code 39 at 2:1	Code 128
ABC-1234	159 modules	129 modules	112 modules
ASSET-0472	191 modules	155 modules	134 modules
123456789012	223 modules	181 modules	101 modules

A 2 to 1 ratio saves about a fifth of the width. The cost is a smaller gap between wide and narrow. So choose 3 to 1 unless space forces your hand, and avoid 2 to 1 at small sizes.

Full ASCII mode and the trouble with $ / + %

Full ASCII carries all 128 characters by writing the extras as two symbols. Lowercase a is +A, an exclamation mark is /A, and Tab is $I. The bars are ordinary Code 39, so a scanner with the mode off prints the pair literally.

The same trick causes the opposite problem. A scanner with Full ASCII on will read a plain A/B as something other than A/B, because /B is a pair. The tool warns you when your text has one of those four symbols before a letter. Check what your scanner is set to, or avoid those symbols.

Why some systems still ask for Code 39

It dates from 1974 and needs no check calculation. It was standardized as ANSI MH 10.8 M-1983 and as the US defense standard MIL-STD-1189, since replaced by ANSI/AIM BC1/1995. Older equipment and contracts may still name it, and if yours does, this is the format to give them.

If nothing asks for it, the Code 128 generator makes shorter barcodes from the same text. For a list of stock codes, the SKU generator builds them and draws each one. Not sure which fits? The barcode generator tests your text against every format.

Sizes

A 0.33 mm narrow bar is a sensible default, and ASSET-0472 at 3 to 1 comes out 69.63 mm wide with its clear space. Go down to 0.25 mm if your label is small and your printer is at least 300 dpi. Leave the full 10 modules of clear space at each end, since a scanner needs it to find where the code begins.

Code 39 fonts and Word

Code 39 is the one common format a plain font can draw, as long as the text is wrapped in asterisks. That is why most free barcode fonts are Code 39. Making a barcode in Word covers Word's own barcode field. What a scanner does with those wide and narrow bars is in how barcodes work. ISO sells the Code 39 standard as a paid document, so the open reference listed at the end is the Wikipedia entry.