Code 39 barcode in a PDF

Where the symbol comes from

This API renders HTML to PDF. It does not generate barcodes, so the Code 39 symbol itself is produced by an encoder in your own stack and placed in the markup, either as inline SVG or as a data URI. That split matters more than it sounds: the encoders are mature and correct, and almost every barcode that will not scan failed for a layout reason rather than an encoding one. Everything below is about the layout half.

What it can encode

Forty-three characters: digits, uppercase A to Z, space, and the symbols minus, full stop, dollar, slash, plus and percent. No lower case. An extended mode encodes full ASCII by using pairs of characters, at double the width, which is rarely worth it.

The check digit, and who computes it

Optional, and that is unusual. Code 39 is self-checking, meaning a single printing defect produces an invalid character rather than a wrong one, so a check digit is not required. A modulo 43 check character exists and some standards mandate it, so whether you need one is a question about the specification you are working to rather than about the symbology.

Size and quiet zone

The two measurements that decide whether a scanner accepts the symbol, and the two most often lost when a design is adjusted to fit.

• X-dimension: Typically 0.19mm or larger. Because each character needs nine elements rather than the three of Code 128, the same data is substantially wider, and the practical constraint is usually the label rather than the minimum.
• Quiet zone: 10 times the X-dimension at each end, with a common minimum of 6.4mm. The start and stop characters are both an asterisk, which appears in the symbol but is not part of your data.

Where it is used

• defence, automotive and aerospace standards that were written decades ago and have not changed
• internal asset tagging, where the human-readable value is uppercase alphanumeric anyway
• laboratory and library systems with long-lived scanner estates

Placing it in the markup

The quiet zone is padding on the container rather than part of the encoder's output, so it survives a change of encoder. Dimensions are in millimetres, because a bar width is a physical measurement and a percentage makes it depend on whatever the container happens to be.

<figure class="code39">
  <img src="data:image/svg+xml;base64,PHN2ZyB4bWxucz0i..." alt="Asset tag RVH-AHU-003">
  <figcaption>*RVH-AHU-003*</figcaption>
</figure>

<style>
.code39 {
  margin: 0;
  padding: 0 7mm;
}
.code39 img {
  /* Code 39 is wide. Twelve characters at a usable X-dimension is
     already most of the width of a small label, which is the real
     reason to reach for Code 128 instead when you have the choice. */
  width: 88mm;
  height: 16mm;
  display: block;
}
.code39 figcaption {
  font: 9pt/1.2 "IBM Plex Mono", monospace;
  letter-spacing: 0.1em;
  margin-top: 1.5mm;
}
</style>

What goes wrong on paper

Underestimating the width. Code 39 needs roughly half again the space of Code 128 for the same content, so a label that was designed around a Code 128 symbol and then switched to Code 39 for compatibility either overflows or gets scaled down past a usable X-dimension. Decide the symbology before the label, not after.

First thing to check when it will not scan

Check for lower case in the data. Code 39 has no lower case characters, and encoders handle that differently: some uppercase silently, some drop the character, some fail. If the scanned value does not match what you sent, character set is the first place to look.

Vector, not a bitmap

Encode to SVG wherever the encoder offers it. A rasterised barcode is resampled when the PDF is printed, and at 203 or 300 dots per inch the bar edges land between printer dots, so the ratio of bar to space drifts by a fraction of a millimetre. That fraction is the whole tolerance. An SVG stays vector inside the PDF and is rendered at the printer's own resolution, which removes the entire failure mode. Where a bitmap is unavoidable, generate it at the final physical size and set image-rendering so nothing smooths the edges.

Frequently asked

Other symbologies

Codes of the same kind, and one from each of the other kinds.