Code 39 Format Specification

📚 Introduction & History

Code 39 (also known as Alpha39, Code 3 of 9, Code 3/9, Type 39, USS Code 39, or USD-3) is a variable-length, discrete barcode symbology that was the first specification allowing encoding of both alphabetic and numeric characters. Developed in 1974 by Dr. David Allais and Ray Stevens of Intermec Corporation, it revolutionized barcode technology by expanding beyond numeric-only limitations.

Why Code 39 Matters

Code 39 was developed to address several key needs:

• Alphanumeric encoding - First barcode to support letters, not just numbers
• Self-checking capability - Single character errors are naturally detectable
• Variable length - No fixed data length requirements
• Easy implementation - Simple encoding without mandatory checksum calculation
• Wide scanner compatibility - Readable by virtually all barcode scanners

Key Milestones

• 1974 - Developed by Intermec Corporation
• 1983 - Standardized as ANSI MH 10.8 M-1983
• 1984 - Adopted by U.S. Department of Defense as MIL-STD-1189
• 1995 - Updated to ANSI/AIM BC1/1995
• Current - Defined in ISO/IEC 16388:2023

Modern Usage

Today, Code 39 remains one of the most widely used barcode symbologies, particularly in:

🏗️ Barcode Structure

Every Code 39 barcode follows a precise structure with defined quiet zones, start/stop characters, and data characters. Understanding this structure is essential for proper implementation and scanning.

Component Details

1. Quiet Zones

The quiet zone (or clear area) is the blank space on either side of the barcode. For Code 39, the minimum quiet zone width is 10 times the narrow element width (10x). This space ensures reliable reading by providing a clear boundary for the scanner.

2. Start and Stop Characters

Code 39 uses the asterisk (*) character as both the start and stop delimiter. This same character pattern marks both the beginning and end of the barcode, which is why the asterisk cannot be used in the data portion of the barcode.

3. Data Characters

The data section contains the actual information being encoded. Each character is separated by a narrow inter-character gap (space), which is critical for proper decoding. Code 39 supports variable length data, typically ranging from 1 to 20-23 characters in practical applications.

4. Check Character (Optional)

Unlike Code 128, the check digit in Code 39 is optional. When used, it's calculated using the Modulo 43 algorithm and placed immediately before the stop character. This is often referred to as "Code 39 Mod 43."

Dimension Specifications

🔤 Character Set & Encoding

Code 39 supports a set of 43 characters plus the start/stop asterisk. Each character is encoded using a unique pattern of 9 elements (5 bars and 4 spaces), where 3 elements are wide and 6 are narrow. This "3 of 9" pattern is the origin of the alternate name "Code 3 of 9."

Complete Character Value Table

⚙️ Encoding Pattern

Each Code 39 character consists of exactly 9 elements: 5 bars and 4 spaces. Of these 9 elements, exactly 3 are wide (binary 1) and 6 are narrow (binary 0). This consistent "3 of 9" pattern enables self-checking capability.

Pattern Notation

Bar patterns are typically documented using the notation where:

• n = narrow element (bar or space)
• w = wide element (bar or space)

Elements alternate between bars and spaces, always starting with a bar. So a pattern like wnnwnnnnw means: wide bar, narrow space, narrow bar, wide space, narrow bar, narrow space, narrow bar, narrow space, wide bar.

Example Character Patterns

Inter-Character Gap

Between each character (including start/stop characters), there must be a narrow inter-character gap. This gap is typically the same width as a narrow element. Without these gaps, the barcode cannot be decoded properly.

Encoding Example

To encode the text HELLO, the full barcode structure would be:

✓ Checksum Calculation (Mod 43)

While Code 39's self-checking property provides basic error detection, an optional Modulo 43 check digit can be added for enhanced data integrity. When used, this creates "Code 39 Mod 43."

Calculation Algorithm

1. Look up the numeric value for each character in the data (see table above)
2. Sum all the character values together
3. Divide the sum by 43
4. The remainder is the check digit value
5. Convert the remainder back to its corresponding character
6. Append this character before the stop code

Worked Example

Calculate the check digit for CODE39:

Validation Process

When a scanner reads a Code 39 Mod 43 barcode:

1. Decode all characters including the check character
2. Recalculate the expected check character from the data
3. Compare calculated value with the encoded check character
4. Accept only if they match

🔡 Full ASCII Mode (Extended Code 39)

Standard Code 39 is limited to 43 characters. Extended Code 39 (also called Full ASCII Code 39) expands this to all 128 ASCII characters by using two-character combinations from the base character set.

How It Works

Extended Code 39 uses four special prefix characters ($, /, +, %) followed by a letter to encode additional characters:

Examples

Considerations

• Increased Length: Each extended character requires two barcode characters, doubling the barcode length for lowercase text
• Compatibility: Not all scanners support Extended Code 39 mode
• Alternative: Code 128 natively supports full ASCII with better density

📋 Industry Standards

Code 39 has been adopted as the foundation for several important industry standards, each adapting the base symbology for specific requirements.

LOGMARS (U.S. Military)

Logistics Applications of Automated Marking and Reading Symbols is the U.S. Department of Defense standard for barcode marking. It uses Code 39 as its symbology.

• Standard: MIL-STD-1189B (now ANSI/AIM BC1/1995)
• Related: MIL-STD-129, MIL-STD-2073-1C
• Use: Military supply chain, shipping containers, equipment tracking

HIBC (Healthcare)

The Health Industry Bar Code standard uses Code 39 (and Code 128) for labeling healthcare products, medical devices, and pharmaceuticals.

• Organization: Health Industry Business Communications Council
• Features: Includes special data identifiers for product/manufacturer info
• Use: Medical devices, pharmaceuticals, patient identification

AIAG (Automotive)

The Automotive Industry Action Group uses Code 39 for parts labeling and supply chain tracking in the automotive industry.

• Standard: AIAG B-1 through B-10 label standards
• Use: Parts containers, shipping labels, inventory management

ISO/IEC Standards

🚀 Applications & Use Cases

Common Applications

When to Use Code 39

Choose Code 39 when you need:

• Alphanumeric encoding (letters and numbers)
• Wide scanner compatibility
• Simple implementation without checksum requirements
• Compliance with LOGMARS, HIBC, or AIAG standards
• Variable length data encoding
• Interoperability with legacy systems

Consider alternatives when:

• Space is limited - Code 128 offers 50% better density
• Full ASCII required - Code 128 supports it natively
• High data capacity needed - Use QR Code or Data Matrix
• GS1 compliance required - Use GS1-128

Scanner Compatibility

Code 39 enjoys universal scanner support:

• ✅ Laser scanners (all types)
• ✅ CCD scanners
• ✅ Linear imagers
• ✅ 2D imagers
• ✅ Mobile phone cameras
• ✅ Fixed industrial scanners

⚖️ Code 39 vs Code 128

Code 39 and Code 128 are both popular linear barcodes. Understanding their differences helps you choose the right symbology for your application.

📖 Related Documentation

External Resources

• ISO/IEC 16388:2023 - Code 39 Standard
• GS1 Global Standards

Tools

• Code 39 Breakdown Visualizer - See character-by-character encoding patterns
• Generate Code 39 Barcodes - Create your own barcodes
• API Documentation - Programmatic barcode generation