When purchasing proximity cards for an access control system, one of the most important details to know is the card format. Prox cards can look nearly identical on the outside, but the information programmed into them can be very different.

Three formats customers commonly encounter are 26-bit, 34-bit, and 37-bit. Understanding the differences between these formats can help organizations choose compatible credentials and avoid ordering cards that won't work with their existing access control system.

What Is a Prox Card Format?

A prox card format describes how credential information is structured and transmitted between the card, reader, and access control system.

The term "26-bit" or "37-bit" refers to the number of bits in the data format. Depending on the format, that data can include information such as:

• Facility or site code
• Card or credential number
• Parity bits used for data checking
• Other format-specific information

The exact structure depends on the credential format being used.

This is important because having the correct physical card is not enough. The card also needs to be programmed with a format that the customer's access control system recognizes.

26-Bit Prox Cards

26-bit Wiegand, commonly identified as H10301, is one of the most widely used proximity card formats. It is an open format and is supported by many access control systems.

The standard 26-bit format contains:

• 8-bit facility code
• 16-bit card number
• 2 parity bits
• Facility codes ranging from 0–255
• Card numbers ranging from 0–65,535

Why Choose 26-Bit?

26-bit cards remain popular because they are:

• Widely supported
• Commonly available
• Relatively straightforward to order
• Compatible with many existing access control systems
• Available from multiple credential manufacturers
• A practical choice for many standard access control applications

However, the available facility-code and card-number ranges can make standard 26-bit formats less suitable for some larger or more complex deployments.

34-Bit Prox Cards

34-bit credentials provide a larger data structure than standard 26-bit cards. A commonly encountered format is H10306, which uses a 16-bit facility code and 16-bit card number, along with parity information.

A 34-bit format may be appropriate when an organization's access control system is specifically configured to use that credential format.

Potential Benefits of 34-Bit Credentials

• Larger facility-code field than standard 26-bit H10301
• Compatible with access control systems specifically configured for 34-bit credentials
• Can provide additional numbering flexibility
• Available in several different 34-bit configurations

It's important to remember that more bits doesn't automatically mean a card is better. The access control system determines which credential formats it can accept.

Also, not every 34-bit credential uses the exact same data structure. Different 34-bit formats may be associated with different manufacturers or access control systems.

37-Bit Prox Cards

37-bit credentials provide an even larger data structure, but there is an important distinction: "37-bit" does not necessarily describe one single format.

Two commonly encountered formats are H10302 and H10304.

37-Bit H10304

The H10304 format uses:

• 16-bit facility code
• 19-bit card number
• Parity information
• A significantly larger card-number range than the standard 26-bit format

37-Bit H10302

H10302 is different because it does not use a facility code. Instead, it uses a much larger card-number field.

This is a great example of why simply telling a supplier "I need 37-bit cards" may not be enough information.

The exact format needs to match the customer's access control system.

26-Bit vs. 34-Bit vs. 37-Bit

Key Differences at a Glance

• 26-bit H10301 is a widely used open format.
• 34-bit formats can provide a larger facility-code structure.
• 37-bit formats can provide substantially more numbering capacity, depending on the specific format.
• 37-bit H10302 and H10304 are not the same format.
• A card's bit count alone does not determine compatibility.
• The credential format must match what the access control system is configured to accept.

Does a Higher Bit Count Mean a Better Card?

Not necessarily.

It's easy to assume that a 37-bit credential is better than a 26-bit credential because it contains more bits. However, the appropriate format depends on the customer's access control system, credential requirements, and existing card population.

If a customer's access control system is configured for 26-bit H10301 credentials, purchasing 37-bit cards won't automatically improve functionality.

The correct question isn't:

"Which bit format is best?"

Instead, ask:

"Which credential format is compatible with your access control system?"

How Do You Know Which Prox Card Format You Need?

Before ordering replacement or additional prox cards, customers should identify the exact format already being used by their access control system.

Information to Check Before Ordering

• Existing card format: Check the packaging or documentation from previous card orders.
• Facility code: If applicable, identify the facility or site code currently being used.
• Card number range: Check the numbering scheme used by existing credentials.
• Access control software: Look for the credential format configured in the system.
• Access control panel: Identify the specific panel or controller being used.
• Reader technology: Confirm the reader is compatible with the credential technology.
• Existing credentials: Compare the new cards against credentials already working in the system.
• Manufacturer requirements: Some systems use proprietary or manufacturer-specific formats.

The format may be listed on the original card packaging, while access control software can also indicate which formats the system supports.

Why Matching the Format Matters

Ordering the wrong prox card format can create unnecessary problems.

A card may physically fit the reader but still fail to provide access because the credential data isn't structured in the format the access control system expects.

Before placing an order, verify:

• Card technology
• Bit format
• Format code
• Facility/site code, when applicable
• Card number range
• Reader compatibility
• Access control system compatibility
• Whether the format is open or proprietary

For organizations purchasing credentials in large quantities, checking these details before ordering can help prevent costly compatibility problems.

Prox Card Format vs. Card Technology

It's also important to distinguish between card format and card technology.

A card's technology describes how the credential communicates with the reader, while the format describes how the credential data is structured.

For example, 125 kHz proximity credentials can be available in different Wiegand formats, including 26-bit and 37-bit formats.

This means two cards can use similar proximity technology while still having different credential formats.

When Ordering Prox Cards, Consider Both:

• Technology: What type of credential does the reader support?
• Frequency: What frequency does the system use, when applicable?
• Format: What Wiegand or other credential format is required?
• Numbering: What facility and card numbers need to be programmed?
• Physical card: What size, style, or form factor is required?
• Compatibility: Will the credential work with the existing access control system?

Open vs. Proprietary Card Formats

Another important consideration is whether the credential format is open or proprietary.

Open formats can generally be sourced from multiple credential manufacturers. Proprietary formats, on the other hand, may be controlled by a particular manufacturer or system and may require credentials from a specific source.

Open Formats

Common examples include:

• 26-bit H10301
• 34-bit H10306
• 37-bit H10304

Proprietary or Restricted Formats

Some systems use:

• Manufacturer-specific formats
• Registered credential programs
• Custom facility structures
• Proprietary numbering schemes

This is another reason why identifying the exact format is more important than simply knowing the number of bits.

Common Mistakes to Avoid When Ordering Prox Cards

Customers can avoid many credential-ordering problems by avoiding these common mistakes:

• Assuming all 26-bit cards are identical
• Ordering a card based only on its appearance
• Assuming every 37-bit card uses the same format
• Failing to provide the facility code when one is required
• Confusing card technology with card format
• Replacing a proprietary credential with a standard open-format card
• Not checking existing card numbers
• Failing to verify reader and access control system compatibility

The Bottom Line

26-bit, 34-bit, and 37-bit prox cards aren't simply different versions of the same card. They use different data structures and may be designed for different access control systems.

26-bit H10301 remains a widely used open format, while 34-bit and 37-bit credentials can provide different data structures and greater numbering flexibility depending on the specific format.

Before ordering prox cards, always verify the:

• Exact credential format
• Card technology
• Facility/site code
• Card number requirements
• Access control system
• Reader compatibility

When in doubt, checking existing credential information or consulting the access control provider can help prevent compatibility issues.