What Is a Check Digit? The Key to Barcode Accuracy & Validation

What Is a Check Digit? The Key to Barcode Accuracy & Validation

BY: TRAVIS WAYNE / DATE: 19/06/2025 / TOPICS: AUTOMOTIVE, FOOD & BEVERAGE, HEALTHCARE, MANUFACTURING, MEDICAL DEVICE, PHARMACEUTICAL, RETAIL, SHIPPING & RECEIVING, WAREHOUSING & DISTRIBUTION, BARCODE BASICS, REGULATORY COMPLIANCE

What is a check digit? Simply put, a check digit is an extra number added to a barcode to help detect errors. Barcode check digits are calculated using algorithms during both barcode creation and scanning. If the scanned check digit matches the created check digit, the barcode was scanned correctly.

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Check digits are foundational to barcode data integrity, validation, and quality. In this blog, we'll dive into what check digits are, how they work, and why they're important.

What is a check digit?

A check digit is a simple method of error detection with a lot of math behind it that ensures barcodes are scanned correctly. A barcode is made up of numbers, and the check digit is the last number in the barcode.

It's not part of the actual data of the barcode but is calculated using a complex equation. This means that different barcode values will generate different check digit values.

The check digit is calculated and written when the barcode is created. When the barcode is scanned, the scanner calculates the check digit using the same equation. If the result matches the check digit printed in the barcode, the barcode was scanned correctly. If it doesn't match, the scanner will display an error.

Most common types of one dimensional (1D) barcodes use check digits, including:

• Code 39
• UPC-A / UPC-E
• EAN-13 / EAN-8
• ITF-14
• GS1-128
• ISBN and ISSN

Two dimensional (2D) barcodes, such as QR codes and Data Matrix barcodes, don't use check digits because they have more complex methods of error detection.

Why are check digits important?

Barcode accuracy is critical to every aspect of the supply chain, from manufacturing to shipping to retail. If a barcode is scanned incorrectly and the error isn’t detected, bad data enters supply chain information systems. This can cause a ripple effect of issues throughout the supply chain. For example, if a scanner in the receiving department misreads a "5" as an "8" in the product barcode, the wrong SKU would be logged into the inventory system. This would cause fulfillment issues, quality control failure, and delays. Additionally, manual investigation would be needed to trace the discrepancy and correct it.

Check digits are the first line of defense against barcode errors. They protect against common scanning mistakes and manual data entry errors. That’s why GS1 barcode standards require check digits.

How check digits prevent scanning errors

Traditional barcodes are read in one dimension (left to right) with a laser scanner. The scanner emits a beam of light at the barcode. The black bars absorb the light, and the white spaces reflect the light back at the scanner. The scanner measures the reflected light to read the data encoded in the barcode. This analog process can be affected by dirt, damage, or anything that prevents the light from being accurately reflected into the scanner.

If dirt or damage causes the barcode scanner to read different data than what is actually encoded in the barcode, the scanner will calculate a different check digit than what is printed. The mismatch will cause the scanner to display an error.

How check digits prevent manual data entry errors

If a barcode is damaged so badly that a scanner can't read it at all, it becomes necessary for the worker to manually enter the barcode data into the system. Check digits protect this process too.

Imagine the worker accidentally types "4" instead of "5" while entering the barcode number. The worker will enter the printed check digit last, but it won't match the check digit expected by the system. The error will be identified and flagged for correction.

Check digits in GS1 barcode standards

GS1 is the global not-for-profit organization that defines and maintains barcode standards that are used across industries. GS1 barcodes make it possible for barcodes created by a company in one country to be read by another company around the world.

GS1 requires check digits on all fixed-length numeric GS1 Identification Keys such as Global Trade Item Number (GTIN), Global Location Number (GLN), and Serial Shipping Container Code (SSCC).

Companies in retail, transportation & logistics, healthcare, and food & beverage industries all benefit from GS1 compliance, including the use of check digits.

How do check digits work?

Check digits are calculated using math formulas called algorithms. When the barcode is created using barcode software, the check digit is automatically calculated and added to the end of the barcode. When the barcode is scanned, the scanner instantly calculates the check digit and compares it to the scanned check digit. If it matches, the barcode is successfully scanned.

One popular check digit algorithm is the Modulo 10 algorithm. Computers can process this algorithm in milliseconds, but it takes a human a little longer to calculate.

Take a UPC barcode with the digits 012345678905. Here’s a simplified version of how its check digit (the last number, 5) is calculated using the common Modulo 10 algorithm:

1. Add the digits in odd positions (1st, 3rd, 5th, etc.): 0 + 2 + 4 + 6 + 8 + 0 = 20
2. Multiply by 3: 20 × 3 = 60
3. Add the digits in even positions (2nd, 4th, 6th, etc.): 1 + 3 + 5 + 7 + 9 = 25
4. Add the results: 60 + 25 = 85
5. Subtract the result (85) from the nearest equal or higher multiple of 10 (90): 90 – 85 = 5
   
   • This is why it's called Modulo 10—we're looking for the difference between our result and the nearest equal or higher multiple of 10.
6. The check digit is 5

For a visual representation, take a look at GS1's webpage.

If a scanner misread one of the digits, for example, 012245678905, it would calculate the check digit as 6. The scan would result in an error because 6 does not match the last digit of the printed barcode: 5.

Luckily, we don't need to manually calculate check digits. GS1 provides a check digit calculator on their website, and barcode printing software such as LABELVIEW automatically calculates check digits for barcodes.

Best practices for managing barcode check digits

The best way to print barcodes with accurate check digits is to use barcode label printing software. LABELVIEW barcode label software includes barcode generator wizards that automatically calculate and print barcode check digits.

In LABELVIEW, you can configure the properties of each barcode, including the check digit.

How to configure the check digit in the machine-readable barcode:

1. Right click on your barcode and select Properties or double-click the barcode to open Properties.
2. Select the Code tab.
3. Configure the Check character setting to 1 digit or None.

Configuring the check digit in the human readable text:

1. Right click on your barcode and select Properties or double-click the barcode to open Properties.
2. Select the Code tab.
3. Open the Human Readable dropdown window.
4. Configure the Include the check character setting by checking or unchecking the box.

Best practices to reduce manual data entry

If workers at your company manually enter barcode data into a system instead of scanning it, look for ways to improve this process.

• Integrate your label printing system so that data is connected to the source
• Automate label printing to remove manual steps from the process
• Use barcode verification hardware to identify poorly printed barcodes and remove them from production

Frequently asked questions about check digits

• How do I find the check digit?
  The check digit is always the last digit of the barcode. It is calculated using an algorithm. You can use barcode label printing software to calculate the check digit or use GS1's website. If you like doing math, you can calculate it manually.
  
  
• Is a check digit always required?
  GS1 barcode standards require a check digit on all fixed-length GS1 Key Identifiers. Many industries rely on GS1 standards for supply chain communication.
   
  If you don't use GS1 barcodes, refer to your barcode label specifications to understand whether a check digit is required or not.
  
  
• Can I manually override the check digit?
  When using barcode creation software, it's not possible to override the check digit because it's a calculated value. Overriding the check digit will cause a scanning error because the scanner expects to see the correct check digit. Creating a barcode using a downloaded barcode font would make it possible to use an incorrect check digit, leading to scanning errors. This is why it's recommended to always use professional barcode label software to create your barcodes.
  
  
• What types of errors do check digits detect?
  Check digits can detect differences between the printed barcode and the scanned or entered data. This may include missing, switched, or mistyped numbers. Errors are identified when the scanner or system calculates a different check digit than is printed on the label.
  
  
• What happens if the check digit is wrong?
  If the printed check digit is wrong (for instance, if it was overridden or wasn't calculated correctly), the barcode won't scan. An error will be indicated instead.
  
  
• Can a check digit fail at detecting an error?
  Very rarely, a check digit can fail to detect an error. For example, if the scanner misreads two digits that cancel each other out in the check digit algorithm, the scanner will calculate the correct check digit based on wrong data and not detect the error. The check digit algorithm is designed to make this possibility exceedingly rare.
  
  
• Where else are check digits used?
  Besides barcodes, check digits are used in a variety of industries where numerical accuracy is required. Bank account numbers, credit card numbers, and identification numbers all may use check digits as a form of error detection.
  
  
• Do stronger error detection methods exist?
  Yes, methods like Cyclic Redundancy Check (CRC) offer stronger detection but are more complex. QR codes use built-in error correction instead of check digits. They can still be read even if part of the code is missing or damaged.

Use compliant barcode printing software to automatically calculate check digits

Let’s recap what we've learned about check digits. Key takeaways include:

• Check digits are a form of error detection that ensure a barcode is scanned correctly
• Check digits strengthen barcode accuracy, protecting the global supply chain from far-reaching errors
• GS1 barcode standards require a check digit on all fixed-length GS1 Key Identifiers
• Barcode label printing software enabled users to automatically calculate and print accurate check digits

Companies use barcode label software like LABELVIEW because it easily and accurately prints barcodes with check digits. LABELVIEW can generate over 100 types of barcodes, supports GS1 compliance, and can print to all common label printers. Download a free 30-day trial to find out how easy it can be to protect your supply chain data.

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