The checksum for a UPC-E barcode is calculated using the compressed 6-digit product code, following the same modulo 10 method as for UPC-A. Here’s how it works:

1. Expand UPC-E to UPC-A: Convert the UPC-E code into its equivalent UPC-A format. This expansion involves reconstructing the full 12-digit number.

2. Identify the Digits: Separate the digits into odd and even positions from the UPC-A number.

3. Sum Odd Positions: Add up the numbers in the odd positions (1st, 3rd, 5th, etc.).

4. Sum Even Positions: Add up the numbers in the even positions (2nd, 4th, 6th, etc.) and multiply this sum by three.

5. Add Both Sums: Combine the odd and even totals.

6. Modulo Operation: Find the result modulo 10 (remainder when divided by 10).

7. Calculate Checksum Digit: Subtract this result from 10. If the result is 10, the checksum digit becomes 0.

Let’s look at an example to understand this better:

Let’s say the UPC-E code is 042526.

1. Expand to UPC-A: Convert it to its equivalent UPC-A code: 042000052610.

2. Identify Odd and Even Digits:

   • Odd positions: 0, 2, 0, 5, 6 (sum = 13)

   • Even positions: 4, 0, 0, 2, 1 (sum = 7)

3. Multiply Even Sum by 3: 7×3=217 \times 3 = 217×3=21

4. Add Both Sums: 13+21=3413 + 21 = 3413+21=34

5. Modulo 10: 34mod  10=434 \mod 10 = 434mod10=4

6. Checksum Digit: 10−4=610 – 4 = 610−4=6

Thus, the complete UPC-A code becomes 042526, where 6 is the check digit.