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Calculating the Check Digit of a Routing Number from an Illegible Check
If the routing number on a check is illegible, the first step to getting this important nine-digit number is to use the check itself. The routing number typically appears in the bottom left hand corner and the check digit is the last number, which is the key to validating the authenticity of the routing number.
To calculate the check digit of a routing number, first write out the first eight digits as they appear on the check. Then, apply the Luhn formula, a checksum algorithm to each of the digits
- Write out the first eight digits of the check
- Starting from the right, multiply each of the odd digit positions (second-to-last, fourth-to-last, etc.) by two
- If any of the multiplied digits are larger than nine, subtract nine from the product
- Add up all the digits from the previous two steps
- Divide the final sum by 10 and take the remainder
- Subtract the remainder from 10 to obtain the check digit
For example, if the first eight digits of an illegible check are 063-8511-0, then the check digit can be calculated as follows:
- 063 8511 0
- 0 6 3 8 5 1 1 0
- 0 12 3 16 5 1 1 0
- 0 3 3 7 5 1 1 0
- 43
- 4 / 10 = remainder 4
- 10 - 4 = 6
Therefore, the check digit for this routing number is 6. Once the full nine-digit routing number including the check digit is validated with this algorithm, then the number can be used to process the transaction.
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